Skip to main content

Some NLM-NCBI services and products are experiencing heavy traffic, which may affect performance and availability. We apologize for the inconvenience and appreciate your patience. For assistance, please contact our Help Desk at info@ncbi.nlm.nih.gov.

BMC Pregnancy and Childbirth logoLink to BMC Pregnancy and Childbirth
. 2019 Feb 15;19:72. doi: 10.1186/s12884-019-2192-z

Epidemiology of influenza in pregnant women hospitalized with respiratory illness in Moscow, 2012/2013–2015/2016: a hospital-based active surveillance study

Svetlana Trushakova 1,, Lidiya Kisteneva 1, Beatriz Guglieri-López 2, Evgenia Mukasheva 1, Irina Kruzhkova 1, Ainara Mira-Iglesias 2, Kirill Krasnoslobodtsev 2, Ekaterina Morozova 2, Ludmila Kolobukhina 1, Joan Puig-Barberà 2,#, Elena Burtseva 1,#
PMCID: PMC6377748  PMID: 30770729

Abstract

Background

To better understand the impact of seasonal influenza in pregnant women we analyzed data collected during four seasons at a hospital for acute respiratory infection that specializes in treating pregnant women.

Methods

This was a single-center active surveillance study of women 15–44 years of age hospitalized for acute respiratory diseases between 2012/2013 and 2015/2016 in Moscow, Russian Federation. Women had to have been hospitalized within 7 days of the onset of symptoms. Swabs were taken within 48 h of admission, and influenza was detected by reverse transcription-polymerase chain reaction.

Results

During the four seasons, of the 1992 hospitalized women 1748 were pregnant. Laboratory-confirmed influenza was detected more frequently in pregnant women (825/1748; 47.2%) than non-pregnant women (58/244; 23.8%) (OR for influenza = 2.87 [95% CI, 2.10–3.92]; p <  0.001). This pattern was homogenous across seasons (p = 0.112 by test of homogeneity of equal odds). Influenza A(H1N1)pdm09 was the dominant strain in 2012/2013, A(H3N2) in 2013/2014, B/Yamagata lineage and A(H3N2) in 2014/2015, and A(H1N1)pdm09 in 2015/2016. Influenza-positive pregnant admissions went to the hospital sooner than influenza-negative pregnant admissions (p <  0.001). The risk of influenza increased by 2% with each year of age and was higher in women with underlying conditions (OR = 1.52 [95% CI, 1.16 to 1.99]). Pregnant women positive for influenza were homogeneously distributed by trimester (p = 0.37 for homogeneity; p = 0.49 for trend). Frequencies of stillbirth, delivery, preterm delivery, and caesarean delivery did not significantly differ between influenza-positive and influenza-negative hospitalized pregnant women or between subtypes/lineages.

Conclusions

Pregnant women are at increased risk for hospitalization due to influenza irrespective of season, circulating viruses, or trimester.

Electronic supplementary material

The online version of this article (10.1186/s12884-019-2192-z) contains supplementary material, which is available to authorized users.

Keywords: Influenza, Pregnancy, Hospitalization, Surveillance

Background

Pregnant women are at increased risk of severe influenza illness and influenza-related death [13] and, during all trimesters, are at increased risk of hospital admission due to influenza infection [4]. Influenza illness during pregnancy also appears to be associated with increased rates of stillbirth, neonatal death, preterm delivery, and low birth weight [57]. In 2012, the World Health Organization expanded its recommendations for seasonal influenza vaccination to all pregnant women [8]. Maternal influenza vaccination does not pose a risk to the developing fetus [9] and may reduce stillbirth, growth restriction, and preterm birth [1012].

Due to small study populations and designs that limit interpretation, the real impact of influenza on pregnant women remains uncertain [4, 13, 14]. Also, many of the severe cases of influenza analyzed occurred during the 2009 A(H1N1)pdm09 pandemic, when women may have been hospitalized for precautionary reasons [4]. Additional data are therefore needed to evaluate and support vaccination policies for pregnant women.

The present investigation was conducted as part of the Global Influenza Hospital Surveillance Network (GIHSN) that aims to generate data on the impact of influenza virus infection during hospitalization. The GIHSN is an international collaboration launched in 2012 to improve understanding of influenza epidemiology and better inform public health policy decisions [15]. The GIHSN has run a multinational, prospective, hospital-based active surveillance study to collect epidemiological data over several consecutive years. Sites included in the GIHSN use a standardized protocol and standard operating procedures, allowing results to be compared and pooled [16].

Since 2012, the Federal Budget Institute of Health “Clinical Hospital for Infectious Diseases No. 1” (CHID#1) in Moscow, Russian Federation has participated in the GIHSN. CHID#1 is one of two reference hospitals for acute respiratory diseases in Moscow and specializes in treating pregnant women. Here, we analyzed data collected from CHID#1 during the four seasons since its inclusion in the GIHSN (2012/2013 to 2015/2016) to describe the epidemiology of influenza in hospitalized pregnant women, and evaluate the clinical symptoms and outcomes of influenza-associated acute respiratory illness in this population.

Methods

Study design

This was a prospective, active surveillance hospital-based study conducted during the 2012/2013 to 2015/2016 influenza seasons at CHID#1 (Moscow, Russian Federation). CHID#1 is a unique, specialized department hospital for pregnant women with infectious diseases. The study was performed due to the study site’s participation in the GIHSN international influenza surveillance project and its use of the standardized GIHSN protocols [16].

Study conduct

Patients admitted to the participating hospital were included, after written consent, if they were residents in the predefined hospital’s catchment area, presented with an acute illness possibly related to influenza, were not institutionalized, and were admitted within 7 days of the onset of symptoms. Patients discharged during the previous 30 days were excluded. Swabs were collected within 48 h from patients meeting the inclusion criteria and tested by reverse transcription-polymerase chain reaction (RT-PCR) for influenza. Influenza-positive samples were sub-typed by RT-PCR to identify A(H1N1)pdm09, A(H3N2), B/Yamagata-lineage, and B/Victoria-lineage strains.

The present analysis was limited to women 15–44 years of age admitted with an acute respiratory infection, in line with the age range used by others [3, 17, 18]. All other aspects of patient selection were in accordance with the GIHSN study protocol [16]. RT-PCR was conducted at the World Health Organization National Influenza Centre at the Ivanovsky Institute of Virology (Moscow, Russian Federation) using Amplisens® Ribo-sorb and Ribo-prep (Federal Budget Institute of Science “Central Research Institute for Epidemiology”, Moscow, Russian Federation) or a PREP-NA DNA/RNA extraction kit (DNA-Technology, Moscow, Russian Federation) to extract RNA; a Reverta-L kit (Federal Budget Institute of Science “Central Research Institute for Epidemiology”) for reverse transcription; and kits from Federal Budget Institute of Science “Central Research Institute for Epidemiology” and DNA-Technology to amplify influenza A, A(H1N1), A(H3N2), A(H1N1)pdm09, and B genes.

Socio-demographic and clinical information were collected by face-to-face interviews with patients or attending physicians or by reviewing clinical records. Information collected included socio-demographic characteristics, the major complaint at admission, smoking habits, underlying conditions, vaccination status, pregnancy outcome during the current admission, clinical course, and major diagnosis at discharge. Registered pregnancy outcomes included abortion (terminated at < 20 weeks gestational age), stillbirth (≥ 20 weeks gestational age with no heartbeat or respiratory effort), delivery (birth at any gestational age with heartbeat or respiratory effort), live preterm birth (< 37 weeks of gestation), low birth weight (< 2500 g), perinatal death (i.e. during the mother’s current admission), and caesarean delivery. Main discharge diagnoses were recorded by the physician using ICD-10 codes.

Statistical analysis

Statistical analyses were restricted to women recruited in periods with continuous influenza circulation, defined as ≥2 admissions positive for influenza in ≥2 consecutive weeks. Calendar time (admission week) was modeled using restricted cubic splines with four knots. The number of knots was set based on the Akaike information criterion [19]. The odds ratio (OR) of admission with influenza was calculated by bivariate logistic regression using the category with the lowest value as the reference. To estimate the significance of differences among groups, chi-squared, likelihood ratio, t, and nonparametric K-sample tests were used. For comparisons of continuous variables among multiple categories, equality of medians and one-way analysis of variance were used with Scheffe correction for multiple comparisons. Likelihood ratio tests were used to check for confounding, interaction, linearity, and clustering. The adjusted odds ratio (aOR) of influenza among pregnant women was estimated by multivariate logistic regression using minimal sufficient adjustment by variables identified as confounders by causal diagrams (e.g. age, underlying conditions, smoking habits, admissions in the previous year, time to swab, season, and epidemiological week at admission). The goodness of fit of the models was assessed using Akaike and Bayesian information criteria. Conditional plots [20] of the average predicted probability of influenza positive admission and of pregnancy outcome during admission were used to assess complex relationships between the pregnant women’s age in years, their infant’s gestational age, underlying conditions, influenza infection, and infection by A subtype or B lineage. The predicted probabilities of either influenza infection or pregnancy outcomes during admission were adjusted by age, smoking habits, chronic underlying conditions, admission in previous 12 months, pregnancy trimester, time to swab, and calendar time (season-week) as restricted cubic splines. All p-values were two-tailed. A p-value < 0.05 was considered to indicate statistical significance. Heterogeneity in the effects of risk factors were quantified using the I2 test, with heterogeneity defined as an I2 > 50%. All statistical analyses were performed with Stata/SE version 14 (College Station, TX, USA).

Results

Included population

The study included 1992 women 15–44 years old admitted for acute respiratory diseases between 2012/2013 and 2015/2016. Of these admissions, 1748 were pregnant (Table 1). Influenza was detected in 47.2% (825/1748) of pregnant admissions and 23.8% (58/244) of non-pregnant admissions (OR for influenza = 2.87 [95% confidence interval (CI), 2.10–3.92]; p <  0.001; data not shown). Proportions of pregnant admissions with influenza were similar during the four influenza seasons included (48.7% in 2012/2013, 44.5% in 2013/2014, 52.4% in 2014/2015, and 44.9% in 2015/2016; p = 0.112 by test for homogeneity of equal odds; data not shown). Proportions of non-pregnant women with influenza were not analyzed further because of insufficient numbers. The main comparative assessment and conclusions were made by comparing hospitalized influenza-positive pregnant women with hospitalized influenza-negative pregnant women.

Table 1.

Influenza infection status in pregnant admissions

n (%)
2012/2013 2013/2014 2014/2015 2015/2016 All seasons
RT-PCR result N = 520 N = 335 N = 296 N = 597 N = 1748
RT-PCR result
 Positivea 253 (48.7) 149 (44.5) 155 (52.4) 268 (44.9) 825 (47.2)
 Negative 267 (51.3) 186 (55.5) 141 (47.6) 329 (55.1) 923 (52.8)
Influenza type
 A(H1N1)pdm09 155 (61.3) 13 (8.7) 11 (7.1) 182 (67.9) 361 (43.8)
 A(H3N2) 41 (16.2) 100 (67.1) 62 (40.0) 21 (7.8) 224 (27.2)
 B/Yamagata lineage 6 (2.4) 34 (22.8) 66 (42.6) 0 (0.0) 106 (12.8)
 B/Victoria lineage 10 (4.0) 2 (1.3) 5 (3.2) 59 (22.0) 76 (9.2)
 A not subtyped 5 (2.0) 0 (0.0) 1 (0.6) 2 (0.7) 8 (1.0)
 B undetermined lineage 36 (14.2) 0 (0.0) 10 (6.5) 4 (1.5) 50 (6.1)

Abbreviation: RT-PCR reverse transcription-polymerase chain reaction

aTest of homogeneity (equal odds) between seasons: p = 0.1176

Influenza circulation

During the four influenza seasons included in this study, influenza was detected during similar periods, although the season varied from as short as 13 weeks in 2014/2015 to as long as 24 weeks in 2015/2016, and the peak occurred as early as week 3 in 2015/2016 and as late as week 11 in 2013/2014 (Fig. 1). Influenza A(H1N1)pdm09 was the dominant strain in 2012/2013, A(H3N2) in 2013/2014, B/Yamagata lineage and A(H3N2) in 2014/2015, and A(H1N1)pdm09 in 2015/2016 (Table 1 and Fig. 1).

Fig. 1.

Fig. 1

Number of pregnant admissions with acute respiratory infection by influenza subtype/lineage and season

Characteristics of influenza-positive pregnant admissions

Influenza-positive pregnant admissions were slightly older than influenza-negative pregnant admissions (median = 28.5 vs. 28.0 years; OR = 1.02 [95% CI, 1.00–1.04]; p = 0.021) (Table 2). The risk of a positive influenza result increased with each year by 2% (95% CI, 0–4%). When analyzed by subtype/lineage, the probability of influenza infection increased with each additional year by 3% (95% CI, 1–6%; p = 0.012) for A(H1N1)pdm09 and by 9% (4–13%; p <  0.001) for B/Yamagata lineage but was unaffected by age for A(H3N2) and B/Victoria lineage (Table 4). Median ages were higher for pregnant admissions positive for B/Yamagata-lineage viruses (30 years) than for those positive for A(H1N1)pdm09 (29 years; p = 0.01), A(H3N2) (28 years; p = 0.006), or B/Victoria lineage (28 years; p = 0.006) (Table 3). Minor but significant differences in age were found for pregnant admissions by influenza subtype/lineage (p = 0.028).

Table 2.

Characteristics of pregnant admissions by influenza infection status

Characteristic Influenza positive Influenza negative OR (95% CI) P-value
N = 825 N = 923
Age group, n (%)
 15–19 years 27 (3.3) 42 (4.6) 1
 20–24 years 153 (18.6) 214 (23.2) 1.11 (0.66–1.88) 0.692
 25–29 years 327 (39.6) 341 (36.9) 1.49 (0.90–2.48) 0.122
 30–34 years 206 (25.0) 214 (23.2) 1.50 (0.89–2.52) 0.128
 35–39 years 89 (10.8) 94 (10.2) 1.47 (0.84–2.59) 0.178
 40–44 years 23 (2.8) 18 (2.0) 1.99 (0.91–4.35) 0.086
Median age in years (range) 28.5 (15–44) 28 (15–44) 1.02 (1.00–1.04) 0.021
Underlying conditions, n (%)
 One or more 136 (16.5) 106 (11.5) 1.52 (1.16–1.99) 0.003
 Cardiovascular disease 28 (3.4) 23 (2.5) 1.37 (0.78–2.40) 0.268
 Chronic obstructive pulmonary disease 12 (1.5) 8 (0.9) 1.68 (0.69–4.14) 0.256
 Asthma 10 (1.2) 6 (0.7) 1.87 (0.68–5.17) 0.227
 Diabetes a 0 (0.0) 3 (0.3) 0.29 (0.00–2.71) 0.294
 Renal impairment 68 (8.2) 54 (5.9) 1.44 (0.99–2.09) 0.053
 Cirrhosis 9 (1.1) 8 (0.9) 1.26 (0.48–3.28) 0.637
 Neuromuscular a 1 (0.1) 0 (0.0) 1.12 (0.03–∞) 0.944
 Neoplasm 2 (0.2) 2 (0.2) 1.12 (0.16–7.95) 0.912
 Rheumatic disease a 0 (0.0) 1 (0.1) 1.12 (0.00–43.63) 1.000
 Antiviral treatment before admission, n (%) 8 (1.0) 11 (1.2) 0.81 (0.32–2.02) 0.652
 Influenza vaccination, n (%) 3 (0.4) 3 (0.3) 0.99 (0.97–1.01) 0.392
Trimester of pregnancy, n (%)
 First (0–13 weeks) 177 (21.5) 221 (23.9) 1
 Second (14–26 weeks) 355 (43.0) 372 (40.3) 1.19 (0.93–1.52) 0.162
 Third (27–42 weeks) 291 (35.3) 325 (35.2) 1.12 (0.87–1.44) 0.388
 Gestational age at admission (weeks), median (range) 23 (1–41) b 23 (3–42) c 1.00 (0.99–1.01) 0.872
Smoking habits, n (%)
 Never smoked 563 (68.2) 602 (65.2) 1
 Past smoker 225 (27.3) 268 (29.0) 0.90 (0.73–1.11) 0.317
 Current smoker 37 (4.5) 53 (5.7) 0.75 (0.48–1.15) 0.188
Socioeconomic class, n (%)
 Managerial 359 (43.5) 383 (41.5) 1
 Skilled 360 (43.6) 425 (46.0) 0.9 (0.74–1.10) 0.324
 Unskilled 34 (4.1) 52 (5.6) 0.7 (0.44–1.10) 0.121
 Other 72 (8.7) 63 (6.8) 1.22 (0.84–1.76) 0.29
Time from onset of symptoms to swabbing, n (%)
 0–2 days 559 (67.8) 541 (58.6) 1
 3–4 days 228 (27.6) 280 (30.3) 0.79 (0.64–0.97) 0.027
 5–7 days 38 (4.6) 102 (11.1) 0.36 (0.24–0.53) 0.001
 Days from onset of symptoms to swabbing, median (range) 2 (0–7) 2 (0–7) 0.87 (0.82–0.93) 0.001

aOdds ratio estimated by exact logistic regression

bN = 823

cN = 918

Table 4.

Adjusted estimates of risk factors for influenza in pregnant admissions overall and by subtype/lineage

Adjusted odds ratio (95% CI) a
Any influenza A(H1N1)pdm09 A(H3N2) B/Yamagata B/Victoria
Characteristic N = 825 N = 361 N = 224 N = 106 N = 76
Age (in years) b 1.02 (1.00–1.04) 1.03 (1.01–1.06) 1.00 (0.97–1.03) 1.09 (1.04–1.13) 0.96 (0.91–1.00)
Trimester
 First (0–13 weeks) 1 1 1 1 1
 Second (14–26 weeks) 1.19 (0.92–1.53) 1.17 (0.82–1.66) 0.89 (0.58–1.35) 1.34 (0.75–2.51) 2.28 (1.10–4.74)
 Third (27–42 weeks) 1.12 (0.86–1.46) 0.82 (0.56–1.19) 1.23 (0.82–1.87) 1.57 (0.86–2.88) 1.58 (0.72–3.44)
Underlying conditions
 No 1 1 1 1 1
 Yes 1.56 (1.16–2.08) 1.78 (1.19–2.67) 1.91 (1.25–2.93) 1.24 (0.66–2.34) 2.01 (0.39–3.15)

aAdjusted for age, trimester, comorbidity, hospital admission in the previous 12 months, time to swab, and season-week

bComparison group was influenza-negative pregnant women

Table 3.

Characteristics of pregnant admissions by influenza subtype/lineage

Characteristic A(H1N1)pdm09 A(H3N2) B/Yamagata B/Victoria P-value
N = 361 N = 224 N = 106 N = 76
Age group, n (%) 0.028
 15–19 years 11 (3.0) 10 (4.5) 2 (1.9) 2 (2.6)
 20–24 years 69 (19.1) 40 (17.9) 17 (16.0) 19 (25.0)
 25–29 years 146 (40.4) 95 (42.4) 31 (29.2) 30 (39.5)
 30–34 years 91 (25.2) 51 (22.8) 31 (29.2) 23 (30.3)
 35–39 years 37 (10.2) 21 (9.4) 18 (17.0) 1 (1.3)
 40–44 years 7 (1.9) 7 (3.1) 7 (6.6) 1 (1.3)
Age (years), median (range) 29 (17–44) 28 (15–43) 30 (19–42) 28 (19–45) 0.004
Underlying conditions a, n (%)
 None 300 (83.1) 182 (81.3) 92 (86.8) 63 (82.9) 0.666
 One or more 61 (16.9) 42 (18.8) 14 (13.2) 13 (17.1) 0.666
 Cardiovascular disease 16 (4.4) 7 (3.1) 1 (0.9) 3 (3.9) 0.376
 Chronic obstructive pulmonary disease 7 (1.9) 5 (2.2) 0 (0.0) 0 (0.0) 0.276
 Asthma 4 (1.1) 3 (1.3) 1 (0.9) 1 (1.3) 0.988
 Renal impairment 31 (8.6) 20 (8.9) 7 (6.6) 8 (10.5) 0.819
 Cirrhosis 3 (0.8) 2 (0.9) 2 (1.9) 1 (1.3) 0.803
 Neoplasm 2 (0.6) 0 (0.0) 0 (0.0) 0 (0.0) 0.521
Trimester of pregnancy, n (%) 0.005
 First (0–13 weeks) 87 (24.1) 46 (20.5) 17 (16.0) 11 (14.5)
 Second (14–26 weeks) 167 (46.3) 81 (36.2) 44 (41.5) 40 (52.6)
 Third (27–42 weeks) 106 (29.4) 97 (43.3) 45 (42.5) 25 (32.9)
 Gestational age at admission (weeks), median (range) 21 (2–41) 25 (4–40) 24 (7–41) 27 (7–38) 0.017
Time from onset of symptoms to swabbing, n (%)
 0–2 days 263 (72.9) 167 (74.6) 57 (53.8) 42 (55.3) 0.001
 3–4 days 83 (23.0) 48 (21.4) 47 (44.3) 29 (38.2)
 5–7 days 15 (4.2) 9 (4.0) 2 (1.9) 5 (6.6)
 Days from onset of symptoms to swabbing, median (range) 2 (0–7) 2 (0–6) 2 (0–5) 2 (0–6) 0.001

Abbreviation: NC not calculated

aDiabetes, neuromuscular disease, and rheumatic disease are not listed because of low numbers

Pregnant admissions who were positive for influenza more frequently had underlying conditions than those who were negative for influenza (OR = 1.52 [95% CI, 1.16–1.99], p = 0.003; aOR = 1.56 [95% CI, 1.16–2.08]) (Tables 2 and 4). This was confirmed for influenza A(H3N2) and A(H1N1) but not for the two B lineages (Table 4). However, significant differences between influenza-positive and influenza-negative pregnant admissions were not detected for the individual underlying conditions (Tables 2 and 3).

Gestational age distribution was similar in influenza-positive and influenza-negative pregnant admissions (p = 0.872) (Table 2). Pregnant admissions positive for influenza were homogeneously distributed by trimester (p = 0.37 for homogeneity in the distribution of estimates and p = 0.49 for trend, data not shown). The OR of admission with any influenza did not differ between the first and second trimesters (1.19 [95% CI, 0.92–1.53]; p = 0.16) or between the first and third trimesters (1.12 [95% CI, 0.86–1.46]; p = 0.39). Most of the pregnant admissions infected with influenza A(H1N1)pdm09 or B/Victoria lineage were in the first or second trimester, whereas most of those infected with influenza A(H3N2) or B/Yamagata lineage were in the second or third trimester, resulting in a significant difference in strain distribution by trimester (p = 0.005) (Table 3); however, aORs for each strain did not differ between trimesters (Table 4). Likewise, distributions of gestational age at admission differed significantly by virus subtype/lineage, and median gestational ages were lower for admissions positive for A(H1N1)pdm09 (21 weeks) than for admissions positive for A(H3N2) (25 weeks; p = 0.032 [data not shown]) or B/Yamagata lineage (24 weeks; p = 0.027 [data not shown]) (Table 3). Conditional plots did not reveal interactions between trimester and patient age or presence of underlying conditions for the risk of admission with any influenza or with each subtype/lineage (Additional file 1).

Rates of influenza vaccination and antiviral use before admission were ≤ 1% and did not differ between influenza-positive and -negative pregnant admissions (Table 2). Smoking habits and socioeconomic class also did not differ between pregnant admissions positive or negative for influenza.

The probability of laboratory-confirmed influenza decreased with the time between symptom onset and swabbing (Table 2). The probability was lowest in pregnant admissions with samples taken 5–7 days after the onset of symptoms (OR = 0.36 [95% CI, 0.24–0.53]; p = 0.001). The adjusted probability of a positive result decreased 13% (95% CI, 7–18%) for each day between onset of symptoms and swabbing (aOR = 0.87 [95% CI, 0.82–0.93]). Time to swabbing differed significantly between subtypes/lineages, although the median was 2 days in all cases (Table 3).

Clinical manifestations of influenza in pregnant admissions and pregnancy outcomes

Fever (p <  0.001), cough (p <  0.001), and myalgia (p <  0.001) were reported as presenting complaints more often in influenza-positive than influenza-negative pregnant admissions (Table 5). Overall, fever, reported by 97.1%, was the most common presenting complaint in pregnant admissions positive for influenza. The aOR for influenza-positive vs. influenza-negative pregnant admissions was 6.34 (95% CI, 4.01–10.03) for fever and 2.76 (95% CI, 2.13–3.43) for cough (Table 6). Cough was a common presenting complaint in pregnant admissions infected with B/Victoria lineage (86.8%) and A(H1N1)pdm09 (82.0%) but less common for those infected with A(H3N2) (69.2%) or B/Yamagata lineage (72.6%) (p <  0.001) (Table 7). Proportions of pregnant admissions reporting all other symptoms (headache, malaise, myalgia, sore throat, and dyspnea) also differed significantly by subtype/lineage. For example, dyspnea was a major presenting complaint in 18.0% of admissions with A(H1N1)pdm09 but in less than 5% of admissions with A(H3N2) or B/Yamagata-lineage and in 6.6of admissions with B/Victoria-lineage.

Table 5.

Clinical manifestations and outcomes in pregnant admissions by influenza infection status

Influenza positive Influenza negative Odds ratio (95% CI) a
Clinical manifestation/outcome N = 825 N = 923 P-value
Signs/symptoms, n (%)
 Fever 801 (97.1) 781 (84.6) 6.07 (3.89–9.46) <  0.001
 Headache 445 (53.9) 489 (53.0) 1.04 (0.86–1.25) 0.688
 Malaise 497 (60.2) 550 (59.6) 1.03 (0.85–1.24) 0.781
 Myalgia 354 (42.9) 244 (26.4) 2.09 (1.71–2.56) <  0.001
 Cough 630 (76.4) 511 (55.4) 2.60 (2.12–3.20) <  0.001
 Sore throat 546 (66.2) 689 (74.6) 0.66 (0.54–0.82) <  0.001
 Dyspnea 86 (10.4) 75 (8.1) 1.32 (0.95–1.82) 0.098
Time from onset of symptoms to admission, n (%)
 1 day 392 (47.5) 384 (41.6) 1
 2 days 209 (25.3) 194 (21.0) 1.06 (0.83–1.34) 0.661
 3 days 138 (16.7) 158 (17.1) 0.86 (0.65–1.12) 0.254
  ≥ 4 days 86 (10.4) 187 (20.3) 0.45 (0.34–0.60) <  0.001
 Time from onset of symptoms to admission, median (range) 2 (0–6) 2 (0–7) 0.85 (0.80–0.91) <  0.001
Length of hospital stay, n (%) b
 0–4 days 254 (30.8) 342 (37.1) 1
 5 days 155 (18.8) 128 (13.9) 1.63 (1.23–2.17) <  0.001
 6–7 days 261 (31.6) 259 (28.1) 1.36 (1.07–1.72) <  0.001
 Median length of hospital stay (range) 6 (0–35) 5 (0–31) 1.00 (0.98–1.04) 0.627
 Intensive care unit admission, n (%) 1 (0.1) 2 (0.2) 0.56 (0.05–6.17) 0.635
Pregnancy outcome, n (%)
 Aborted (<  20 weeks) 14 (1.7) 39 (4.2) 0.39 (0.21–0.73) 0.003
 Stillborn (≥ 20 weeks) 12 (1.5) 11 (1.2) 1.22 (0.54–2.79) 0.631
 Live delivery during the current admission, n (%) 78 (9.5) 99 (10.7) 0.87 (0.64–1.19) 0.379
 Preterm (< 37 weeks) c 17 (21.8) 19 (19.2) 1.17 (0.56–2.45) 0.669
 Cesarean‡ 14 (17.9) 16 (16.2) 1.13 (0.52–2.50) 0.753
 Low birth weight (<  2500 g) c 3 (3.8) 12 (12.1) 0.29 (0.08–1.07) 0.062
Discharge diagnosis, n (%)
 Influenza 617 (74.8) 108 (11.7) 23.34 (18.07–30.13) <  0.001
 Pneumonia 9 (1.1) 13 (1.4) 0.77 (0.33–1.82) 0.553
 Respiratory disease 187 (22.7) 736 (79.7) 0.74 (0.06–0.09) <  0.001
 Other 12 (1.5) 66 (7.2) 0.19 (0.10–0.36) <  0.001

aEstimated by exact logistic regression

bProportions are relative to deliveries during the current admission

N = 825 for influenza positive, N = 922 for influenza negative

Table 6.

Adjusted estimates of risks for clinical manifestations and outcomes in pregnant admissions overall and by subtype/lineage

Adjusted odds ratio (95% CI) a
Any influenza A(H1N1)pdm09 A(H3N2) B/Yamagata B/Victoria
Characteristic N = 825 N = 361 N = 224 N = 106 N = 76
Signs and symptoms
 Fever 6.34 (4.01–10.03) 5.81 (3.02–11.17) 6.36 (2.88–10.65) 11.0 (2.65–45.73) 5.56 (1.31–23.51)
 Headache 1.03 (0.84–1.25) 1.35 (1.02–1.77) 0.84 (0.62–1.14) 0.72 (0.47–1.09) 1.41 (0.83–2.40)
 Malaise 1.05 (0.86–1.29) 1.67 (1.26–2.22) 0.72 (0.53–0.98) 0.56 (0.37–0.85) 1.84 (1.01–3.33)
 Myalgia 1.83 (1.48–2.26) 2.62 (1.98–3.47) 1.16 (0.83–1.62) 1.71 (1.11–2.63) 2.12 (1.28–3.52)
 Cough 2.76 (2.13–3.43) 4.21 (3.04–5.83) 2.03 (1.47–2.82) 2.14 (1.35–3.38) 4.72 (2.35–9.45)
 Sore throat 0.64 (0.51–0.80) 0.53 (0.39–0.71) 0.71 (0.50–1.00) 0.57 (0.36–0.89) 0.78 (0.44–1.39)
 Dyspnea 1.10 (0.78–1.54) 2.31 (1.54–3.49) 0.37 (0.18–0.77) 0.42 (0.16–1.08) 0.98 (0.37–2.63)
Days between symptom onset and admission
 Below median 1 1 1 1 1
 Median or above 1.43 (0.88–2.32) 2.06 (1.12–3.79) 1.29 (0.56–2.99) 1.15 (0.43–3.13) 0.96 (0.26–3.51)
Length of stay
 Below median 1 1 1 1 1
 Median or above 0.94 (0.77–1.14) 1.34 (1.02–176) 0.72 (0.53–0.98) 0.52 (0.34–0.80) 1.08 (0.65–1.77)
Pregnancy outcome
 Abortion 0.32 (0.16–0.64) 0.44 (0.18–1.09) 0.30 (0.09–1.05) 0.36 (0.80–1.67)
 Stillbirth 1.10 (0.47–2.59) 0.35 (0.07–1.72) 1.14 (0.30–4.35) 3 (0.87–10.28) 1.34 (0.25–7.06)
 Live delivery 0.74 (0.52–1.06) 0.78 (0.47–1.29) 0.64 (0.37–1.10) 0.90 (0.45–1.78) 0.62 (0.20–1.94)
 Preterm delivery 0.89 (0.45–1.78) 1.07 (0.45–2.56) 0.67 (0.22–2.06) 0.42 (0.05–3.31) 2.42 (0.47–2.38)
 Cesarean delivery 0.96 (0.45–2.06) 0.46 (0.12–1.70) 1.22 (0.44–3.42) 0.87 (0.18–4.07)
 Low birth weight 0.34 (0.10–1.10) 0.28 (0.03–2.13) 0.42 (0.09–2.06) 2.85 (0.29–1.07)

aAdjusted for age, trimester, comorbidity, hospital admission in the previous 12 months, time to swab, and season-week

Table 7.

Clinical manifestations and outcomes in pregnant admissions by influenza subtype/lineage

A(H1N1)pdm09 A(H3N2) B/Yamagata B/Victoria
Manifestation/outcome N = 361 N = 224 N = 106 N = 76 P-value
Signs/symptoms, n (%)
 Fever 350 (97.0) 217 (96.9) 104 (98.1) 74 (97.4) 0.924
 Headache 209 (57.9) 110 (49.1) 48 (45.3) 52 (68.4) 0.003
 Malaise 240 (66.5) 114 (50.9) 51 (48.1) 60 (78.9) <  0.001
 Myalgia 191 (52.9) 73 (32.6) 42 (39.6) 35 (46.1) <  0.001
 Cough 296 (82.0) 155 (69.2) 77 (72.6) 66 (86.8) <  0.001
 Sore throat 215 (59.6) 160 (71.4) 71 (67.0) 56 (73.7) 0.009
 Dyspnea 65 (18.0) 9 (4.0) 5 (4.7) 5 (6.6) <  0.001
Time from onset of symptoms to admission, n (%) <  0.001
 1 day 195 (54.0) 115 (51.3) 29 (27.4) 31 (40.8)
 2 days 98 (27.1) 58 (25.9) 32 (30.2) 13 (17.1)
 3 days 40 (11.1) 31 (13.8) 35 (33.0) 16 (21.1)
  ≥ 4 days 28 (7.8) 20 (8.9) 10 (9.4) 16 (21.1)
 Time from onset of symptoms to admission (days), median (range) 1 (0–6) 1 (0–6) 2 (0–5) 2 (0–5) <  0.001
Length of hospitalization stay, n (%) a <  0.001
 0–4 days 124 (34.3) 61 (27.2) 22 (20.8) 26 (34.2)
 5 days 85 (23.5) 32 (14.3) 14 (13.2) 16 (21.1)
 6–7 days 100 (27.7) 84 (37.5) 39 (36.8) 26 (34.2)
  ≥ 8 days 52 (14.4) 46 (20.5) 30 (28.3) 8 (10.5)
 Length of hospitalization stay (days), median (range) 5 (0–35) 6 (0–14) 6 (1–16) 5 (0–11) <  0.001
 Admission to an intensive care unit, n (%) 1 (0.3) 0 (0.0) 0 (0.0) 0 (0.0) 0.771
Pregnancy outcome, n (%)
 Abortion (<  20 weeks) 8 (2.2) 3 (1.3) 2 (1.9) 0 (0.0) 0.667
 Stillbirth (≥ 20 weeks) 2 (0.6) 3 (12.5) 4 (3.8) 2 (2.6) 0.050
 Live delivery during the current admission 31 (8.6) 24 (10.7) 14 (13.2) 4 (5.3) 0.261
 Preterm (< 37 weeks gestational age) b 9 (29.0) 4 (16.7) 1 (7.1) 2 (50.0) 0.177
 Cesarean† 3 (9.7) 6 (25.0) 2 (14.3) 0 (0.0) 0.391
 Low birth weight (<  2500 g) c 1 (3.2) 1 (4.2) 0 (0.0) 1 (25.0) 0.276

aN = 223 for A(H3N2) and N = 105 for B/Yamagata

bProportions are relative to deliveries

Hospital admission occurred sooner after the onset of symptoms in influenza-positive than influenza-negative pregnant admissions (p <  0.001) (Table 5). This was particularly the case for pregnant admissions positive for influenza A(H1N1)pdm09 and influenza A(H3N2), where more than half went to the hospital on the first day (Table 7). In contrast, more than half of pregnant admissions positive for influenza B went to the hospital by the second day. As a result, the time to symptom onset differed significantly by strain (p <  0.001).

The overall length of hospital stay was not significantly longer for pregnant admissions positive for influenza than those negative for influenza (Table 5); however, pregnant women hospitalized for > 4 days had a higher probability of laboratory-confirmed influenza than those hospitalized for ≤4 days (OR = 1.62 [95%CI, 1.21–2.16]; p = 0.001; data not shown). The median hospital stay also differed by subtype/lineage (Table 7) and was longer for pregnant women infected with B/Yamagata lineage (6.4 days) than for those infected with influenza B/Victoria lineage (4.9 days; p = 0.025 [data not shown]) or A(H1N1)pdm09 (5.4 days; p = 0.074 [data not shown]) but did not differ for those infected with A(H3N2) (5.9 days; p > 0.05 [data not shown]).

At discharge, the most common diagnosis was other respiratory infections (n = 923; 52.8%) followed by influenza (n = 725; 41.5%) (Table 5). Pneumonia was the main discharge diagnosis in 22 cases (0.1%) and did not differ between influenza-positive and influenza-negative pregnant women.

Only one influenza-positive pregnant admission and only two influenza-negative pregnant admissions were hospitalized in an intensive care unit (Table 5). No deaths were reported.

Pregnancy outcomes

Pregnancy outcomes recorded during the hospital stay included 177 live births, 53 abortions (pregnancy stopped before 20 weeks), and 23 stillbirths (Table 5). No perinatal deaths were reported (data not shown).

Abortion was more frequent in influenza-negative than influenza-positive pregnant admissions (4.2% vs. 1.7%; p = 0.003), although the frequency did not significantly differ by subtype/lineage (Table 7). Frequencies of other pregnancy outcomes did not differ between strains, but predicted probabilities were higher for stillbirth in women infected with B/Yamagata, for cesarean delivery in women infected with A(H3N2), and for preterm delivery and low birth weight in women infected with B/Victoria (Table 8 and Fig. 2).

Table 8.

Predicted probability of pregnancy outcomes during admission according to influenza infection status

Outcome RT-PCR result Adjusted predicted probability a (95% CI) n
Abortion (<  20 weeks) No influenza 0.07 (0.05, 0.09) 39
A(H1N1)pdm09 0.03 (0.01, 0.06) 8
A(H3N2) 0.02 (0.00, 0.05) 3
B/Yamagata-lineage 0.03 (−0.01, 0.06) 2
Stillbirth (≥ 20 weeks) No influenza 0.02 (0.01, 0.03) 11
A(H1N1)pdm09 0.01 (0.00, 0.02) 2
A(H3N2) 0.02 (0.00, 0.05) 3
B/Yamagata-lineage 0.06 (0.00, 0.12) 4
B/Victoria-lineage 0.03 (−0.01, 0.07) 2
Live delivery during admission No influenza 0.11 (0.09, 0.13) 99
A(H1N1)pdm09 0.09 (0.06, 0.12) 31
A(H3N2) 0.09 (0.05, 0.12) 24
B/Yamagata-lineage 0.11 (0.06, 0.16) 14
B/Victoria-lineage 0.08 (0.01, 0.15) 4
Preterm (< 37 weeks gestational age) No influenza 0.13 (0.07, 0.20) 19
A(H1N1)pdm09 0.27 (0.06, 0.48) 9
A(H3N2) 0.19 (0.01, 0.38) 4
B/Yamagata-lineage 0.06 (−0.05, 0.17) 1
B/Victoria-lineage 0.54 (− 0.05, 1.13) 2
Cesarean delivery No influenza 0.18 (0.09, 0.26) 16
A(H1N1)pdm09 0.09 (−0.02, 0.21) 3
A(H3N2) 0.29 (0.09, 0.49) 6
B/Yamagata-lineage 0.15 (−0.05, 0.36) 2
Low birth weight (<  2500 g) No influenza 0.14 (0.07, 0.21) 12
A(H1N1)pdm09 0.05 (−0.06, 0.16) 1
A(H3N2) 0.06 (−0.06, 0.17) 1
B/Victoria-lineage 0.55 (−0.05, 1.15) 1

Abbreviation: RT-PCR reverse transcription-polymerase chain reaction

aAdjusted by age, smoking habits, chronic underlying conditions, admission in last 12 months, trimester, time to swab and season-week

Fig. 2.

Fig. 2

Predicted probabilities of birth outcomes according to the mother’s age and influenza strain. Predicted probabilities of abortion (a), stillbirth (b), live delivery (c), preterm delivery (d), cesarean delivery (e), and low birth weight (f) in influenza-positive pregnant admissions. Probabilities were adjusted by age, smoking habits, chronic underlying conditions, previous admissions, trimester, time to swab, and season-week

Age had a noticeable impact on pregnancy outcomes in influenza-positive admissions. The probabilities of abortion and stillbirth increased with the mother’s age, whereas the probability of live birth decreased with the mother’s age (Fig. 2). Probabilities of preterm delivery, caesarean delivery, and low birth weight were highest in the youngest mothers, although a second smaller increase in probability occurred in women 35–40 years of age.

Discussion

In 2012, the World Health Organization identified pregnant women and newborns as priority risk groups for seasonal influenza [8]. However, the full burden of influenza in pregnant women and their infants is poorly understood due to differences in the design and conduct of epidemiological studies, and the small numbers of pregnant women included [2123]. The present study describes new epidemiological data of influenza infection among a large cohort of pregnant women, and the impact of influenza on clinical outcomes in these women and their infants. Using a prospective, active-surveillance study design as part of the GIHSN’s hospital-based surveillance [16], we collected data from more than 1700 pregnant women (825 with confirmed influenza) admitted to a hospital specializing in acute respiratory infections.

We restricted the analysis to women aged 15–44 years. A standard age range (often ages 16–49) is usually [24], but not always [25], chosen to define women of childbearing age. This age-range includes women who are less likely to be pregnant [24, 26], as was the case in our investigation – the number of pregnant women aged 45–50 was exceedingly small (< 1%, data not shown). In addition, the distribution of pregnant women in the lowest (15–19 year) and highest (40–44 year) age groups in our study was similar, and we restricted the childbearing age range (15 to < 44) according to pregnancy probability in our dataset to ensure consistency and reasonable estimates.

Influenza infection, confirmed by RT-PCR, accounted for nearly half of the admissions over the four influenza seasons. The risk of influenza infection was higher in pregnant than non-pregnant women (OR = 2.87 [95% CI, 2.10–3.92]), irrespective of subtype/lineage and trimester. This agrees with a larger multi-country study by the GIHSN during the 2012/2013 influenza season, which found an aOR of 3.84 (95% CI, 2.48–5.94) for influenza-related hospitalization in pregnant vs. non-pregnant women [16]. It also agrees with a meta-analysis of influenza A infection, which found a combined OR of 2.44 (95% CI 1.22–4.87) for influenza-related hospitalization in pregnant vs. non-pregnant women, although most of the included studies were during the 2009 A(H1N1) pandemic season [4].

Underlying conditions, especially anemia, obesity, and asthma, increase the risk of influenza-related hospitalization in pregnant women [27, 28]. Although we confirmed this in the present study, we were unable to detect significant differences for individual conditions, probably because of insufficient numbers. The present study also confirmed that the risk of hospitalization with influenza does not differ by trimester or influenza subtype, as described by others [29].

Influenza-positive pregnant admissions were hospitalized sooner after the onset of symptoms and stayed slightly longer in the hospital than influenza-negative pregnant admissions. Pregnant admissions positive for influenza also more frequently complained of cough, myalgia, and especially fever than those who were negative for influenza. This suggests that influenza causes more severe illness in pregnant women than other kinds of acute respiratory infection.

Influenza viruses varied substantially between seasons, although all subtypes and lineages resulted in hospitalization. In addition, demographics, clinical manifestations, and rates of stillbirth differed slightly between subtypes/lineages. For example, the risk of influenza infection increased slightly with the mother’s age. Also, admissions with influenza A(H1N1)pdm09 or B/Victoria lineage occurred mostly in the first or second trimester, whereas admissions with influenza A(H3N2) or B/Yamagata lineage occurred mostly in the second or third trimester. Although influenza B has been reported to be more frequent in the second trimester than influenza A [29], these results suggest that the two B lineages cannot be considered equivalent.

Influenza, especially A(H1N1), is considered a risk for stillbirth and low birth weight [27, 3032]. However, we did not find differences in rates of stillbirth, preterm delivery, or caesarean delivery between influenza-positive and -negative pregnant admissions or between subtypes/lineages. In agreement with this, a recent meta-analysis reported a computed pooled OR of 1.24 (95% CI, 0.96–1.59) for small for gestational age, suggesting that influenza does not affect birth weight [23]. Unexpectedly, however, abortion before 20 weeks was more frequent in influenza-negative than influenza-positive women, suggesting that other respiratory infections pose a higher risk of abortion.

These data confirm that pregnant women are at increased risk from seasonal influenza A and B viruses. With more than 1700 pregnant admissions, this study provides important and detailed information about the impact of influenza in pregnant women that can be used to inform and support vaccination policies in this susceptible population. Furthermore, our study provides pregnancy outcome data, which are rarely included in epidemiological studies of influenza in pregnant women, and have not been published before.

However, this study had some limitations. The main analysis was based on comparing hospitalized influenza-positive with hospitalized influenza-negative pregnant women. We were unable to compare pregnant admissions to non-pregnant admissions because of insufficient numbers of non-pregnant women, who are more frequently admitted for influenza-like illness to other hospitals in Moscow. Nonetheless, the unique nature of the CHID#1 study site allowed us to recruit a substantial number of pregnant women. CHID#1 receives the most pregnant admissions from any of the hospitals in the GIHSN network (over 97% of the total pregnant admissions based on unpublished GIHSN data from the 2015/2016 season). Another limitation was that we could not assess the long-term effects of influenza on pregnant women, or pregnancy outcome beyond the current admission, because data were collected only from women while they were hospitalized, and follow-up evaluations until the end of pregnancy were not within the study protocol. Finally, the study could have been limited by increasing rates of hospitalization for pregnant women following the 2009 pandemic due to increased awareness of the risks. However, this was probably accounted for by recruiting consecutive admissions without previous knowledge of influenza status. Furthermore, influenza positivity rates were similar over the four influenza seasons, suggesting that this was not a problem.

Conclusions

Our results confirm that pregnant women are at increased risk from influenza infection irrespective of season, circulating viruses, or trimester. This supports recommendations by the World Health Organization [8] and many countries [33, 34] that pregnant women be prioritized for seasonal influenza vaccination. Despite these recommendations and evidence that influenza vaccination is considered safe and effective for pregnant women [7, 10, 12, 35, 36], vaccination uptake by pregnant women is generally poor [37], as found in the present study, where < 1% of pregnant women were vaccinated. Additional efforts are therefore needed to educate healthcare workers, public health officials, and pregnant women about the risks of seasonal influenza and the importance of vaccination.

Additional file

Additional file 1: (35.5KB, pdf)

Predicted probability of admission with influenza by (a) trimester, (b-e) subtype/lineage, and (f) overall according to age group and presence of underlying conditions. Conditional plots examining interactions between trimester and patient age or presence of underlying conditions for the risk of admission with any influenza or with each subtype/lineage. (PDF 35 kb)

Acknowledgements

Medical writing support was provided by Drs. Phillip Leventhal and Jonathan M. Pitt (4Clinics, France) and paid for by Sanofi Pasteur.

Funding

The work reported in this manuscript was funded by Sanofi Pasteur.

Availability of data and materials

Qualified researchers may request access to patient-level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi’s data sharing criteria, eligible studies, and process for requesting access can be found at: https://www.clinicalstudydatarequest.com.

Abbreviations

aOR

Adjusted odds ratio

CHID#1

Federal Budget Institute of Health “Clinical Hospital for Infectious Diseases No. 1”

CI

Confidence interval

GIHSN

Global Influenza Hospital Surveillance Network

OR

Odds ratio

RT-PCR

Reverse transcription-polymerase chain reaction

Authors’ contributions

LK1, IK, and LK2 contributed to data acquisition and enrolling participants; ST, EM1, KK, and EM2 contributed to laboratory data testing, BG, AM, and ST contributed to data processing, analysis, or interpretation; JP and EB provided overall study oversight and JP wrote the first draft of the manuscript. All authors provided critical review or revisions of the manuscript, approved the final draft, and agree to be accountable for its accuracy and integrity.

Ethics approval and consent to participate

The study was approved by the “Ethics Committee of Hospital No.1 for Infectious Diseases”, CHID#1, Moscow Health Department, Russian Federation. All participants provided written consent. Written consent was obtained from the parents or legal guardians of participants under the age of 16 years, in line with the GIHSN protocol and local legislation.

Consent for publication

Not relevant.

Competing interests

ST, LK1, EM1, IK, KK, EM2, LK2, and EB report grants from Fondation Merieux; grants and non-financial support from Sanofi Pasteur; non-financial support from The Foundation for the Promotion of Health and Biomedical Research of Valencia Region (FISABIO) during the conduct of the study (scientific counselling, discussion on methods, data quality management and analysis, and provision of the study protocol and questionnaires); and grants from the Influenza Division of the US Centers for Disease Control and Prevention outside of the submitted work. The remaining authors have no competing interests to declare.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Svetlana Trushakova, Email: s.trushakova@gmail.com.

Lidiya Kisteneva, Email: lborisovna2007@yandex.ru.

Beatriz Guglieri-López, Email: gugtriz@gmail.com.

Evgenia Mukasheva, Email: mukasheva_evgeniya@mail.ru.

Irina Kruzhkova, Email: irina-kru@yandex.ru.

Ainara Mira-Iglesias, Email: amisqrt2@gmail.com.

Kirill Krasnoslobodtsev, Email: kkg_87@mail.ru.

Ekaterina Morozova, Email: katerina.garina@gmail.com.

Ludmila Kolobukhina, Email: lkolobuchina@yandex.ru.

Joan Puig-Barberà, Email: jpuigb55@gmail.com.

Elena Burtseva, Email: elena_burceva@mail.ru.

References

  • 1.Dodds L, McNeil SA, Fell DB, Allen VM, Coombs A, Scott J, et al. Impact of influenza exposure on rates of hospital admissions and physician visits because of respiratory illness among pregnant women. CMAJ. 2007;176(4):463–468. doi: 10.1503/cmaj.061435. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Mullooly JP, Barker WH, Nolan TF., Jr Risk of acute respiratory disease among pregnant women during influenza a epidemics. Public Health Rep. 1986;101(2):205–211. [PMC free article] [PubMed] [Google Scholar]
  • 3.Neuzil KM, Reed GW, Mitchel EF, Simonsen L, Griffin MR. Impact of influenza on acute cardiopulmonary hospitalizations in pregnant women. Am J Epidemiol. 1998;148(11):1094–1102. doi: 10.1093/oxfordjournals.aje.a009587. [DOI] [PubMed] [Google Scholar]
  • 4.Mertz D, Geraci J, Winkup J, Gessner BD, Ortiz JR, Loeb M. Pregnancy as a risk factor for severe outcomes from influenza virus infection: a systematic review and meta-analysis of observational studies. Vaccine. 2017;35(4):521–528. doi: 10.1016/j.vaccine.2016.12.012. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 5.Fell DB, Buckeridge DL, Platt RW, Kaufman JS, Basso O, Wilson K. Circulating influenza virus and adverse pregnancy outcomes: a time-series study. Am J Epidemiol. 2016;184(3):163–175. doi: 10.1093/aje/kww044. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Hewagama S, Walker SP, Stuart RL, Gordon C, Johnson PD, Friedman ND, et al. 2009 H1N1 influenza a and pregnancy outcomes in Victoria, Australia. Clin Infect Dis. 2010;50(5):686–690. doi: 10.1086/650460. [DOI] [PubMed] [Google Scholar]
  • 7.Louie JK, Acosta M, Jamieson DJ, Honein MA. Severe 2009 H1N1 influenza in pregnant and postpartum women in California. N Engl J Med. 2010;362(1):27–35. doi: 10.1056/NEJMoa0910444. [DOI] [PubMed] [Google Scholar]
  • 8.World Health Organization Vaccines against influenza WHO position paper - November 2012. Wkly Epidemiol Rec. 2012;87(47):461–476. [PubMed] [Google Scholar]
  • 9.Fell DB, Platt RW, Lanes A, Wilson K, Kaufman JS, Basso O, et al. Fetal death and preterm birth associated with maternal influenza vaccination: systematic review. BJOG. 2015;122(1):17–26. doi: 10.1111/1471-0528.12977. [DOI] [PubMed] [Google Scholar]
  • 10.Madhi SA, Cutland CL, Kuwanda L, Weinberg A, Hugo A, Jones S, et al. Influenza vaccination of pregnant women and protection of their infants. N Engl J Med. 2014;371(10):918–931. doi: 10.1056/NEJMoa1401480. [DOI] [PubMed] [Google Scholar]
  • 11.Savitz DA, Fell DB, Ortiz JR, Bhat N. Does influenza vaccination improve pregnancy outcome? Methodological issues and research needs. Vaccine. 2015;33(47):6430–6435. doi: 10.1016/j.vaccine.2015.08.041. [DOI] [PubMed] [Google Scholar]
  • 12.Zaman K, Roy E, Arifeen SE, Rahman M, Raqib R, Wilson E, et al. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med. 2008;359(15):1555–1564. doi: 10.1056/NEJMoa0708630. [DOI] [PubMed] [Google Scholar]
  • 13.Lambach P, Hombach J, Ortiz JR. A global perspective of maternal influenza immunization. Vaccine. 2015;33(47):6376–6379. doi: 10.1016/j.vaccine.2015.08.036. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Mertz D, Kim TH, Johnstone J, Lam PP, Science M, Kuster SP, et al. Populations at risk for severe or complicated influenza illness: systematic review and meta-analysis. BMJ. 2013;347:f5061. doi: 10.1136/bmj.f5061. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Puig-Barbera J, Tormos A, Trushakova S, Sominina A, Pisareva M, Ciblak MA, et al. The global influenza hospital surveillance network (GIHSN): a new platform to describe the epidemiology of severe influenza. Influenza Other Respir Viruses. 2015. [DOI] [PMC free article] [PubMed]
  • 16.Puig-Barbera J, Natividad-Sancho A, Launay O, Burtseva E, Ciblak MA, Tormos A, et al. 2012-2013 seasonal influenza vaccine effectiveness against influenza hospitalizations: results from the global influenza hospital surveillance network. PLoS One. 2014;9(6):e100497. doi: 10.1371/journal.pone.0100497. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 17.Hartert TV, Neuzil KM, Shintani AK, Mitchel EF, Jr, Snowden MS, Wood LB, et al. Maternal morbidity and perinatal outcomes among pregnant women with respiratory hospitalizations during influenza season. Am J Obstet Gynecol. 2003;189(6):1705–1712. doi: 10.1016/S0002-9378(03)00857-3. [DOI] [PubMed] [Google Scholar]
  • 18.Dolan GP, Myles PR, Brett SJ, Enstone JE, Read RC, Openshaw PJ, et al. The comparative clinical course of pregnant and non-pregnant women hospitalised with influenza a(H1N1)pdm09 infection. PLoS One. 2012;7(8):e41638. doi: 10.1371/journal.pone.0041638. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 19.Motulsky HCA. Fitting models to biological data using linear and nonlinear regression: a practical guide to curve fitting. New York: Oxford University Press; 2004. [Google Scholar]
  • 20.Kohler U, Kreuter F. Data analysis using Stata. 3. College Station, TX: Stata press; 2012. Conditional-effects plots; pp. 321–324. [Google Scholar]
  • 21.Katz MA, Gessner BD, Johnson J, Skidmore B, Knight M, Bhat N, et al. Incidence of influenza virus infection among pregnant women: a systematic review. BMC Pregnancy Childbirth. 2017;17(1):155. doi: 10.1186/s12884-017-1333-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 22.McMillan M, Porritt K, Kralik D, Costi L, Marshall H. Influenza vaccination during pregnancy: a systematic review of fetal death, spontaneous abortion, and congenital malformation safety outcomes. Vaccine. 2015;33(18):2108–2117. doi: 10.1016/j.vaccine.2015.02.068. [DOI] [PubMed] [Google Scholar]
  • 23.Fell DB, Savitz DA, Kramer MS, Gessner BD, Katz MA, Knight M, et al. Maternal influenza and birth outcomes: systematic review of comparative studies. BJOG. 2017;124(1):48–59. doi: 10.1111/1471-0528.14143. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Parker J, Branum A, Axelrad D, Cohen J. Adjusting National Health and nutrition examination survey sample weights for women of childbearing age. Vital Health Stat 2 2013(157):1–20. [PubMed]
  • 25.Thompson MG, Kwong JC, Regan AK, Katz MA, Drews SJ, Azziz-Baumgartner E, et al. Influenza vaccine effectiveness in preventing influenza-associated hospitalizations during pregnancy: a multi-country retrospective test negative design study, 2010-2016. Clin Infect Dis. 2018. [DOI] [PubMed]
  • 26.Axelrad DA, Cohen J. Calculating summary statistics for population chemical biomonitoring in women of childbearing age with adjustment for age-specific natality. Environ Res. 2011;111(1):149–155. doi: 10.1016/j.envres.2010.10.002. [DOI] [PubMed] [Google Scholar]
  • 27.He J, Liu ZW, Lu YP, Li TY, Liang XJ, Arck PC, et al. A systematic review and meta-analysis of influenza a virus infection during pregnancy associated with an increased risk for stillbirth and low birth weight. Kidney Blood Press Res. 2017;42(2):232–243. doi: 10.1159/000477221. [DOI] [PubMed] [Google Scholar]
  • 28.Schanzer DL, Langley JM, Tam TW. Influenza-attributed hospitalization rates among pregnant women in Canada 1994-2000. J Obstet Gynaecol Can. 2007;29(8):622–629. doi: 10.1016/S1701-2163(16)32559-2. [DOI] [PubMed] [Google Scholar]
  • 29.Regan AK, Moore HC, Sullivan SG. N DEK, Effler PV. Epidemiology of seasonal influenza infection in pregnant women and its impact on birth outcomes. Epidemiol Infect. 2017;145(14):2930–2939. doi: 10.1017/S0950268817001972. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Yates L, Pierce M, Stephens S, Mill AC, Spark P, Kurinczuk JJ, et al. Influenza a/H1N1v in pregnancy: an investigation of the characteristics and management of affected women and the relationship to pregnancy outcomes for mother and infant. Health Technol Assess. 2010;14(34):109–182. doi: 10.3310/hta14340-02. [DOI] [PubMed] [Google Scholar]
  • 31.Pierce M, Kurinczuk JJ, Spark P, Brocklehurst P, Knight M. Perinatal outcomes after maternal 2009/H1N1 infection: national cohort study. BMJ. 2011;342:d3214. doi: 10.1136/bmj.d3214. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 32.Naresh A, Fisher BM, Hoppe KK, Catov J, Xu J, Hart J, et al. A multicenter cohort study of pregnancy outcomes among women with laboratory-confirmed H1N1 influenza. J Perinatol. 2013;33(12):939–943. doi: 10.1038/jp.2013.110. [DOI] [PubMed] [Google Scholar]
  • 33.GOV.UK. Annual flu programme. 2018. https://www.gov.uk/government/collections/annual-flu-programme. Accessed 18 Jun 2018.
  • 34.US Advisory Committee on Immunization Practices. Prevention and control of seasonal influenza with vaccines Recommendations of the advisory committee on immunization practices--United States, 2013-2014. MMWR Recomm Rep. 2013;62(RR-07):1–43. [PubMed] [Google Scholar]
  • 35.Naleway AL, Smith WJ, Mullooly JP. Delivering influenza vaccine to pregnant women. Epidemiol Rev. 2006;28:47–53. doi: 10.1093/epirev/mxj002. [DOI] [PubMed] [Google Scholar]
  • 36.Tapia MD, Sow SO, Tamboura B, Teguete I, Pasetti MF, Kodio M, et al. Maternal immunisation with trivalent inactivated influenza vaccine for prevention of influenza in infants in Mali: a prospective, active-controlled, observer-blind, randomised phase 4 trial. Lancet Infect Dis. 2016;16(9):1026–1035. doi: 10.1016/S1473-3099(16)30054-8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 37.Yuen CY, Tarrant M. Determinants of uptake of influenza vaccination among pregnant women - a systematic review. Vaccine. 2014;32(36):4602–4613. doi: 10.1016/j.vaccine.2014.06.067. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Additional file 1: (35.5KB, pdf)

Predicted probability of admission with influenza by (a) trimester, (b-e) subtype/lineage, and (f) overall according to age group and presence of underlying conditions. Conditional plots examining interactions between trimester and patient age or presence of underlying conditions for the risk of admission with any influenza or with each subtype/lineage. (PDF 35 kb)

Data Availability Statement

Qualified researchers may request access to patient-level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi’s data sharing criteria, eligible studies, and process for requesting access can be found at: https://www.clinicalstudydatarequest.com.


Articles from BMC Pregnancy and Childbirth are provided here courtesy of BMC

RESOURCES