Comparison of SARS-CoV-2 total and neutralizing antibody transfer to neonates from vaccinated mothers vs. mothers infected during pregnancy: an observational study

Abstract

Background

Differences in antibody production against SARS-CoV-2 arise from both, infection and immunization, leading to variations in neutralization capacity and transplacental antibody transfer during pregnancy. A deeper understanding of these mechanisms can provide valuable insights into neonatal protection against viral exposure and inform the evaluation of vaccination strategies during pregnancy. This study aimed to compare the transfer of IgG and neutralizing antibodies between neonates born to mothers vaccinated against SARS-CoV-2 either during or before gestation and those born to mothers infected during pregnancy.

Methods

A multicenter cohort study was conducted across four hospitals in Bogotá, Colombia, from March to October 2023. The study assessed IgG and neutralizing antibodies in blood samples from pregnant women and umbilical cord blood from 86 mother-child pairs. The cohort comprised 40 mothers vaccinated during pregnancy (46.5%), 35 vaccinated before pregnancy (40.6%), and 11 infected with SARS-CoV-2 during pregnancy (12.8%). Statistical analysis involved Fisher’s exact test for categorical variables and ANOVA for continuous variables. The Pearson correlation coefficient was employed to evaluate the correlation between maternal and umbilical cord neutralizing antibodies. Additionally, logistic regression was utilized to examine the association between neutralizing antibodies in maternal and umbilical cord blood.

Results

The proportion of patients with positive neutralizing antibodies (defined as > 20%) differed significantly between groups, with the highest percentage observed in pregnant women vaccinated before pregnancy (85.7%). Notably, the transfer rate of neutralizing antibodies was significantly higher in infected mothers (mean = 1.0, Interquartile Range = 1.0–1.2, p = 0.014). A moderate-to-strong correlation was identified between maternal and cord neutralization percentages (r = 0.6003), irrespective of the group. Furthermore, vaccination before pregnancy (Log Odds ratio (LOR) 3.18 [95% CI 1.95–4.40]) or infection during pregnancy (LOR 2.89 [95% CI 1.17–4.61]) was associated with a higher probability of achieving a higher neutralization percentage compared to those vaccinated during pregnancy.

Conclusion

This study provided evidence of maternal antibody mediated immune response to both, SARS-CoV-2 infection and COVID-19 vaccination. The findings suggest that vaccination prior to pregnancy elicits more effective antibody responses compared to vaccination during pregnancy, highlighting the effectiveness of anti-COVID-19 vaccination prior to pregnancy.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s12884-026-09539-7.

Background

The SARS-CoV-2 pandemic has exerted a profound impact on the population health globally, with pregnant women and their newborns being the more affected groups. This novel coronavirus has led to substantial mortality and morbidity rates across various demographics. In Colombia, from the initial detection of SARS-CoV-2 cases on March 6, 2020, to December 2023, a total of 40,779 pregnant women and 2,593 newborns tested positive for the virus. Tragically, 247 pregnant women and 10 newborns succumbed to complications associated with the infection [1].

Pregnant women are particularly vulnerable to complications arising from respiratory infections, including those caused by SARS-CoV-2 and influenza viruses [2–4]. Research focusing on COVID-19 infection in this population has revealed an elevated risk of several adverse outcomes, including admission to intensive care units (ICUs), hospitalization of both mother and newborn, and the necessity for invasive mechanical ventilation or extracorporeal membrane oxygenation (ECMO) [5]. Furthermore, these studies [5–7] have identified an increased likelihood of obstetric complications, such as preterm birth (OR 1.17, 95% CI 1.06–1.29), preeclampsia (OR 1.21, 95% CI 1.11–1.33), and thromboembolic events (OR 4.47, 95% CI 2.87–6.96) [8].

Given the substantial risks and potential complications associated with SARS-CoV-2 infection in pregnant women, coupled with the evidence demonstrating the safety and efficacy of COVID-19 vaccines (with efficacy rates ranging from 66.1% to 95%, depending on the specific vaccine formulation) for both mothers and newborns, immunization stands out as a crucial strategy for mitigating maternal morbidity and mortality [9]. Vaccination has proven to be a highly effective approach in reducing the severity of COVID-19, with studies indicating an impressive effectiveness of up to 94% in preventing severe outcomes [10].

Shimabukuro et al. [11]analyzed data from the v-safe system from December 14, 2020, to February 28, 2021 and found that the incidence of adverse events during pregnancy in women vaccinated against COVID-19 was similar to pre-pandemic rates, with stillbirths (0.1%) and congenital malformations (2.2%) comparable to those reported before the pandemic. Blakewat H. et al. [12] conducted a cohort study with 1,328 vaccinated pregnant women and 1,188 unvaccinated women, showing no significant differences in outcomes like low weight according to the gestational age, congenital anomalies, or NICU admissions.

A retrospective cohort study by Goldshtein et al. [13] in Israel found that vaccinated pregnant women had a lower risk of SARS-CoV-2 infection compared to unvaccinated women, indicating a protective effect of vaccination. These findings support the efficacy of COVID-19 vaccines during pregnancy, as highlighted by organizations like ACOG, SMFM, and WHO [14–16]. In Colombia, vaccination for pregnant women began in July 2021, with the Pfizer vaccine chosen due to its safety and efficacy [17].

Research on vaccine immunogenicity and maternal-fetal antibody transfer is ongoing. IgG antibodies cross the placenta around the 13th week of pregnancy, providing newborns with passive immunity [18, 19]. Studies by Collier et al. [20] and Beharier et al. [21] found that mRNA vaccines generate high IgG-RBD titers in pregnant women and demonstrate efficient antibody transfer to the fetus, supporting the safety and effectiveness of vaccination during pregnancy.

The current body of research exhibits a notable gap in our understanding of SARS-CoV-2-specific antibody production following infection, their post-immunization stability, and the variability in their neutralizing potential. This knowledge deficit highlights the critical need to investigate these immunological characteristics in pregnant Colombian women. Such an investigation would establish a crucial baseline data that, when extrapolated to the wider population, could potentially inform and refine vaccination strategies for this particular population group. The objective of this study was to compare the transfer of IgG antibodies targeting the receptor-binding domain (RBD) and neutralizing antibodies (NAbs) in newborns of mothers who were vaccinated against SARS-CoV-2 during pregnancy, those vaccinated prior to pregnancy, and those who were infected during pregnancy.

Methods

Study type

A multicenter cohort study was conducted across four tertiary-care hospitals in Bogotá, Colombia, from March to October 2023. The study protocol received approval from the Ethics Committee of Universidad El Bosque on April 25, 2022 (Minute No. 008-2022). Additionally, ethical clearance was obtained from the respective Ethics Committees of Hospital Universitario Clínica San Rafael, Los Cobos Medical Center, Centro Policlínico del Olaya, and the Subred Integrada de Servicios de Salud Suroccidente of Bogotá’s Health Secretary.

Sample size calculation

The sample size for the study was calculated using Real Statistics V7.9 software, with a medium effect error of 0.5, based on the study groups reported by Cassaniti et al. [22] with a statistical power of 80% and a 95% confidence interval for a total of 128 participants.

This phase of the study was considered a pilot study, with three groups of patients designated: 15 pregnant women with infection during pregnancy, 30 pregnant women vaccinated during pregnancy, and 45 pregnant women vaccinated before pregnancy. The results have demonstrated an initial approach to responding to the stated objective.

Study population

The study population comprised pregnant women aged 18 years and older who were admitted for childbirth. Inclusion criteria encompassed three groups. (1) Women who received the SARS-CoV-2 vaccine prior to pregnancy (with the last dose administered within 36 months before conception, regardless of the immunization scheme) (2) Women vaccinated during pregnancy (specifically those who received the Pfizer vaccine for the Wuhan variant during their first, second, or third trimester). (3) Women with SARS-CoV-2 infection during pregnancy (likely due to the Omicron variant lineage XBB.1.5, which was predominant during the study period) who had also been vaccinated against SARS-CoV-2 before pregnancy. Participants were asked to report on the following: If they had COVID-19 between 2020 and 2023, if they had moderate symptoms (such as fever or rhinorrhea) but lacked a COVID-19 confirmatory diagnosis due to limited access to diagnostic tests, or if they had an asymptomatic infection. Additionally, if they had been in contact with a confirmed SARS-CoV-2 positive case.

Pregnant women with multiple gestations or suspected active SARS-CoV-2 infection were excluded from the study. Recruitment took place in outpatient clinics, delivery rooms, and inpatient areas of the participating institutions. Eligible pregnant women were approached after the study’s objectives and characteristics were explained to them. Those who agreed to participate signed an informed consent form, which included authorization for the collection of clinical data from both mother and neonate, as well as biological samples from each.

Following this protocol, women were admitted for hospitalization in the labor ward. Sociodemographic information and medical histories, including comorbidities and diagnostic data during pregnancy, were collected. Patients were evaluated by medical staff and gynecologists according to standard care protocols. Clinical data were recorded in a format specifically designed for this study. Subsequently, maternal blood samples (6 mL in yellow-top tubes) were obtained, followed by cord blood samples at birth. These samples were promptly transported from the delivery room to the clinical laboratory for centrifugation. The resulting serum was aliquoted and frozen until transfer to the molecular diagnostic laboratory at Universidad El Bosque. Concurrently, information from the clinical records during childbirth was collected, including delivery method, neonatal data (birth weight and Apgar scores), and any requirement for Neonatal Intensive Care Unit (NICU) admission. Additionally, a three-month postpartum telephone follow-up was conducted with participants. This follow-up included questions regarding visits to healthcare facilities for emergency services, hospitalizations, PCR testing for SARS-CoV-2, or infant mortality due to respiratory infections.

Sample processing of maternal and umbilical cord blood samples

Samples were processed to identify IgG antibody levels from pregnant individuals and cord blood using the VIDAS^® SARS-CoV-2 IgG (9COG) kit from bioMérieux SA (Ref 424114). This technique is based on detecting specific IgG antibodies against the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein using the Enzyme-Linked Fluorescent Assay (ELFA) with an index greater than 1.00 considered positive (The index value is proportional to IgG levels for SARS-CoV-2 reported as BAU/mL). Additionally, the samples were processed by Enzyme-Linked ImmunoSorbent Assay (ELISA) to detect NAbs using the SARS-CoV-2 Neutralizing Antibody Kit from Invitrogen (Cat. # BMS2326). The sample processing for each applied test was carried out following the manufacturer’s instructions [23]. In the ELISA test, the neutralization percentage was calculated from the absorbances obtained from the samples according to the following formula:

In accordance with the manufacturer’s instructions [23], samples with a neutralization percentage higher than 20% were considered positive for NAbs. Samples with a neutralization percentage lower than 20% were defined as negative for NAbs, so only samples with a positive result were included in the statistical analysis.

The transfer rate of NAbs from the pregnant woman to the neonate was calculated using the following formula:

For this calculation, a reference value of greater than 1 was taken as the expected efficiency of transplacental antibody transfer [24, 25].

Statistical analysis

Continuous variables are presented as means with standard deviations (SD), while categorical variables are reported as absolute frequencies and percentages. For categorical variables, we employed the Chi-square test and Fisher’s exact test as appropriate. The relationship between neutralizing antibodies (NAbs) in maternal and umbilical cord blood was assessed using the Pearson correlation coefficient, with correlations of 0.5–0.7 considered strong and those above 0.7 very strong. We analyzed continuous variables using one-way ANOVA without adjustment. Logistic regression was utilized to calculate the Odds Ratio (OR) and 95% confidence intervals for the presence of NAbs in maternal and cord blood, using mothers vaccinated during pregnancy as the reference group. Furthermore, we calculated and incorporated the duration of exposure to SARS-CoV-2 vaccine or infection into our analysis. Statistical significance was determined for values of p < 0.05. Statistical analysis was performed using STATA software^® version 18.0 (College Station, TX, USA: StataCorp LLC), and graph design was performed using GraphPad Prism^® version 10.0.0.

Results

Baseline characteristics

The total population included in the study was 102 pregnant women admitted to labor and delivery and outpatient services of the four participating hospitals. Data from 16 mother-child pairs were excluded due to factors such as the omission of cord blood sample collection during delivery and the loss of the sample during transport from the delivery room to the hospital’s laboratory. Therefore, information from 86 mother-child pairs was analyzed: 35 in the vaccination group before pregnancy (40.6%), 40 in the vaccination group during pregnancy (46.5%), and 11 in the SARS-CoV-2 infection group during pregnancy (12.8%). The evaluation of IgG antibodies in pregnant women and newborns was performed in 55 mother-child pairs (who met criteria of sample volume and pairs respectively), whereas the evaluation of NAbs was performed in 86 mother-child pairs that had the complete information required by the study (Fig. 1). All women included had at least one doses of SARS-CoV-2 vaccine.

Fig. 1.

Flowchart of study participants. A total of 55 patients, each providing samples with adequate volume for IgG antibody detection, was included in the study. Exclusion criteria were applied to samples with insufficient volume or those lost during transit from the delivery room to the laboratory, specifically maternal and umbilical cord samples

During the analysis, differences were observed in the age and weight of the pregnant women when comparing the three study groups, with p = 0.036 and p = 0.021, respectively. The group with infection during pregnancy had higher values for these two variables (30.1 years and 79 kg) than the vaccinated groups. The same group also had differences in the distribution of pregnant women according to socioeconomic status (p = 0.003)and education level (p = 0.001), with 63% of these women reporting access to higher education. Among the reported medical histories, the most common condition among the patients was gestational hypertension (12.8%), followed by gestational diabetes (10.5%). However, no statistically significant differences were observed (Table 1).

Table 1.

Maternal and neonatal clinical characteristics

Variable		Vaccine		SARS-CoV-2 Infection	Total
		During n (%)	Before n (%)	n (%)	n (%)	p
		40 (46.5%)	35 (40.6%)	11 (12.8%)	86 (100%)
Maternal age (years)	Mean (SD)	25.1 (5.9)	26.3 (5.5)	30.1 (5.1)	26.2 (5.8)	0.036 ꝉ
Nationality	Foreign	1 (2.4%)	0 (0%)	0 (0%)	1 (1.2%)
	Colombian	39 (97.5%)	35 (100%)	11 (100%)	85 (98.8%)	0.559
Occupation	Employee	19 (47.5%)	25 (71.4%)	10 (90.9%)	54 (62.8%)
	Independent	1 (2.5%)	2 (5.7%)	0 (0%)	3 (3.5%)	0.070
	Student	2 (5.0%)	1 (2.9%)	0 (0%)	3 (3.5%)
	Housewife	18 (45.0%)	7 (20.0%)	1 (9.1%)	6 (30.2%)
Socioeconomic stratification⸉	Status 1	14 (35.0%)	7 (20.0%)	0 (0%)	21 (24.4%)
	Status 2	19 (47.5%)	19 (54.3%)	3 (27.3%)	41 (47.7%)	0.003
	Status 3	7 (17.5%)	9 (25.7%)	5 (45.4%)	21 (24.4%)
	Status 4	0 (0%)	0 (0%)	2 (18.2%)	2 (2.3%)
	Status 5	0 (0%)	0 (0%)	1 (9.01%)	1 (1.2%)
Education level	Primary school (completed)	2 (5.0%)	0 (0%)	0 (0%)	2 (2.3%)
	Primary school (incomplete)	1 (2.5%)	0 (0%)	0 (0%)	1 (1.2%)
	Secondary school (completed)	16 (40.0%)	24 (68.6%)	2 (18.2%)	42 (48.8%)
	Secondary school (incomplete)	4 (10.0%)	1 (2.9%)	0 (0%)	5 (5.8%)
	Technical	13(32.5%)	8 (22.9%)	2 (18.2%)	23 (26.7%)	0.001
	College	4 (10.0%)	2 (5.7%)	7 (63.6%)	13 (15.1%)
Marital status	Single	11 (27.5%)	6 (17.1%)	1 (9.1%)	18 (20.9%)
	Married	10 (25.0%)	7 (20.0%)	5 (45.4%)	22 (25.6%)	0.352
	Common law couples	19 (47.5%)	22(62.9%)	5 (45.4%)	46 (53.5%)
Comorbidity	Gestational Diabetes	4 (10.0%)	5 (14.3%)	0 (0%)	9 (10.5%)	0.559
	Diabetes Mellitusř	0 (0%)	0 (0%)	1 (9.1%)	1 (1.2%)	0.128
	Chronic Hypertension	0 (0%)	1 (2.9%)	1 (9.1%)	2 (2.3%)	0.120
	Gestational Hypertension	5 (12.5%)	3 (8.6%)	3 (27.3%)	11 (12.8%)	0.259
	Hypothyroidism	0 (0%)	1 (2.9%)	1 (9.1%)	2 (2.3%)	0.120
	Smoking	2 (5.0%)	1 (2.8%)	1 (9.1%)	4 (4.6%)	0.619
BMI (Kg/m2) at Birth	Mean (SD)	28.1 (4.3)	29.2 (3.4)	30.6 (3.4)	28.9 (3.9)	0.400
Weight (Kg) at Birth	Mean (SD)	69.1 (9.6)	73.6 (9.8)	79 (12.9)	72.2 (10.5)	0.021 ꝉ
Gestational ageƥ (weeks)	Mean (SD)	38.4 (1.3)	37.9 (1.9)	37.8 (0.9)	38.1 (1.6)	0.013
Mode of delivery	Vaginal	21 (52.5%)	14 (40.0%)	6 (54.6%)	41 (47.7)	0.522
	Cesarean	19 (47.5%)	21 (60.0%)	5 (45.4%)	45 (52.3)
Birth weight (g)	Mean (SD)	2863 (366.5)	2979 (398.0)	2838 (272.7)	2907.4 (370.7)	0.391
APGAR score ȶ	Mean (SD)	8 (0.7)	8 (0.8)	8 (0.4)	8 (0.7)	0.101
Hospitalization in NICU (yes)	n (%)	7 (17.5%)	7 (20.0%)	0 (0%)	14 (16.3)	0.360

Differences were calculated using the Chi-square test (Fisher’s exact test)

ꝉ One-way ANOVA (Bonferroni adjustment). Statistical significance was determined for values of p<0.005

ř Pre-pregnancy diagnosis

ȶ Scale 0 to 10

Abbreviations: BMI; Body max index; NICU, Neonatal intensive care unit.SD, standard deviation

⸉ Socioeconomic strata represent a categorical variable based on the diversity and quality of housing. This classification typically comprises six distinct strata: stratum one (lower-low), stratum two (low), stratum three (upper-low), stratum four (medium), stratum five (medium-high), and stratum six (high)

^ƥ Gestational age at birth moment

The mean gestational age at the time of vaccination in the pregnant women vaccinated during pregnancy group was 23 weeks (SD = 7.9). However, no significant differences were identified when evaluating variables such as the type of delivery and newborn weight between the study groups (p = 0.522 and p = 0.391, respectively). No maternal or neonatal deaths occurred among the analyzed population during the study period (Table 1). The time elapsed since the last SARS-CoV-2 vaccine dose differed significantly (p = 0.0001) among the three groups (S1 Table). However, no statistically significant differences were observed in maternal or cord blood IgG levels within each group based on the timing of SARS-CoV-2 vaccination (S1. Figure).

Titers and transfer of IgG and NAbs

In order to characterize the levels of IgG antibodies in pregnant women and neonates and the transfer of these specific antibodies, a blood sample from the pregnant woman obtained prior to delivery (hospitalization or labor room) and a blood sample from the umbilical cord obtained at the time of delivery were processed to evaluate humoral immune response between the three analysis groups (Fig. 2). The results showed that all the samples from pregnant women and the cord (100%) were positive for IgG antibodies. The levels of these IgG antibodies (binding antibody units, BAU/mL) did not show differences when comparing pregnant women (p = 0.607) and cord blood samples (p = 0.184) in the same groups (Table 2). However, it was found that the levels of IgG were higher in cord blood samples than in samples from pregnant women (Median value in BAU/mL 28.8 vs. 25.5, respectively, p = 0.009), particularly when evaluating IgG levels in the group of pregnant women vaccinated during pregnancy (Median in BAU/mL 31.5 vs. 24.3,p = 0.020).

Fig. 2.

Correlation of the percentages of maternal and umbilical cord neutralizing antibodies for SARS-CoV-2. The correlation of NAbs in maternal and umbilical cord blood was evaluated using Pearson’s correlation coefficient, considering a strong correlation between 0.5 - 0.7 and a very strong correlation higher than 0.7. A. Evaluation of the correlation coefficient in the three research groups. B. Evaluation of the correlation coefficient in patients with vaccination during pregnancy. C. Evaluation of the correlation coefficient in patients vaccinated before pregnancy. D. Evaluation of the correlation coefficient in patients with SARS-Cov-2 infection during pregnancy

Table 2.

Neutralizing antibodies and specific IgG antibodies for SARS-CoV-2

Evaluation of antibodies		Vaccine		SARS-CoV-2 Infection n (%)	Total n (%)	p
		During n (%)	Before n (%)
		40 (46.5%)	35 (40.6%)	11 (12.8%)	86 (100%)
Positive maternal IgG	n (%)	25 (100%)	19 (100%)	11 (100%)	55 (100%)
Positive Cord IgG	n (%)	25 (100%)	19 (100%)	11 (100%)	55 (100%)
Maternal IgG levels (BAU/ mL)	Median (IR)	24.3 (14.9 -29-9)	26.4 (18.2 -29.2)	20.9 (14.6 – 27.4)	25.5 (15.9 – 29.1)	0.607‡
Cord IgG levels (BAU/mL)	Median (IR)	31.5 (22.5 – 34.4)	29.5 (21.1 –32.7)	23.5 (19.7 –30.3)	28.9 (21.5- 33.6)	0.184‡
IgG antibodies transfer rate	Median (IR)	1.3 (1.0-1.4)	1.2 (1.0-1.3)	1.2 (0.7-1.4)	1.2 (1.0-1.4)	0.5794
Positive maternal NAbs	n (%)	8 (20.0%)	30 (85.7%)	9 (81.8%)	47 (54.6%)	0.000ꬸ
Positive Cord NAbs	n (%)	7 (17.5%)	22 (62.9%)	8 (72.7%)	37 (43.0%)	0.000ꬸ
Positive maternal NAbsꬹ	Mean (SD)	42.51 (10.8)	43.3 (13.1)	39.3 (14.8)	42.4 (12.9%)	0.706
Positive Cord NAbsꬹ	Mean (SD)	42.6 (12.6)	37.4 (10.7)	41.3 (11.1)	39.1 (11.1)	0.881
Neutralizing antibodies (NAbs) transfer rate	Median (IR)	0.9 (0.8-1.1)	0.7 (0.5-0.9)	1.0 (1.0-1.2)	0.8 (0.6-1.0)	0.0142

Continuous variables are presented as mean (Standard Deviation, SD) and median (Interquartile Range IR)

ꬸDifferences were calculated by Chi-square test (Fisher’s exact test)

ꝉ One-way ANOVA (Bonferroni adjustment) statistical significance was considered p<0.005

‡ Differences were calculated by the Kruskal-Wallis test

^ꬹ Positive NAbs were presented as percentage. Positive samples were defined if percentage neutralization ≥20%

Statistical analysis of neutralizing antibodies was performed on samples with a neutralization percentage higher than 20% (positive)

Abbreviations: NAbs Neutralizing antibodies, BAU/mL Binding antibody units

When evaluating the presence of positive NAbs (higher than 20%), significant differences were found between the three exposure groups for the maternal and cord blood respectivelyInterestingly, the positivity value in the group of pregnant women vaccinated before pregnancy and those who had an infection during pregnancy was higher than 80%. In the cord blood samples, the values obtained in these groups were higher than 60% (Table 2). No significant differences were found regarding the time of vaccine administration (which was categorized as < 12 months or older than 12 months), nor when evaluating the number of doses received in this group (vaccinated before pregnancy with positive result in NAbs assay) (Table 3).

Table 3.

Relationship between SARS-CoV-2–2 neutralizing antibodies, vaccination time, and number of vaccine doses

Variable	Category	Positive maternal NAbs n (%)	Negative maternal NAbs n (%)	Total	p	Positive cord NAbs n (%)	Negative cord NAbs n (%)	Total	p
Time between vaccination and pregnancy	<12 months	18 (85.7%)	3 (14.3%)	21	0.679	12 (57.1%)	9 (42.9%)	21	0.311
	>12 months	12 (85.7%)	2 (14.3%)	14		10 (71.4%)	4 (28.6%)	14
Doses	1	1 (50.0%)	1 (50.0%)	2	0.229	1 (50.0%)	1 (50.0%)	2	0.592
	2	10 (76.9%)	3 (23.1%)	13		7 (53.8%)	6 (46.2%)	13
	3	17 (94.4%)	1 (5.6%)	18		13 (72.2%)	5 (27.8%)	18
	4	2 (100%)	0 (0%)	2		1 (50.0%)	1 (50.0%)	2

Differences between categorical variables were evaluated using Fisher’s exact test

This analysis was performed for the group of women vaccinated before pregnancy

In the analysis of the transfer of NAbs and IgG antibodies from the pregnant woman to the neonate, significant differences were observed between the groups regarding the transfer of NAbs (p = 0.0142). The group with infection during pregnancy had a slightly higher transfer rate of neutralizing antibodies, 1.0 compared to the vaccinated groups, 0.9 during pregnancy, and 0.7 before pregnancy (Table 2). The results indicate a linear relationship between neutralizing antibodies (NAbs) in maternal blood and those in the umbilical cord, with a moderate to strong correlation coefficient (r = 0.6003; Fig. 2). Additionally, a moderate negative correlation was observed between NAbs and IgG levels in cord blood samples from the group of vaccinated before pregnancy (r= -0.6789,p = 0.0014; Figure S2). We found a higher association between receiving the vaccine before pregnancy or having an infection during gestation and having a higher percentage of neutralization in maternal blood (LOR 3.18[95% CI 1.95–4.40] y LOR 2.89 [95% CI 1.17–4.61], respectively) (Table 4).

Table 4.

Log Odds ratio of having NAb in maternal and umbilical cord blood, according to the groups

Group		LORŦ (95% Confidence Interval)
Pregnant women	Vaccine before pregnancy	3.18 (1.95 - 4.40)
	SARS-CoV-2 Infection	2.89 (1.17 – 4.61)
Cord	Vaccine before pregnancy	2.07 (1.01 – 3.14)
	SARS-CoV-2 Infection	2.53 (0.97 - 4.08)

The analysis was performed considering the group of women vaccinated during pregnancy as the reference group

^ŦAbbreviations: LOR Log Odds Ratio

In the follow-up of infants under three months of age, no significant differences were found between the three groups in the outcomes related to respiratory infections (Table 5). It was reported that 11 infants had to visit the emergency services for consultation, 8 had to be hospitalized, and 4 had a PCR test for SARS-CoV-2, which were all negative. There were no infant deaths during the follow-up period.

Table 5.

Infants’ follow-up description according to the origin group

Outcomes		Vaccine		SARS-CoV-2 Infection	Total	p
		During	Before
Emergency room visit	n (%)	4 (10.0%)	6 (17.1%)	1 (9.1%)	11 (12.8%)	0.736
Hospitalization	n (%)	3 (7.5%)	5 (14.3%)	0 (0%)	8 (9.3%)	0.449
PCR test for SARS-CoV-2	n (%)	3 (7.5%)	1 (2.8%)	0 (0%)	4 (4.6%)	0.781
Positive PCR test for SARS-CoV-2	N (%)	0 (0%)	0 (0%)	0 (0%)	0 (0%)

Differences were calculated by the Chi-square test (Fisher’s exact test)

Abbreviations: PCR Polymerase Chain Reaction

Discussion

In the present study, we evaluated IgG antibody and NAb levels in the serum of pregnant women and umbilical cord blood in three study groups of patients: (1) Pregnant women vaccinated against COVID-19 before pregnancy; (2) Pregnant women vaccinated against COVID-19 during pregnancy; (3) Pregnant women with SARS-CoV-2 infection during pregnancy (specifically, those who had previously been vaccinated against SARS-CoV-2). IgG antibodies against RBD were analyzed using the ELFA technique. NAbs were analyzed using a competitive ELISA test, which is currently a simpler and more cost-effective alternative to neutralization tests with viruses or pseudoviruses. The main findings of this study encompass the detection of IgG antibodies in all evaluated samples, with no discernible differences between groups. Notably, a higher frequency of neutralizing antibodies was observed in two distinct cohorts: women who received vaccination prior to gestation and those who experienced COVID-19 infection during pregnancy. This comprehensive analysis provides valuable insights into the immune response patterns in these specific populations, contributing to our understanding of antibody dynamics in the context of pregnancy and COVID-19.

The analyses showed the presence of IgG antibodies in all pregnant women evaluated and in all umbilical cord blood samples without finding differences in IgG levels in the mother-child pairs between the research groups. These results are similar to those obtained in the cohort study by Muñoz et al. [26], which included women who received two doses of Pfizer or Moderna or a booster dose during pregnancy, that reported the presence of IgG antibodies in all maternal and cord blood samples. Jacobs et al. [27] studied the production of IgG antibodies in infected pregnant and non-pregnant women and found similar IgG levels in both groups. However, when comparing pregnant women who received a dose of the vaccine during pregnancy with pregnant women infected with SARS-CoV-2, the IgG levels found were significantly higher in the group of vaccinated women.

Malek A. et al. [19]found that fetal IgG levels progressively increased during pregnancy, reaching a peak in the third trimester, whereas maternal antibodies decreased. They also found that antibody transfer occurred through a process of selective transfer, being higher for IgG1 > IgG2> IgG3 > IgG4, which could explain our finding of significantly higher cord IgG levels than maternal levels in all research groups [28].

The transfer of antibodies from the mother to the neonate can be assessed by the rate of transplacental transfer, which has been studied using different vaccination schemes for other pathogens such as Bordetella pertussis and the influenza virus. Van Den Berg. et al. [29] evaluated the transfer of antibodies to the umbilical cord of Bordetella pertussis, Haemophilus influenzae, Clostridium tetani, and Neisseria meningitidis in vaccinated pregnant women, finding transfer rates between 0.67 and 1.34 with lower rates in premature newborns.

The study by Liu S. et al. [30] about COVID-19 found a higher rate of IgG antibody transfer when evaluating pregnant women vaccinated with three doses (1.71) compared to those vaccinated with two doses (1.53) and infected pregnant women (1.21) without these differences being statistically significant. These findings are similar to those found in this study, in which no differences were observed in the rate of IgG transfer between the research groups.

Nabs have been associated with symptomatic COVID-19 prevention. They have also allowed scientists to determine effective immunity against SARS-CoV-2 when evaluating vaccine immunogenicity [31, 32]. Antibody production occurs between 0 and 28 days after vaccination, with higher levels in patients with a complete vaccination schedule or a history of infection. According to evidence, the levels of these antibodies decrease within 2 to 3 years [33–36].

Our results showed that the percentage of women with positive Nabs (defined as > 20%) was higher in the group of pregnant women vaccinated before pregnancy (85.7%) and in the infected group (81.8%) than in the group of pregnant women vaccinated during pregnancy (20%).

Studies on the NAbs response in pregnant women have focused on evaluating infection during pregnancy or in pregnant women who receive the vaccine during pregnancy. However, to the authors’ knowledge, there are no reports comparing Nabs in groups of pregnant women vaccinated before and during pregnancy.

One hypothesis explaining our finding of higher levels of NAbs in pregnant women with pre-pregnancy vaccination is that the physiological changes of pregnancy condition an anti-inflammatory environment with the aim of achieving immunomodulation and generating tolerance for the fetus. However, it remains controversial whether the immune response to vaccination in pregnant women differs from that of general population [27, 37, 38].

On the other hand, the study of Jiang R et al. analyzed the dynamics of the different types of antibodies, and found that IgG levels decrease after 15 days post-vaccination, in contrast to neutralizing antibodies, which increased continuously over time after vaccination and there is not a similar behavior between IgG antibodies and neutralizing antibodies [39]These results could explain our finding that NAbs were higher in the research group of vaccinated before pregnancy. Several studies have demonstrated a correlation between neutralizing antibody levels and the severity of COVID-19 infections [40–42]. However, in our study, data on infection severity were unavailable, precluding this analysis.

Treoning et al. [43], evaluated the early immune response to vaccination and found no changes in the gene expression involved in regulating the innate immune response in both mice and pregnant and non-pregnant women [36]. However, cellular immunity also plays a significant role; although more information is needed in this area, regulatory B cells, which have an anti-inflammatory function, increase during the first and third trimesters, potentially leading to suppression of some aspects of the response during pregnancy [44, 45].

When analyzing the percentage of neutralization, we did not find significant differences between the research groups, which agrees with the results reported in the literature, where similar or higher values have been found in vaccinated pregnant women than in women who have had the infection [20, 46, 47]. These results are similar to those obtained by Matsui et al., who analyzed 64 mother-child pairs and found no significant differences when evaluating positive NAbs (defined as > 50%) between the vaccination and infection groups [48].

On the other hand, a moderate correlation was observed between the percentage of maternal and umbilical cord blood NAbs (Pearson r = 0.6003), similar to the findings of da Silva et al. [49], who identified a linear relationship between maternal and umbilical cord NAbs, although with a Pearson correlation coefficient of 0.95.

In this study, we found a higher transfer rate of NAbs in the research group of infected pregnant women or pregnant women vaccinated prior to the pregnancy, similar to that reported by Muñoz et al. [26], who found efficient transplacental transfer with primary and booster vaccinations, with transfer rates between 1.00 and 1.78 for NAbs. Similarly, Murphy et al. [50] reported greater antibody transfer in pregnant women with an infection during gestation or who received a booster dose. This differs from what was reported in another study that analyzed the association between the time of administration of the last vaccine dose (latency) and NAbs transfer without finding statistically significant differences. Authors also found the presence of NAbs in the cord blood up to 16 months after the last dose [51].

Considering the characteristics of the women included in the study (age, weight), we could suggest that differences were not related with health status. Furthermore, pregnant women in Colombia have the same conditions to assure and complete the vaccination, without the type of insurance (private or public), and are considered between the group of high risk in infectious diseases. The Colombian Federation of Obstetrics and Gynecology participates in national and international consensus initiatives to strengthen vaccination coverage in women, prioritizing maternal immunization to protect both the mother and the newborn.

The evaluation of neonates and the following-up of the infants, did not found SARS-CoV-2 infections in the participants samples, which may be related to the protection induced by the transfer of antibodies from the mother. The results of Halasa et al., which included the evaluation of 379 hospitalized infants < 6 months, showed that the status of complete maternal vaccination schedule during pregnancy, reduces the risk of hospitalization in infants by 61% [52]. This result is limited due to the low power of this study.

One of the strengths of this study was the inclusion of pregnant women vaccinated before pregnancy in the analysis groups, a population whose antibody dynamics and behavior for this virus were not known, and the comparison with previously analyzed and widely reported groups. Likewise, this study highlights the importance of SARS-CoV-2 vaccination in pregnant women and the transfer of IgG antibodies to neonates. However, this study had some limitations, such as the small sample size of the group of pregnant women infected during pregnancy, which made it difficult to establish homogeneous research groups for comparison. Additionally, there was a lack of information regarding the specific timing of administration of each dose in the research group of pregnant women vaccinated before pregnancy.

Conclusion

The results obtained in this study demonstrate the dynamics of SARS-CoV-2 specific IgG antibody production in response to both vaccination and natural infection, as well as their transplacental transfer from pregnant women to their newborns. The observed differences in positivity rates and percentages of neutralizing antibodies (NAbs) among the analysis groups were intrinsically linked to the group definition criteria. These findings underscore that time is a critical dependent variable, potentially influencing the degree of antibody specialization. Consequently, time must be considered the central axis of interpretation when determining the interval between exposure (either through vaccine administration or infection) and delivery. This temporal factor is crucial for understanding the efficacy of maternal immunity transfer and its implications for neonatal protection against SARS-CoV-2.

Abbreviations

ICU

Intensive Care Unit

ECMO

Extracorporeal membrane oxygenation

ACOG

American College of Obstetricians and Gynecologists

SMFM

Society for Maternal-Fetal Medicine

WHO

World Health Organization

RBD

Receptor-binding domain

NAbs

Neutralizing Antibodies

NICU

Neonatal Intensive Care Unit

ELFA

Enzyme-Linked Fluorescent Assay

ELISA

Enzyme-Linked ImmunoSorbent Assay

BMI

Body Max Index

BAU

Binding Antibody Units

PCR

Polymerase Chain Reaction

LOR

Log Odds Ratio

Authors’ contributions

AO Conceptualization, Methodology, Investigation, Funding acquisition, Writing - Original Draft. CR Methodology, Formal analysis, Investigation, Writing - Review & Editing. XR Conceptualization, Methodology, Writing - Review & Editing, Supervision. JC Conceptualization, Methodology, Resources, Writing - Review & Editing, Supervision. JH Methodology, Formal analysis, Writing - Review & Editing. LB Resources, Supervision. SS Resources, Supervision. RG Resources, Supervision. GR Resources, Supervision. SB Resources, Supervision. PP Resources, Supervision. SLB Resources, Supervision.

Funding

This work was supported by Universidad El Bosque, Bogotá, Colombia [PCI-2022-11095].

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

In accordance with the ethical principles of the Declaration of Helsinki, the study was approved by the Ethics Committee of the Universidad El Bosque on April 25, 2022 (Minute No. 008-2022). The study was also approved by the Ethics Committees of the Hospital Universitario Clínica San Rafael, Los Cobos Medical Center, Centro Policlínico del Olaya, and the Subred Integrada de Servicios de Salud Suroccidente of Bogotá’s Health Secretary.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.