Pertussis: a retrospective analysis of cases observed in Mersin Türkiye, 2020 to 2024 evaluation of pertussis cases

Abstract

Background

Whooping cough is endemic in many regions in Türkiye and the world, and is an important cause of morbidity and mortality, especially in children and adolescents. The aim of this study was to evaluate the epidemiological findings of pertussis cases in a region.

Methods

The cross-sectional study was conducted in 2024–2025. The study evaluated the relationship between the frequency of pertussis cases reported in 2020 to 2024 and the demographic characteristics of the patients (age, gender, nationality, district of residence, etc.), the season of notification, and vaccination status. The definitive diagnosis of the cases was made by isolating Bordotella pertussis from nasopharyngeal sample cultures or by detecting the Bordotella pertussis gene in nasopharyngeal samples by PCR.

Results

When the distribution of cases in Mersin province by years was examined, 2 cases were detected in 2020, 1 in 2021, 0 in 2022, 3 in 2023, and 92 cases in 2024. It was determined that there was a significant increase in the number of cases in 2024 compared to previous years. Of the 92 cases reported in 2024, 46 were laboratory confirmed and the case rate in 2024 was determined as 2.35 per hundred thousand. 57 cases (62.0%) had never been vaccinated with pertussis vaccine. Of the 46 cases with positive results, 30 (65.2%) were unvaccinated and 42 (91.3%) were younger than 6 months. The frequency of case positivity was significantly higher in infants younger than 6 months (p=.028).

Conclusions

The increase in cases in 2024 is thought to be linked to migration movements.Protective measures, especially vaccination studies, should be increased in infants younger than 6 months and pregnants where the frequency of cases is high.

Clinical trial number

Not applicable.

Introduction

Pertussis is a highly contagious respiratory tract infection caused by Bordetella pertussis (B. pertussis), characterized by paroxysmal coughing episodes. In the disease, a prodromal stage resembling the common cold (catarrhal period) is followed by severe coughing (the paroxysmal stage). Pertussis is a serious public health problem. It can result in death, especially in communities with inadequate immunization and nutritional services [1–3].

Pertussis is transmitted through airborne droplets released by the infected individuals. A single infected individual can transmit the disease to an average of 12–17 people. The minimum immunization percentage for herd immunity is 92–94%. Although infants are a high-risk group, the course and incidence of the disease vary according to age and vaccination status [4].

The disease is more dangerous in infants and is a significant cause of illness and death in this age group. Initial symptoms include mild fever, nasal discharge, and cough. Subsequently, the characteristic coughs of pertussis begin. Pneumonia is a relatively common complication. Seizures and encephalopathy are rare [5]. According to the CDC (Centers for Disease Control and Prevention), approximately 24.1 million people worldwide develop pertussis each year, and about 160,700 children under the age of 5 die from the disease [6].

The primary preventive measure against pertussis is vaccination [7]. The universal pertussis vaccination program, administered from infancy and at regular intervals, plays a key role in pertussis control [1]. In vaccinated children, all stages of the disease are shorter, and the severity of the paroxysms is reduced [2]. In 2023, 14.5 million children worldwide missed all vaccinations. The coverage of the third dose of the diphtheria, tetanus, and pertussis (DTP3) vaccine was reported by the World Health Organization as 84% in 2023 [5].

According to 2018 data, the notification rate of pertussis in European countries ranges from 0.0 to 46.0 per 100,000. The frequency of notification is higher in those under the age of 1 and in the month of August. Among the cases with known vaccination status, 35% had never been vaccinated. Increasing vaccination rates, awareness efforts in the community, and expansion of the laboratory network are essential strategies for disease prevention [8].

The Turkish Ministry of Health, in its letter dated 13.03.2025, planned to vaccinate mothers during pregnancy with a vaccine containing a pertussis component to protect infants against pertussis in the first months of life. Thus, in order for the antibodies formed in the mother to be transferred to the baby during birth and thereby protect the newborn, it was decided, upon the recommendation of the Immunization Advisory Board, to administer one dose of Tdap vaccine during pregnancy. For this purpose, it was decided that one dose of the currently administered tetanus and diphtheria (Td) vaccine would be replaced with the Tdap vaccine during each pregnancy period, according to the schedule attached [9].

Despite pertussis vaccination efforts, it still remains a significant and priority public health issue [10–15]. This study aimed to evaluate the characteristics of suspected and confirmed pertussis cases in Mersin province in 2024 and the measures taken to prevent the disease.

Methods

Study setting and design

This cross-sectional study was conducted in Mersin province between June 2024 and June 2025. Mersin is a large city and port on the Mediterranean coast of southern Turkey. It is made up of four district governorates, each having its own municipality: Akdeniz, Mezitli, Toroslar and Yenişehir. Mersin lies on the western side of Çukurova, a Turkish economic hub housing Turkey’s largest seaport. Eastward, a larger metropolitan region which will combine Mersin with Tarsus and Adana (the Adana-Mersin Metropolitan Area) is in the making with more than 3.3 million inhabitants. The urban population of the city is 1,084,789 (Mezitli, Yenisehir, Toroslar, Akdeniz), with a metropolitan population of 1,954,279.

Study population and sampling

The population of the study consisted of patients with suspected pertussis in Mersin province in 2024. No sampling was performed, as the aim was to include the entire population. Clinical samples taken from suspected cases by clinicians working in hospitals in Mersin were sent to the reference laboratory for analysis, and nasopharyngeal samples reported as positive in PCR or culture results were classified as confirmed cases.

Data collection

Data collection was carried out between December 15, 2024, and January 15, 2025. Suspected pertussis case notification records from the Mersin Provincial Health Directorate for 2024 were reviewed. The data were obtained from the notification system records.

Data analysis

Descriptive statistics were used to summarize the data; chi-square and Mann-Whitney U tests were used for analysis, and p<.05 was considered significant. In the study, the sample result (positive or negative) was used as the dependent variable. Independent variables included: age group (0–3 months, 3–6 months, > 6 months), nationality (Turkish or Syrian), gender (male or female), risk level of the district of residence (high or low risk), and vaccination status (unvaccinated or at least one dose).Areas with high concentrations of Syrian migrant populations have been identified as high-risk.In our country and in our province, the Tdap (tetanus, diphtheria, and acellular pertussis) vaccine has been administered to pregnant women since April 2, 2025. Since the data analyzed in our study cover only the year 2024, the vaccination schedule for pregnant women during this period was as follows: one dose between 18 and 24 weeks of gestation and one dose between 28 and 32 weeks of gestation, for a total of two doses of the Td vaccine. In the first stage, all independent variables were included in the model and a full logistic regression analysis was performed (Model 1). By removing non-significant variables, a more parsimonious and well-fitted model was developed (Model 2). Model fit was assessed using Akaike Information Criterion (AIC) and Nagelkerke R² values. Odds ratios (OR) and 95% confidence intervals (CI) were calculated to interpret the effect of independent variables on the likelihood of a positive sample result. Analyses were performed using R statistical software (version 4.5.1) within the RStudio IDE (version 2025.05.1).

Ethical issues

Ethical approval for the study was obtained from the Toros University Scientific Research and Publication Ethics Committee (Decision date and decision number: 30.10.2024/172). Since the study was reported as a retrospective study of medical records, the ethics committee waived the requirement for informed consent. Additionally, written permission was received from the Research Applications Review and Evaluation Commission of the Mersin Provincial Health Directorate. The authors had access to information that could identify individual participants during data collection.

Limitations

Limitations of the study were that it only included reported cases; clinically compatible but unreported cases could not be included.

Operational definitions

According to the Ministry of Health’s Field Guide for the Control of Pertussis Disease, the diagnostic criteria for pertussis are:

Suspected case

A person with a cough lasting at least 2 weeks, accompanied by at least one of the following.

• Severe coughing spells.

• Inspiratory whoop.

• Vomiting immediately after coughing.

• No other apparent cause of cough (e.g., pneumonia, pleurisy, sinusitis…).

Confirmed case

Along with the criteria for a suspected case.

• Isolation of B. pertussis from nasopharyngeal culture samples, or.

• Detection of the B. pertussis gene in a nasopharyngeal sample by PCR [16].

Results

In 2024, 92 suspected pertussis cases were reported in Mersin province. A review of annual case reports revealed 2 cases in 2020, 1 in 2021, 0 in 2022, and 3 in 2023 (Fig. 1).

Fig. 1.

Distribution of pertussis reports in Mersin province by year, 2020–2024

The average age of the cases was 8.1 months (min: 0.5 months, max: 17 years). Of the cases, 70 (76.1%) were of Turkish nationality, 22 (23.9%) were Syrian nationals. Thirty-four cases (37.0%) were from the Akdeniz district, and 25 cases (27.2%) from the Toroslar district. 47 cases (51.1%) were female and 45 (48.9%) were male. 76 cases (82.6%) were under 6 months of age. All cases were hospitalized, and 1 case resulted in death.The deceased patient was an unvaccinated female infant under 3 months of age.The case fatality rate (CFR) was 2.17%, with 1 death among 46 confirmed cases. Out of the 92 suspected cases, 46 were identified as pertussis-positive (50.0%) and 46 as pertussis-negative based on laboratory results. 57 cases (62.0%) had not received any pertussis vaccination. Of the 46 positive cases, 30 (65.2%) were unvaccinated and 42 (91.3%) were under 6 months of age. The incidence rate of pertussis was 2.35 per 100,000. (2024 Mersin population: 1.954.279; 46 pertussis-positive cases) (Table 1).

Table 1.

Sociodemographic and other characteristics of the cases

Sociodemographic characteristics	Number	Percentage (%)
Nationality
Turkish	70	76.1
Syrian	22	23.9
Gender
Male	45	48.9
Female	47	51.1
Age
<3 months	46	50.0
3–6 months	30	32.6
>6 months	16	17.4
Child under 10 at home
Yes	64	69.6
No	28	30.4
District
Akdeniz	34	37.0
Toroslar	25	27.2
Other districts	33	35.8
Season
Spring	46	50.0
Summer	23	25.0
Autumn	3	3.3
Winter	20	21.7
Vaccination status
Unvaccinated	57	62.0
Vaccinated	35	38.0
Total	92	100.0

Regarding seasonal distribution, 46 cases (50.0%) occurred in spring, 23 cases (25.0%) in summer, 3 cases (3.3%) in autumn, and 20 cases (21.7%) in winter (Fig. 2).

Fig. 2.

Distribution of pertussis reports in Mersin province by month in 2024

No significant relationship was found between pertussis positivity and nationality or gender. However, the frequency of positive results was significantly higher in cases under 6 months compared to those over 6 months (p=.028). No significant difference was found in the frequency of positive results between unvaccinated individuals and those who had received at least one dose of vaccine (Table 2).

Table 2.

Factors associated with the frequency of positivity in cases

Characteristics	Positive	Negative	p
Nationality
Turkish	37 (%80.4)	33 (%71.7)	0.328
Non-Turkish	9 (%19.6)	13 (%28.3)
Gender
Female	26 (%56.5)	21 (%45.6)	0.297
Male	20 (%43.5)	25 (%54.4)
Age Group
< 3 months	26 (%56.5)	20 (%43.5)
3–6 months	16 (%34.8)	14 (%30.4)	0.086
> 6 months	4 (%8.7)	12 (%26.1)
Child under 10 years old at home
Yes	32 (%69.7)	32 (%69.6)	0.999
None	14 (%30.3)	14 (%30.4)
District
Akdeniz	16 (%34.8)	18 (%39.1)	0.806
Toroslar	12 (%26.1)	13 (%28.3)
Other	18 (%39.1)	15 (%32.6)
Season
Spring	23 (%50.0)	23 (%50.0)	0.677
Summer	13 (%28.3)	10 (%21.7)
Autumn	2 (%4.3)	1 (%2.2)
Winter	8 (%17.4)	12 (%26.1)
Vaccination status
Unvaccinated	30 (%65.2)	27 (%58.7)	0.519
Vaccinated	16 (%34.8)	19 (%41.3)
Total	46 (%50.0)	46 (%50.0)

In the study, a two-step logistic regression analysis was applied to examine the predictive power of certain socio-demographic variables on sample results. In Model 1, which was initially constructed, the variables nationality, gender, age, residential district risk level, and vaccination status were included in the model. As a result of this analysis, the variable “age” showed that being over 6 months of age at the time of sample submission significantly decreased the likelihood of a positive result compared to those aged 0–3 months (OR = 0.13, 95% CI [0.04, 0.66], p = .013). The risk category of the residential district was found to be borderline significance (OR = 0.35, 95% CI [0.11, 0.95], p = .081). These findings indicate that individuals over 6 months of age have an odds ratio (OR) of approximately 0.13, corresponding to an 87% reduction in the odds of having a positive sample result compared to those in the 0–3 month age group. Other variables did not show significant effects (Table 3).

Table 3.

Logistic regression analysis results (Model 1 and Model 2)

Variable	β	SE	z	p	OR [%95 G.A]
Model 1
Constant	0.57	0.81	0.71	0.480	1.77
Nationality (Syrian vs. Turkish)	-0.83	0.56	-1.47	0.143	0.44 [0.13, 1.34]
Gender (Female vs. Male)	0.67	0.48	1.39	0.166	1.95 [0.70, 5.40]
Age (3–6 months vs. 0–3 months)	-0.75	0.67	-1.13	0.259	0.47 [0.18, 1.34]
Age (Over 6 months vs. 0–3 months)	-2.02	0.81	-2.48	0.013	0.13 [0.04, 0.66]
Residence Status (High Risk vs. Low Risk)	-1.04	0.59	-1.75	†0.081	0.35[0.11, 0.95]
Vaccination Status (At least 1 vaccination vs. Unvaccinated)	-0.01	0.55	-0.02	0.982	0.99 [0.38,2.28]
Model 2 (Revised)
Constant	1.16	0.51	2.28	0.023	3.21
Age (3–6 months vs. 0–3 months)	-0.80	0.56	-1.41	0.157	0.45 [0.14, 1.34]
Age (Over 6 months vs. 0–3 months)	-1.76	0.73	-2.42	0.016	0.17 [0.04, 0.66]
Residence Risk Status (High Risk vs. Low Risk)	-1.07	0.54	-1.98	0.048	0.34 [0.11, 0.95]
Model Fit	Model 1			Model 2 (iyileştirilmiş)
AIC	133.42			126.03
Nagelkerke R²	0.19			0.13

Note: Logistic regression results for Model 1 (full model) and Model 2 (improved simplified model): coefficients (β), standard errors (SE), z-values, p-values, odds ratios (OR), and 95% confidence intervals. Model 2 was created by removing non-significant variables. AIC and Nagelkerke R² values for both models are also included in the table. A p value of < 0.05 was considered significant.

To create a more parsimonious and better-fitting version of the model, the vaccination status variable, which did not show significance, was removed from the model, resulting in Model 2. In this improved model, the likelihood of having a positive sample result for individuals older than 6 months at the time of sample submission remained significant (OR = 0.17, 95% CI [0.04, 0.66], p = .016), and the residential district risk level variable also became statistically significant (OR = 0.34, 95% CI [0.11, 0.95], .p = .048). Additionally, the AIC value of Model 2 was lower than that of Model 1; this indicates that Model 2 fits the data better and ispreferable. Nagelkerke R² values demonstrate the explanatory power of the models. It was calculated as 0.19 for Model 1 and 0.13 for Model 2. These values show that both models explain a limited portion of the variance in the sample result. Other variables (nationality, gender, 3–6 month age range) did not show a significant effect in either model. Table 3 shows the logistic regression analysis results (Table 3).

Discussion and conclusions

Pertussis notifications have increased over the past 20 years and the disease remains endemic. Factors contributing to its re-emergence and the rise in case numbers may include decreased vaccination rates (due to vaccine refusal, migration, etc.), the inclusion of pertussis among differential diagnoses and improved sample submission, as well as the waning effectiveness of the inactivated vaccine over time.In our study, it was determined that there was a significant increase in the number of cases in 2024.No changes occurred in testing protocols, laboratory capacity, or reporting practices in 2024. The increase in cases is thought to be related to migrant mobility.

In our study, when examining factors associated with case positivity, no significant relationship was found between positivity rates and nationality. This may be due to similar vaccination statuses between Turkish and other citizens.

A significant relationship was found between positivity rates and age groups in our study. The higher positivity rate among cases under six months of age may be related to prioritizing pertussis in clinically compatible cases and ensuring sample submission in this age group. This finding underscores the disproportionate burden of pertussis among pertussis as a health burden in infants under six months. Based on the recommendation of the Immunization Advisory Board in in Turkey, the administration of a dose of Tdap vaccine containing tetanus and diphtheria toxoids along with an acellular pertussis component during pregnancy is considered an effective intervention in reducing cases and controlling pertussis disease.

Among the 46 confirmed cases in our study, 42 (91.3%) were infants younger than 6 months.Similarly, in Poeta’s study conducted in Italy, the majority of cases were also infants under 6 months (median: 3 months) [17]. In Stein Zamir’s study, the median age of cases was 1.5 years, with most being young children [18]. In Rumik’s study, the highest incidence of pertussis (88.1%) was found among individuals aged 10–14 years [19]. Poltorek’s study also reported a high incidence in the 10–14 age group [20]. In Alamaw’s study, the mean age of cases was reported as 3.7 years (range: 3 months to 45 years) [21]. In Nordholm’s study, the highest incidence was identified in adolescents [22]. The concentration of pertussis cases in different age groups across countries may be attributed to the earlier implementation of vaccination programs for pregnant women in some regions. At the time our study was completed, Tdap vaccination had not yet been introduced for pregnant women in our country, and the positivity rate was found to be high in infants under 6 months.

In our study, no significant association was found between case positivity and vaccination status. In Alamaw’s study, being fully vaccinated was identified as a protective factor [21]. Similarly, in Yeshanew’s study, being unvaccinated was identified as a risk factor for pertussis (OR = 4.17) [23]. The lack of a significant association in our study may be due to the inclusion of partially vaccinated individuals in the vaccinated group, or the fact that some infants had not yet received subsequent doses according to the national immunization schedule due to their age.

In conclusion, pertussis remains a current public health concern and continues to be a significant cause of morbidity and mortality among infants and children. Due to the high case incidence in infants under 6 months inMersin province, targeted public health interventions for this age group should be prioritized and strengthened.

In this context, the implementation of new maternal Tdap vaccination policy during pregnancy is expected to provide early-life protection through the cocooning effect and reduce case incidence. Future prospective studies are warranted to evaluate the impact of maternal Tdap vaccination on pertussis incidence in early infancy.

Abbreviations

B. Pertussis

Bordetella pertussis

CDC

Center for Disease Control and Prevention

DTP3

The third dose of the diphtheria, tetanus, and pertussis vaccine

Tdap

Tetanus, diphtheria, acellular pertussis vaccine

Td

Tetanus and Diphtheria vaccine

CFR

Case Fatality Rate

Author contributions

LÜH, TDY and TS were responsible for the conceptualization, formal analysis, ınvestigation, writing of the manuscript. LÜH and TS were responsible for data curation, methodology, supervision, visualization. All authors reviewed and approved the final version of the manuscript.

Funding

None.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Ethical approval for the study was obtained from the Toros University Scientific Research and Publication Ethics Committee (Decision date and decision number: 30.10.2024/172). Since the study was reported as a retrospective study of medical records, ethics committee waived the requirement for informed consent. Additionally, written permission was received from the Research Applications Review and Evaluation Commission of the Mersin Provincial Health Directorate. Authors had access to information that could identify individual participants during data collection. All methods were carried out in accordance with relevant guidelines and regulations, including the Declaration of Helsinki.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.