Sero-Epidemiology of Pertussis in the Municipality of Duque de Caxias, Rio de Janeiro, Brazil: A Case Study

1. Introduction

Pertussis, or whooping cough, is a highly contagious respiratory infection caused by the gram-negative bacterium Bordetella pertussis. It is characterized by prolonged paroxysmal coughing, inspiratory “whoop,” and post-tussive vomiting, often lasting several weeks [1]. Despite being vaccine-preventable, pertussis has re-emerged globally since the 1980s [2], even in countries with high vaccination coverage such as the United States [3], Canada [4], Australia [5], and Brazil [6,7] a phenomenon referred to as pertussis re-emergence.

According to the World Health Organization (WHO), pertussis affects approximately 24 million people and causes 160,700 deaths annually [8], though underreporting likely results in higher true figures. The disease is most prevalent in regions with limited vaccination coverage, such as sub-Saharan Africa and Southeast Asia, but increasing outbreaks in Europe and the Americas indicate that even robust immunization programs face challenges from waning immunity and pathogen evolution [9].

In Brazil, pertussis has been a notifiable disease since 1975, monitored by the National Notifiable Diseases Information System (SINAN). In 2019, 6,012 cases were recorded. Between 2020 and 2024, case numbers fluctuated, with notable increases among children aged 1–9 years. In 2024 alone, 2,663 cases were reported, with the 10–14 age group most affected [10,11].

The National Immunization Program (PNI) recommends the DTP/Hib/HB vaccine at 2, 4, and 6 months, with booster doses at 15 months and 4 years. Despite these measures, declining vaccine coverage and possible waning immunity have been observed, particularly following disruptions during the COVID-19 pandemic [12,13].

Given these concerns, this study sought to evaluate the persistence of vaccine-induced humoral immunity in a pediatric population in Duque de Caxias, a densely populated urban area in Rio de Janeiro. By quantifying serum IgG levels against pertussis toxin (PTx) and B. pertussis antigens, we aimed to assess the real-world effectiveness of vaccination and identify potential immunity gaps that may contribute to pertussis resurgence.

2. Materials and Methods

2.1. Study Population and Serum Samples

This cross-sectional sero-epidemiological study analyzed 220 serum samples collected between January and December 2022 from children vaccinated with DTP/Hib/HB with aged 1–14 years at public health units in Duque de Caxias, Rio de Janeiro, Brazil. Samples were grouped as follows: 1–4 years ( 110 samples), 5–9 years (68 samples), 10–14 yearsm (44 samples). This stratification enabled evaluation of immunity at different post-vaccination intervals.

2.2. ELISA Assays

Commercial ELISA kits (SERION ELISA classic, ESR1201G, Brazil) were used to quantify IgG antibodies against B. pertussis and B. parapertussis antigens, including pertussis toxin (PTx) and filamentous hemagglutinin (FHA). The SERION ELISA classic IgG B. pertussis Toxin I kit was used for PTx-specific detection. Results were expressed in IU/mL, with values >40 IU/mL classified as positive, 40–100 IU/mL as borderline, and <40 IU/mL as negative, following manufacturer recommendations.

2.3. Statistical Analysis

Data were analyzed using GraphPad Prism 9. The Kruskal–Wallis test, followed by Dunn’s multiple comparisons, was applied to evaluate differences in antibody levels among age groups. A p-value < 0.05 was considered statistically significant.

3. Results

3.1. Anti-PTx IgG Levels

Analysis of anti-PTx IgG levels revealed age-related declines in antibody titers. Among children aged 5–9 years (n = 68) and 10–14 years (n = 44), all sera were below the protective threshold (<40 IU/mL). The 1–4-year group displayed greater variability, with titers ranging from 12.7 to 408 IU/mL. Within this group, 5 samples (4.5%) were positive, 17 (15.4%) borderline, and 87 (79.1%) negative. Overall, only 5 of 220 samples (2.17%) exhibited protective levels (>40 IU/mL). The youngest group showed significantly higher antibody levels than older groups (p < 0.05).

Figure 1. Antibody response to Bordetella pertussis toxin in children aged 1–13 years. Serum antibody titers (IU/mL) were measured by ELISA in 220 children and are shown as boxplots with individual data points overlaid. Samples were stratified into three age groups (1–4, 5–9, and 10–13 years). Standard positive and negative sera were provided by the ELISA kit. Statistical analysis was performed using the Kruskal–Wallis test followed by Dunn’s multiple comparisons test. Data are presented as median and interquartile range. Statistical significance was set at p < 0.05; **p < 0.001.

Figure 1. Antibody response to Bordetella pertussis toxin in children aged 1–13 years. Serum antibody titers (IU/mL) were measured by ELISA in 220 children and are shown as boxplots with individual data points overlaid. Samples were stratified into three age groups (1–4, 5–9, and 10–13 years). Standard positive and negative sera were provided by the ELISA kit. Statistical analysis was performed using the Kruskal–Wallis test followed by Dunn’s multiple comparisons test. Data are presented as median and interquartile range. Statistical significance was set at p < 0.05; **p < 0.001.

3.2. IgG Reactivity to B. pertussis Antigens

When tested with B. pertussis antigens (PTx + FHA), seropositivity increased to 36.7% (78/220). The mean antibody levels declined with age: 1–4 years: 100.7 IU/mL (n = 69), 5–9 years: 68.2 IU/mL (n = 8), 10–14 years: 63.1 IU/mL (n = 1)

A significant downward trend in titers across age groups (p < 0.05) indicated waning immunity over time. The majority of samples (63.2%) were negative, suggesting inadequate long-term protection.

Figure 2. Antibody responses to filamentous hemagglutinin (FHA) and pertussis toxin (Ptx) in children aged 1–13 years. Serum antibody titers (IU/mL) were measured by ELISA in 220 children and are shown as boxplots with individual data points overlaid. Samples were stratified into three age groups (1–4, 5–9, and 10–13 years). Standard positive and negative sera were provided by the ELISA kit. Statistical analysis was performed using the Kruskal–Wallis test followed by Dunn’s multiple comparisons test. Data are presented as median and interquartile range. Statistical significance was set at p < 0.05; *p < 0.01, **p < 0.001.

Figure 2. Antibody responses to filamentous hemagglutinin (FHA) and pertussis toxin (Ptx) in children aged 1–13 years. Serum antibody titers (IU/mL) were measured by ELISA in 220 children and are shown as boxplots with individual data points overlaid. Samples were stratified into three age groups (1–4, 5–9, and 10–13 years). Standard positive and negative sera were provided by the ELISA kit. Statistical analysis was performed using the Kruskal–Wallis test followed by Dunn’s multiple comparisons test. Data are presented as median and interquartile range. Statistical significance was set at p < 0.05; *p < 0.01, **p < 0.001.

4. Discussion

The resurgence of pertussis, even in highly vaccinated populations, underscores the complex interplay between vaccine-induced immunity, pathogen evolution, and public health practices. Brazil’s National Immunization Program (PNI), historically successful in achieving >90% coverage, has seen declines in recent years dropping to 77,2% for DTP/Hib/HB in 2022. The consequences of this reduction are now reflected in rising pertussis incidence.[14].

Our study revealed that fewer than 3% of vaccinated children exhibited protective anti-PTx antibody levels. Although 36.7% showed broader reactivity to B. pertussis antigens, the low toxin-specific response highlights insufficient or waning immunity consistent with reports from China, Sri Lanka, and Malaysia, where post-vaccination antibody decline was also observed among older children and adolescents [15,16,17,18].

In Brazil, all pediatric vaccines currently distributed by PNI are whole-cell (wP) formulations. While effective in controlling disease transmission, wP vaccines can elicit variable immune durability and higher reactogenicity compared with acellular vaccines (aP). The low anti-PTx IgG levels observed in this study suggest that protection derived from wP vaccination may wane faster than expected, emphasizing the need for booster doses beyond early childhood. International experience supports this approach. Countries that introduced adolescent boosters or maternal immunization (using dTpa) have seen reduced pertussis incidence in infants and adolescents. In Brazil, similar policies could mitigate waning immunity and protect vulnerable age groups. Although, this study was limited by single-municipality design and absence of clinical or genetic confirmation of infection, the results provide valuable insight into real-world immunity patterns in an urban Brazilian population and reinforce the need for broader surveillance [19,20].

5. Conclusions

The low prevalence of protective antibodies against pertussis toxin among vaccinated children in Duque de Caxias indicates waning immunity and incomplete protection under the current vaccination scheme. Revising the national schedule to include adolescent boosters and incorporating acellular vaccine options are essential steps toward long-term pertussis control in Brazil.