# Incidence of Pertussis in Older Children Underestimated in the Whole-Cell Vaccine Era: A Cross-Sectional Seroprevalence Study

## TL;DR

This study found that pertussis infections were more common in older school-age children in China during the whole-cell vaccine era than previously reported.

## Contribution

The study provides new evidence that pertussis incidence in older children was underestimated during the whole-cell vaccine era in China.

## Key findings

- The positive rate of anti-pertussis toxin IgG antibodies was 4.9% among school-age children.
- Adolescents aged 14–<18 years had significantly higher antibody levels than younger children.
- Pertussis infections were likely underreported due to unrecognized localized outbreaks in schools.

## Abstract

Objectives: China was once a country with a high incidence of pertussis, with reported incidence rates exceeding 100 per 100,000 before the introduction of the pertussis vaccine. After the widespread implementation of the pertussis vaccination program, reported cases of pertussis significantly decreased. This study aimed to investigate the serological prevalence of pertussis among school-age children during the administration of the whole-cell pertussis (wP) vaccine in China. Methods: We selected a representative random sample from different schools, with the inclusion criteria being school-age children without clinical symptoms of pertussis. A total of 368 frozen serum samples were obtained from children aged 6–<18 years at various schools in Guizhou in November 2005 and subsequently analyzed. Results: The positive rate of anti-pertussis toxin (PT) IgG antibodies (>62.5 IU/mL) were 4.9% (16/368) among school-age children. The positive rates of anti-PT IgG antibodies were 3.3%, 3.8%, 4.0%, 3.3%, and 10.8% in children aged 6–<8 y, 8–<10 y, 10–<12 y, 12–<14 y, and 14–<18 y, respectively. The increase in PT-IgG antibody levels among older children was likely due to pertussis infection in these school-age children. The positive rate of anti-PT IgG varied between different schools. The pertussis antibody levels of adolescents aged 14–<18 y were significantly higher than those of school-age children in the younger age group (6–<8 y and 8–<10 y) (p = 0.0097 and p = 0.0007, respectively). Conclusions: During the era of wP vaccine use, pertussis infections were common among school-age children, particularly in adolescents, with potential unrecognized localized or school-based outbreaks.