# Correlation analysis between positivity rate of immunoglobulin G antibodies against pertussis toxin among community-based populations and reported pertussis incidence in Shandong, China: a seven-year seroepidemiological study

## TL;DR

This study in Shandong, China, found that reported pertussis cases may be underestimated, with higher infection rates in young children and significant regional and age differences.

## Contribution

The study provides a seven-year seroepidemiological analysis linking antibody positivity rates to reported pertussis incidence in Shandong.

## Key findings

- The mean PT-IgG antibody positivity rate was 8.05%, with the highest in 2019 and lowest in 2020.
- Children under 3 years old had the highest positivity rate, while 3-5 year olds had the lowest.
- Reported incidence rates showed a strong annual correlation with infection rates but weak age-based correlation.

## Abstract

The reported annual incidence of pertussis in China has shown a marked increase over the past decade, but it may be still underestimated. The purpose of this study was to understand the seroepidemiologic characteristics of pertussis in community-based populations and to assess the level of pertussis infection in the population.

Between 2017 and 2023, one or two cities in each of the eastern, central and western regions of Shandong Province were selected as survey sites, and a total of six age groups of healthy individuals were enrolled by multistage stratified random sampling to carry out the survey. The serum level of pertussis toxin (PT) IgG antibody was quantified by indirect enzyme-linked immunosorbent assay.

A total of 9,238 subjects were enrolled, and the mean positive rate of PT-IgG antibody was 8.05% (95%CI: 7.50%~8.60%), with the highest in 2019 (10.70%, 95% CI: 9.19%~12.21%) and the lowest in 2020 (6.32%, 95% CI: 4.98%~7.66%). The highest positive rate was found in the age group of less than 3 years old (11.46%, 95%CI: 9.87%~13.05%), and the lowest rate was found in the 3 ~ 5 years group (5.40%, 95%CI: 4.28%~6.52%). There was a significant difference in the positive rate between age groups (χ2 = 43.098, p < 0.001). Comparison of trends in recent infection rates and reported incidence rates in the population, that showed a very strong linear correlation in the annual distribution (r = 0.821, p = 0.023), and an extremely weak linear correlation in age distribution (r = 0.086, p = 0.872). Estimated infection rates (/100,000) among people aged ≥ 3 years ranged from 5,257 (95%CI: 3,918 ~ 6,596) to 24,449 (95%CI: 22,157 ~ 26,740) by years, with estimated infection rates in eastern region (31,544.44-fold) and in older age group (292,340.00-fold for ≥ 20 years old group, 216,388.89-fold for 17 ~ 19 years old group) differed significantly from the reported incidence rate.

The annual distribution trend of reported pertussis incidence rate aligns with the infection rate observed among community populations, the actual level of infection is likely to be seriously underestimated. Therefore there is a need to emphasize surveillance and consider additional booster immunizations for adolescents and adults.

The online version contains supplementary material available at 10.1186/s12879-025-11802-9.