[Comparison of pertussis incidence trends before and after the COVID-19 pandemic: Investigation of factors behind the surge in pertussis cases in Kanagawa Prefecture (Week 1, 2018 - Week 33, 2025)].

Abstract

Objective　To investigate the causes of the sharp increase in pertussis cases, by comparing the trends in pertussis incidence in Kanagawa Prefecture before and after the COVID-19 pandemic.Methods　Epidemiological data of pertussis cases reported in Kanagawa Prefecture from epidemiological week 1 of 2018 to week 33 of 2025 were examined based on infectious disease surveillance. Since changes in pertussis trends may have been influenced by behavioral changes that were implemented during the COVID-19 pandemic, such as droplet infection prevention and social distancing, the study period was divided into two sections: before and after the pandemic. The number of cases was compared across variables, including age group, vaccination history, symptoms, and diagnostic methods.Results　A pertussis outbreak was observed between 2018 and 2020, and a large-scale outbreak starting in 2024 continued to persist as of week 33 of 2025. Comparing the periods before and after the COVID-19 pandemic, the age group with the highest number of case reports shifted from children aged 5-10 years to those aged 10-15 years. Moreover, the number of reported cases has decreased in infants under 6 months of age, who are at the highest risk of severe complications from pertussis. Regarding vaccination history, in both time frames, the proportion of cases with "four doses received" was the highest; after the COVID-19 pandemic, an increase in the number of cases for those with four doses of DPT or DPT-IPV vaccines was observed in the 7-15 age group. Regarding symptoms, the percentage of patients with nocturnal cough, dyspnea, staccato, whoop, vomiting, apnea, cyanosis, and leukocytosis decreased. In terms of diagnostic methods, the frequency of genetic testing and rapid diagnostic tests increased compared to serological techniques.Conclusion　Factors contributing to the sharp increase in pertussis cases included the accumulation of susceptible individuals due to droplet and contact infection control measures and behavioral restrictions during the COVID-19 pandemic, waning immunity due to the time elapsed following pertussis vaccination, a possible increase in the number of reported cases due to an expanded set of diagnostic methods, and an increase in macrolide-resistant Bordetella pertussis strains. Advancing the starting age of routine vaccinations to 2 months contributed to the prevention of infant pertussis. However, additional DPT vaccinations at the ages of 5-7 and 11-12 years, as recommended by the Japanese Pediatric Society, are considered necessary.