Epidemiological and spatial-temporal clustering characteristics of pertussis in Southwest China, 2013-2024.

BACKGROUND

Despite high childhood vaccination coverage, pertussis has resurged in China in recent years. Understanding its evolving epidemiology is critical for guiding public health interventions. This study analyzed spatiotemporal trends and identified key drivers of pertussis notification rates in Chongqing, China (2013-2024).

METHODS

We analyzed longitudinal surveillance data of pertussis cases from the National Notifiable Infectious Disease Reporting System (NNDRS) in Chongqing, China (2013-2024). Demographic and vaccination coverage data were obtained from the Basic Information System (a subsystem of the NNDRS) and the National Immunization Program Information System (NIPIS), respectively. We employed joinpoint regression to quantify long-term notification rate trends, interrupted time-series analysis to evaluate the impact of national diagnostic criteria revisions (2017, 2024), spatiotemporal scan statistics to identify high-risk space-time clusters, and multilevel linear mixed-effects models to determine district-level factors associated with notification rates, while accounting for random variations across districts and years.

RESULTS

From 2013 to 2024, 33,226 pertussis cases were notified in Chongqing. A dramatic surge occurred in 2024, accounting for 68.55% (n = 22,777) of all cases. A distinct epidemiologic shift was observed in age distribution: During 2013-2023, infants < 1 year had the highest notification rate and were the largest age group (56.8%, 5,939/10,449), with 44.7% (2,658/5,939) being aged < 3 months. However, in 2024, although infants (<1 year) retained the highest notification rate, children aged 5-10 years constituted the highest group by case count (n = 13,868), a case count 7.4 times greater than that in infants (n = 1,873). Joinpoint regression showed a statistically significant upward trend in the annual pertussis notification rate (average annual percent change [AAPC] = 32.16%; 95% confidence interval [CI]: 5.40-65.72). The most pronounced increase was observed among children aged 5-10 years (AAPC = 94.91%; 95% CI: 55.18-144.81). The 2024 revision of national pertussis diagnostic criteria was associated with an immediate 26.5-fold increase in pertussis notifications (Incidence Rate Ratio [IRR] = 26.46; 95% CI: 17.52-39.95; p < 0.001). Multilevel regression identified higher district-level income levels (β = 0.56, p < 0.001) and the 2024 diagnostic criteria revision (β = 5.52, p = 0.013) as significant determinants of higher pertussis notification rates. No significant association was found with district-level vaccination coverage.

CONCLUSION

This study documents a substantial resurgence of pertussis in Chongqing from 2013 to 2024, characterized by a notable change in age distribution. Although infants (aged <1 year) maintained the highest notification rate, school-aged children (5-10 years) emerged as the largest case group in 2024. Consequently, establishing seroepidemiological surveillance to directly quantify population immunity is essential for designing targeted and effective control measures.