Transmission patterns of pertussis in school-based clustered outbreaks: A retrospective analysis in Zhejiang Province, China, 2024.

We conducted a retrospective study using data from Zhejiang Province's pertussis surveillance system between February 1 and May 1, 2024, defining school-based clusters as ≥5 cases occurring within 21 d in the same class. We identified 124 outbreaks involving 825 confirmed cases, with cluster sizes ranging from 5 to 16 cases (median: 7). Most outbreaks occurred in primary schools (79.03%, 98/124), followed by kindergartens (19.35%, 24/124) and middle schools (1.61%, 2/124). Among cases, 19.3% (159/825) were index cases (predominantly aged 7-8 y) and 80.7% (666/825) were secondary infections. The median serial interval was 9.0 d (IQR: 5.0-13.0); it was significantly longer in kindergarten than in primary school cases (10.0 vs. 9.0 d, P = .007) and shorter with earlier diagnosis (7-14 d post-onset) (7.0 vs 10.0 d, P = .007). Our findings emphasize the importance of protecting high-risk school populations, implementing prompt outbreak control measures, and conducting timely health education to prevent pertussis transmission.