Background: A SARS-CoV-2 Delta variant outbreak in schools occurred in the Vietnamese provinces of Phu Tho, Ha Nam, and Thanh Hoa in September 2021. The epidemiological role of school-aged children during COVID, in particular transmission risks from school-aged children to their household contacts remain poorly understood. We assessed transmission dynamics within households of infected children and the protective effects of COVID-19 vaccination among household contacts.

Methods: This was a secondary data analysis using Vietnam’s National COVID-19 Case Database. We used a multivariate regression model with random effects at household level to estimate vaccine effectiveness (VE) and to identify factors associated with the risk of SARS-CoV-2 transmission to household contacts of PCR-confirmed children from three schools in Phu Tho, Ha Nam, and Thanh Hoa province.

Results: We included data from 157 SARS-CoV-2 infected children and all their 540 household contacts. The overall attack rate among household contacts was 24.6%. COVID-19 incidence was higher among female household contacts compared to males (adjusted relative risk (aRR) 1.35; 95%CI 1.50 to 4.21) and highest among household contacts aged 19-39 years (aRR 2.51; 95%CI 3.11 to 17.05). Fully vaccinated household contacts had substantially lower infection risks than unvaccinated/partially vaccinated household contacts (aRR 0.46; 95%CI 0.26 to 0.84). Overall VE among household contacts was 39% (95%CI -1 to -63), and higher among males (VE 34%; 95%CI -19 to 63) compared to females, and among people older than 40 years (VE 65%; 95%CI 17 to 85).

Conclusions: We found substantial onward transmission from infected schoolchildren to their household contacts. Several household-level and individual-level factors were significantly associated with the risk of SARS-CoV-2 transmission, and vaccination was particularly protective among males and among older adults. Understanding the epidemiological role of school-aged children has important implications for school closures and vaccine prioritization strategies during COVID-19.