Within-Household SARS-CoV-2 Transmission and Vaccine Effectiveness in the First Three COVID-19 School Outbreaks in Northern Vietnam, September to December 2021

Background: SARS-CoV-2 transmission risks from school-aged children to their household contacts and the protective effects of vaccination in household settings remain poorly understood. We assessed transmission dynamics within households of SARS-CoV-2-positive, schoolchildren and the protective effects of COVID-19 vaccination among household contacts in Vietnam.Methods: We estimated attack rates, vaccine effectiveness (VE) and adjusted risk ratios (aRR) of factors associated with SARS-CoV-2 transmission to household contacts of PCR-confirmed children from three schools in Phu Tho, Ha Nam, and Thanh Hoa province using a multivariate regression model with household-level random effects.Findings: This retrospective cohort study included 157 SARS-CoV-2-infected children and their 540 household contacts. The AR among household contacts was 24.6% on average. VE among household contacts was 39% (95%CI -1 to -63) overall, and higher among males than females and adults older than 40. COVID-19 transmission was greater to female household contacts (aRR 1∙35; 95%CI 1∙50 to 4∙21), and highest among 19-39-year-olds (aRR 2∙51; 95%CI 3∙11 to 17∙05). Fully vaccinated household contacts had significantly lower infection risks (aRR 0∙46; 95%CI 0∙26 to 0∙84).Interpretation: We found substantial onward SARS-CoV-2 transmission from schoolchildren to household contacts, and higher protection from vaccination was higher among males and older people. It is important to recognize the role of children in SARS-CoV-2 transmission. We recommend that all household members with school-aged children receive at least two doses of COVID-19 vaccines. This is a lesson learned for Vietnam in order to manage COVID-19 transmission and protect school-aged children.Funding: Trang Thu Vu is funded by the ASEAN-Australia Health Security Fellowship by the Commonwealth, Department of Foreign Affairs and Trade, and the Master of Applied Epidemiology Scholarship by the Australian National University.Declaration of Interest: The authors declare no conflicts of interest.Ethical Approval: The study was approved by the Human Ethic Board at The Australian National University (Identification number: 2022/380). This analysis was exempted from ethics review by the National Institute of Hygiene and Epidemiology as a part of national COVID-19 outbreak investigation and response activities.