Household SARS-CoV-2 transmission during Omicron wave in Chiang Mai, Thailand: a prospective observational study

Background SARS-CoV-2 transmission studies involving children in Thailand have been relatively limited to the early waves with the alpha and delta variants. Our study aims to address these gaps by examining household transmission in Chiang Mai, northern Thailand, during the Omicron wave in a post vaccination period. Methods This prospective observational study enrolled households comprising a confirmed COVID-19 index patient with at least one uninfected contact and a child (<18 years of age who maybe an index or contact). Participant data, nasopharyngeal swabs, and blood samples were collected at entry and final visit. Participants recorded daily symptoms for 21 days and self-administered SARS-CoV-2 antigen tests every other day for 14 days. Incident infections were confirmed by RT-PCR. Secondary attack rates (SARs) were calculated and associated factors were analyzed using multivariable generalized estimating equations models. Phylogenetic analysis was used to confirm intra-household transmission. Findings From July 2022 to May 2024, 93 households (93 index cases, 197 contacts) were enrolled; 52% of index cases and 49% of contacts were <18 years. Among contacts, despite 89% (175/197) having received the SARS-CoV-2 vaccine (of whom 75% > 6 months prior), 44 became infected, yielding a household SAR of 33% (95% CI: 24–44). In phylogenetically-confirmed transmission, SAR was 25% (95% CI: 17–35). Index low viral load (aRR: 0.82, 95% CI: 0.74–0.92) and contacts baseline anti-NCP IgG positivity (aRR: 0.42, 95% CI: 0.22–0.83) were significantly associated with lower household transmission. Interpretation Despite widespread vaccination, household transmission of SARS-CoV-2 remained common. Prior immunity in contacts and lower viral load in index cases reduced risk. These findings underscore the central role of households in ongoing spread and highlight the value of booster vaccination and genomic surveillance to clarify transmission pathways and inform prevention policies. Funding The study was funded by the European Health and Digital Executive Agency (HADEA), European Commission , and by the Institut de Recherche pour le Développement (IRD), France.