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

## TL;DR

This study examines how SARS-CoV-2 spreads within households in Thailand during the Omicron wave, finding that prior immunity and lower viral load reduce transmission risk.

## Contribution

The study provides insights into household transmission dynamics of the Omicron variant in a post-vaccination setting, emphasizing the role of prior immunity and viral load.

## Key findings

- Household SAR was 33% despite high vaccination coverage.
- Prior anti-NCP IgG positivity in contacts reduced transmission risk.
- Lower viral load in index cases was associated with reduced transmission.

## Abstract

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.

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.

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.

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.

The study was funded by the 10.13039/100020655European Health and Digital Executive Agency (HADEA), 10.13039/501100000780European Commission, and by the 10.13039/100012947Institut de Recherche pour le Développement (IRD), France.