Postpandemic Social Contact Patterns: A Cross-Sectional Survey in Japan, South Korea, Thailand, and Vietnam

INTRODUCTION: Social contact patterns are one of the key determinants of infectious disease transmission, but post-coronavirus disease 2019 (COVID-19)-pandemic evidence from East and Southeast Asia remains limited. We examined age-specific contact patterns in Japan, South Korea, Thailand, and Vietnam and assessed whether contact frequencies had returned to prepandemic levels. METHODS: We conducted online, single-day contact surveys in 2025 using quota sampling by age and sex. Participants reported contacts on the preceding day, defined as physical contact or direct conversations of at least three sentences held within 2 m. Age-specific contact matrices were constructed for weekdays and weekends and combined using a 5:2 weighting. Factors associated with daily contact counts were examined using negative binomial regression. RESULTS: The analysis used data from 10,279 individuals: 8,080 in Japan, 1,102 in South Korea, 589 in Thailand, and 508 in Vietnam. Mean daily contact counts were 8.1, 6.2, 6.4, and 8.8, respectively; medians were 4, 4, 4, and 6, respectively. All four countries showed age-assortative mixing, although assortativity was less pronounced in Thailand and Vietnam. In Japan and South Korea, younger age and larger household size were associated with more contacts, whereas greater mask-wearing tendency was associated with more contacts in Japan and fewer contacts in South Korea. Compared with available historical estimates, contact frequencies in Japan and South Korea remained below prepandemic levels. Sparse baseline data and methodological differences precluded firm conclusions being made for Thailand and Vietnam. CONCLUSIONS: Contact frequencies in Japan and South Korea remained below prepandemic levels in 2025, suggesting persistent postpandemic changes in mixing behavior. Evidence for comparable changes in Thailand and Vietnam was inconclusive. Future surveys should collect setting-specific information, in addition to contact frequency and age, to clarify how changes in work, education, and household interaction shape postpandemic social mixing patterns.