There is limited evidence on vaccine effectiveness against asymptomatic or mild Omicron infections. We estimated that recent third doses of messenger RNA or inactivated vaccines reduced the risk of self-reported infection by 52% (95% confidence interval, 17%-73%) among randomly sampled adults during the Omicron BA.2-dominated surge in Hong Kong. Using self-reported vaccination and testing results for coronavirus disease 2019 among a random adult population in Hong Kong, we estimated that third doses of messenger RNA or inactivated vaccines could provide moderate level of protection during the Omicron BA.2 surge.