Antibody efficacy of inactivated vaccine boosters (CoronaVac) against Omicron variant from a 15-month follow-up study

•Nabs and IgG S antibodies peaked 10–14 days post booster and stabilized in 60 days.•Nabs against Omicron decreased 6.4-fold and IgG S antibodies 2.3-fold during peaks.•Two doses of CoronaVac alone can't elicit sufficient Nab responses against Omicron.•Three doses provided efficacy of 97.5% and 65.2% at 14 and 150 days, respectively.