Protection of Homologous and Heterologous Vaccine Boosters Against COVID-19 in Singapore

Despite fully vaccinating over 80% of the total population in Singapore, COVID-19 cases surged in September 2021 with the relaxation of social distancing, isolation and quarantine measures. A COVID-19 vaccine booster program (with 30mcg BNT162b2 or 50mcg mRNA-1273) was launched on September 15, 2021 for older adults aged 60 years and above who completed their primary vaccination at least 6 months prior. Using a Poisson regression, we estimated the incidence rate ratio of confirmed infections and severe disease between booster and non-booster groups, and compared the effectiveness of homologous and heterologous boosting. Rates of confirmed and severe infections were 73% (95% CI, 71% to 75%) and 95% (95% CI, 91% to 97%) lower respectively among the booster group. Heterologous boosting conferred greater protection, with the PPM (Moderna booster after Pfizer-BioNTech primary series) group obtaining 82% (95% CI, 77% to 86%) booster effectiveness, versus 73% (95% CI, 71% to 74%) for the PPP (three Pfizer-BioNTech shots) group. Our study confirms the effectiveness of a mRNA vaccine booster dose against infections and severe diseases.