This study investigated the relative effectiveness of a mix-and-match vaccination strategy, primarily comprising ChAdOx1 nCOV-19, mRNA-1273, BNT162b2, and a protein-based vaccine, MVC-COV1901, against COVID-19 in a healthcare worker (HCW) cohort in Taiwan during a period when the Omicron variant was predominant. The analysis included a total of 21,729 HCWs and recorded 3,672 infections with no severe disease nor death. Two main findings were observed from the study. Firstly, for those with ChAdOx1 nCOV-19 as primary series, a booster dose with BNT162b2 was associated with a small decrease in the risk of acquiring infection compared to those with mRNA-1273 as a booster (Adjust hazard ratio [Adj HR] 0.864; 95% confidence interval [CI] 0.761‒0.981, P = .024). Secondly, for HCWs receiving an mRNA-1273 booster, compared to those receiving ChAdOx1 nCOV-19 as the primary series, mixed primary series and homologous mRNA-1273 primary series were associated with a higher (Adj HR 1.144; 95% CI 1.021‒1.282, P = .021) and lower risk (Adj HR 0.735; 95% CI 0.671‒0.805, P < .001) of acquiring infection, respectively. Our study demonstrated that mix-and-match vaccination strategy may be associated with different level of risk reduction in acquiring infection, and sizable, prospective studies are encouraged to further elucidate our observation.
Keywords: COVID-19; booster; healthcare worker; primary series; vaccination.