Boosting with Multiple Doses of mRNA Vaccine after Priming with Two Doses of Protein Subunit Vaccine MVC-COV1901 Elicited Robust Humoral and Cellular Immune Responses against Emerging SARS-CoV-2 Variants

Confronted with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants, such as Delta and Omicron, with high infectivity and immune evasion capacity, vaccination remains the most effective tool to prevent infection and severe illness. However, heterologous vaccination of mRNA vaccines primed with protein subunit vaccines had not been evaluated before the current study. Since subunit vaccine MVC-COV1901 (MVC) has been granted emergency use authorization in Taiwan, in this study, we explored the humoral and cellular immune responses to additional third (2× MVC/Mod) and fourth (2× MVC/2× Mod) doses of mRNA-1273 (Mod) after priming with two doses of subunit vaccine MVC against the emerging variants. We found a 12.3- to 16.1-fold increase in antibodies targeting the receptor binding domain (RBD) of the Delta variant with 2× MVC/Mod compared to two doses of MVC (2× MVC) or AZD1222 (2× AZ) regimens and a 26- to 32.2-fold improvement in neutralizing potency against the Omicron variant (BA.1). Besides, the numbers of gamma interferon (IFN-γ)-secreting T cells induced by 2× MVC/Mod were also elevated 3.5-fold and 3.7- to 4.3-fold for the wild type and Delta variant. However, boosting with a fourth dose of Mod (2× MVC/2× Mod) after the 2× MVC/Mod regimen failed to significantly improve the immune responses. Moreover, all vaccination schedules showed reduced neutralizing activity against the Omicron variant. Collectively, our results suggested that the third or fourth dose booster vaccination with mRNA vaccine after priming with two doses of protein subunit vaccine could elicit stronger humoral and cellular immune responses. These findings could provide a future global heterologous boosting strategy against COVID-19. IMPORTANCE Vaccination is the most important strategy to combat the COVID-19 outbreak; however, it remains to be determined whether heterologous prime-boost regimens could induce equal or even stronger immune responses against SARS-CoV-2. Here, we showed that boosting the additional doses of mRNA-1273 (Mod) priming with two doses of MVC-COV1901 (MVC) (2× MVC/Mod) improved humoral and cellular immunity compared to two doses of AZD1222 (2× AZ) or MVC (2× MVC) against SARS-CoV-2 variants. However, the Omicron variant showed strong immune evasion ability for all vaccination schedules. Our findings provided evidence supporting that heterologous vaccination by boosting with mRNA vaccine after priming with two doses of protein subunit vaccine could strongly promote humoral and cellular immune responses against the emerging SARS-CoV-2 variants.