Safety and immunogenicity of a third dose of COVID-19 protein subunit vaccine (CovovaxTM) after homologous and heterologous two-dose regimens

ObjectiveTo report the safety and immunogenicity profile of a protein subunit vaccine (Covovax) given as a third (booster) dose to individuals primed with different primary vaccine regimens. MethodsIndividuals primed with two doses of COVID-19 vaccines for at least 3 months were enrolled and assigned to five groups according to their primary vaccine regimens: CoronaVac, BBIBP-CorV, AZD1222, BNT162b2, and CoronaVac/AZD1222. Immunogenicity analysis was performed to determine binding antibodies, neutralizing activity, and the T-cell response. ResultsOverall, 215 individuals were enrolled and boosted with the Covovax vaccine. The reactogenicity achieved was mild-to-moderate. Most participants elicited a high level of binding and neutralizing antibody responses against wild type and omicron variants following the booster dose. The 197 participants were classified by anti-N IgG. Of these, 141/197 (71.6%) were a seronegative population, and neutralizing activity and IFN-{gamma} release were further monitored. A booster dose could elicit neutralizing activity to wild type and omicron variants by more than 95% and 70% inhibition at 28 days, respectively. The Covovax vaccine could elicit a cell-mediated immune response. ConclusionThe protein subunit vaccine (Covovax) can be proposed as a booster dose after two different priming dose regimens. It has strong immunogenicity and good safety profiles.