Added value of this study
infection, infections
ADVERSE EVENTS, INFECTION, VIRUS, INFECTIONS
We conducted a comprehensive comparative assessment of homologous versus heterologous COVID-19 vaccination using inactivated vaccines and mRNA vaccines, after both primary series of inactivated vaccines or mRNA vaccines, and simultaneously included safety (adverse events and hospitalisations), immunogenicity (live virus neutralising antibody response and cell-mediated response), and efficacy (breakthrough infections) outcomes. Our results suggest immune benefit, through antibody-mediated or cell-mediated response, or a combination of these responses, in both administering a homologous or heterologous third dose after two doses of inactivated or mRNA vaccination. All four combinations provide similar levels of protection against infection, even in individuals receiving homologous third-dose inactivated vaccine who have low levels of neutralising antibodies against the (mismatched) circulating strain. This finding suggests that neutralising antibody titres might not be the dominant correlate of protection for inactivated vaccines.