Implications of all the available evidence
Our findings support the benefit of homologous and heterologous booster doses after a primary series of inactivated and mRNA vaccines. All available evidence on vaccine efficacy and effectiveness of inactivated vaccines against variants, compared with the weak neutralising antibody responses conferred by these vaccines, suggest that neutralising antibody levels might not be a sufficient proxy of vaccine efficacy (ie, correlate of protection) in immunogenicity trials or immunobridging studies for inactivated vaccines. Further research is needed to identify how different SARS-CoV-2 vaccine platforms might provide protection via different immune mechanisms. There is an urgent need to identify a correlate of protection for inactivated COVID-19 vaccines, or correlates that might provide information on breadth of protection for all vaccine platforms.