evidence: Most cases assessed results less than six months after the primary vaccination. Most received co-funding from academic institutions and pharmaceutical companies. Most studies compared a COVID-19 vaccine with placebo. Five evaluated the addition of a 'mix and match' booster dose. [...] Different forms of vaccines have been developed to prevent the SARS-CoV-2 virus and subsequent COVID-19 disease. Several are in widespread use globally. To assess the efficacy and safety of COVID-19 vaccines (as a full primary vaccination series or a booster dose) against SARS-CoV-2.