The coronavirus disease 2019 (COVID-19) pandemic has threatened global health and prompted the need for mass vaccination. We aimed to assess the efficacy and effectiveness of COVID-19 vaccines to prevent mortality and reduce the risk of developing severe disease after the 1st and 2nd doses. From conception to 28 June 2021, we searched PubMed, Cochrane, EBSCO, Scopus, ProQuest, Web of Science, WHO-ICTRP, and Google Scholar. We included both observational and randomized controlled trials. The pooled vaccine efficacy and effectiveness following vaccination, as well as their 95 percent confidence intervals (CI), were estimated using the random-effects model. In total, 22 of the 21,567 screened articles were eligible for quantitative analysis. Mortality 7 and 14 days after full vaccination decreased significantly among the vaccinated group compared to the unvaccinated group (OR = 0.10, ([95% CI, 0.04-0.27], I2 = 54%) and (OR = 0.46, [95% CI, 0.35-0.61], I2 = 0%), respectively. The probability of having severe disease one or two weeks after 2nd dose decreased significantly (OR = 0.29 [95% CI, 0.19-0.46], I2 = 25%) and (OR = 0.08 [95% CI, 0.03-0.25], I2 = 74%), respectively. The incidence of infection any time after the 1st and 2nd doses diminished significantly (OR = 0.14 [95% CI, 0.07-0.4], I2 = 100%) and (OR = 0.179 [95% CI, 0.15-0.19], I2 = 98%), respectively. Also, incidence of infection one week after 2nd dose decreased significantly, (OR = 0.04, [95% CI (0.01-0.2], I2 = 100%). After meta-regression, the type of vaccine and country were the main predictors of outcome [non-mRNA type, ß = 2.99, p = 0.0001; country UK, ß = -0.75, p = 0.038; country USA, ß = 0.8, p = 0.02]. This study showed that most vaccines have comparable effectiveness, and it is purported that mass vaccination may help to end this pandemic.
Keywords: COVID-19 vaccine; SARS-CoV-2; effectiveness; efficacy; meta-anlysis; mortality; systematic review.