The prevalence of adaptive immunity to COVID-19 and reinfection after recovery - a comprehensive systematic review and meta-analysis of 12 011 447 individuals

Research purpose: The research aims to estimate the prevalence of detectable SARS-CoV-2 antibodies, T and B memory cells after recovery, to determine the prevalence of SARS-CoV-2 reinfection, and to investigate the protective efficacy of infection with SARS-CoV-2 against reinfection. Methodology: Several online databases were searched for studies conducted between 1 January 2020 to 1 April 2021. Studies which compared COVID-19 infection between individuals with and without prior infection were included and assessed for quality and risk of bias. Pooled estimates of the prevalence of humoral and cellular immunity parameters and reinfection were obtained in a meta-analysis using bias adjusted synthesis methods. Findings: At ? 6 months after recovery, the prevalence of SARS-CoV-2 specific immunological memory remained high; IgG - 90.4% (95%CI 72.2-99.9, I2=89.0%, p<0.01, 5 studies), and CD4+ - 91.7% (95%CI 78.2 - 97.1, one study). The pooled prevalence of reinfection was 0.2% (95%CI 0.0 - 0.7, I2 = 98.8, 9 studies). Individuals previously infected with SARS-CoV-2 had an 81% reduction in odds of a reinfection (OR 0.19, 95% CI 0.1 - 0.3, I2 = 90.5%, 5 studies). Research value: This review of 12 million individuals presents evidence that most individuals who recover from COVID-19 develop immunological memory to SARS-CoV-2, thus, reinfection after recovery was rare