SARS-CoV-2 reinfections: Overview of efficacy and duration of natural and hybrid immunity

Seroprevalence surveys suggest that more than a third and possibly more than half of the global population has been infected with SARS-CoV-2 by early 2022. As large numbers of people continue to be infected, the efficacy and duration of natural immunity in terms of protection against SARS-CoV-2 reinfections and severe disease is of crucial significance for the future. This narrative review provides an overview on epidemiological studies addressing this issue. National surveys covering 2020-2021 documented that a previous SARS-CoV-2 infection is associated with a significantly reduced risk of reinfections with efficacy lasting for at least one year and only relatively moderate waning immunity. Importantly, natural immunity showed roughly similar effect sizes regarding protection against reinfection across different SARS-CoV-2 variants, with the exception of the Omicron variant for which data are just emerging before final conclusions can be drawn. Risk of hospitalizations and deaths was also reduced in SARS-CoV-2 reinfections versus primary infections. Observational studies indicate that natural immunity may offer equal or greater protection against SARS-CoV-2 infections compared to in-dividuals receiving two doses of an mRNA vaccine, but data are not fully consistent. The combination of a previous SARS-CoV-2 infection and a respective vaccination, termed hybrid immunity, seems to confer the greatest protection against SARS-CoV-2 infections, but several knowledge gaps remain regarding this issue. Natural immunity should be considered for public health policy regarding SARS-CoV-2.

Automated summary

Seroprevalence surveys indicate that a significant proportion of the global population has been infected with SARS-CoV-2, and natural immunity plays a crucial role in protecting against reinfections and severe disease. National surveys show that previous infection provides a reduced risk of reinfection for at least one year, with moderate waning immunity. Natural immunity appears to have similar effectiveness against different SARS-CoV-2 variants, except for the Omicron variant which requires further study. Observational studies suggest that natural immunity may provide equal or greater protection compared to two doses of mRNA vaccines, although data are not consistent. Combining previous infection with vaccination seems to offer the greatest protection against SARS-CoV-2, but there are still knowledge gaps in this area.