Protective effectiveness of previous infection against subsequent SARS-Cov-2 infection: systematic review and meta-analysis.

Background: The protective effectiveness provided by naturally acquired immunity against SARS-CoV-2 reinfection remain controversial.

Objective: To systematically evaluate the protective effect of natural immunity against subsequent SARS-CoV-2 infection with different variants.

Methods: We searched for related studies published in seven databases before March 5, 2023.