Natural infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) elicits strong protection against reinfection with the B.1.1.7 (alpha),1,2 B.1.351 (beta),1 and B.1.617.2 (delta)3 variants. However, the B.1.1.529 (omicron) variant harbors multiple mutations that can mediate immune evasion. We estimated the effectiveness of previous infection in preventing symptomatic new cases caused by omicron and other SARS-CoV-2 variants in Qatar. In this study, we extracted data regarding coronavirus disease 2019 (Covid-19) laboratory testing, vaccination, clinical infection data, and related demographic details from the national SARS-CoV-2 databases, which include all results of polymerase-chain-reaction (PCR) testing, vaccinations, and hospitalizations and deaths for Covid-19 in Qatar since the start of the pandemic.
The effectiveness of previous SARS-CoV-2 infection in preventing reinfection was defined as the proportional reduction in susceptibility to infection among persons who had recovered from infection as compared with those who had not been infected.4 Previous SARS-CoV-2 infection was defined as a positive result on PCR assay at least 90 days before a new positive PCR finding.4 We used a test-negative, case–control study design to assess the effectiveness of previous infection in preventing reinfection on the basis of a method that had recently been investigated and validated for derivation of robust estimates for such comparisons4 (Section S1 of the Supplementary Appendix, available with the full text of this letter at NEJM.org). In addition, we performed sensitivity analyses that included adjustment for vaccination status and that excluded vaccinated persons from the analysis. Case patients (defined as persons with positive PCR results) and controls (defined as persons with negative PCR results) were matched according to sex, 10-year age group, nationality, and calendar time of PCR testing to control for known differences in the risk of exposure to SARS-CoV-2 infection in Qatar.4