Protective Effect of Previous SARS-CoV-2 Infection against Omicron BA.4 and BA.5 Subvariants

The BA.4 and BA.5 subvariants of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) B.1.1.529 (omicron) variant have shown the capacity of escaping from neutralizing antibodies.1 These subvariants had an appreciable presence in Qatar by early May 2022 (Fig. S1 in the Supplementary Appendix, available with the full text of this letter at NEJM.org) and had become the dominant subvariants by June 8 (Fig. S2). We estimated the effectiveness of previous SARS-CoV-2 infection in preventing reinfection with BA.4 and BA.5 subvariants using a test-negative, case–control study design (Section S1).2