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

We extracted data regarding SARS-CoV-2 laboratory testing, clinical infection, vaccination, and demographic details from the national SARS-CoV-2 databases, which include all results of polymerase-chain-reaction (PCR) and rapid antigen testing conducted at health care facilities in Qatar. Previous infection was defined as a positive test result at least 90 days before a new positive test finding; persons with negative results were used as controls.² To control for differences in SARS-CoV-2 infection risk in Qatar, we matched cases and controls according to sex, 10-year age group, nationality, number of coexisting medical conditions, calendar week of testing, method of testing, and reason for testing.² Previous infection was further categorized according to its occurrence in Qatar before the December 19, 2021, initiation of the omicron wave (pre-omicron infections) or after that date (post-omicron infections).³

Read more...