Omicron BA.1/1.1 SARS-CoV-2 Infection among Vaccinated Canadian Adults

C o r r e s p o n d e n c e Omicron BA.1/1.1 SARS-CoV-2 Infection among Vaccinated Canadian AdultsTo the Editor: The incidence of the omicron BA.1/1.1 variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which rapidly spread worldwide even among vaccinated persons, is incompletely defined. 1 We quantified the incidence of SARS-CoV-2 infection during the initial omicron BA.1/1.1 variant wave among Canadian adults 2 and the contribution of previous infection and concurrent vaccination to agespecific active immunity (Fig. S1 in the Supplementary Appendix, available with the full text of this letter at NEJM.org).3 From May 2020 through March 2022, in the Action to Beat Coronavirus (Ab-C) study, we conducted four serial assessments of SARS-CoV-2 seropositivity, each involving 5000 to 9000 adults, using the Angus Reid Forum, a nationally representative online polling platform (Fig. S2).Driedblood-spot samples that had been obtained by the participants were tested with highly sensitive and specific chemiluminescence-based enzyme-linked immunosorbent assays targeting the spike protein, receptor-binding domain, and nucleocapsid (N) protein.2,4 Vaccines that have been used in Canada contain only spike protein and thus should not elicit N protein positivity.5 Approval for the study was obtained from the institutional review board at Unity Health Toronto.The 5031 adults who were surveyed in phase 4 of the Ab-C study and whose dried-blood-spot samples were received between January 24 and March 15, 2022, were broadly representative of Canadian adults.The study population had similar prevalences of obesity, smoking, diabetes, and vaccination against SARS-CoV-2 as the general Canadian adult population, but the study population included a lower percentage of lowereducated adults (Table S1).More women than men and more vaccinated adults than unvaccinated adults provided dried-blood-spot samples in phase 4 of the study.