Risk of BA.5 Infection among Persons Exposed to Previous SARS-CoV-2 Variants

To the Editor: In recent months, omicron (B.1.1.529) became the dominant variant of severe acute respiratory syndrome coronavirus 2 (SARSCoV-2), displaying some degree of immune evasion. The initial omicron subvariants, BA.1 and BA.2, are being progressively displaced by BA.5 in many countries, possibly owing to greater transmissibility and partial evasion of BA.1and BA.2-induced immunity. The protection afforded by BA.1 against infection by the BA.5 subvariant is critical because adapted vaccines under clinical trials are based on BA.1. Portugal was one of the first countries affected by a BA.5 predominance. We used the national coronavirus disease 2019 (Covid-19) registry (SINAVE) to calculate the risk of BA.5 infection among persons with documented infection with past variants, including BA.1 and BA.2. The registry includes all reported cases in the country, regardless of clinical presentation. The national SARS-CoV-2 genetic surveillance identified periods when different variants represented more than 90% of the isolates. We identified all persons who had a first infection in periods of dominance of each variant, to calculate their infection risk during the period of BA.5 dominance (Fig. 1A). We pooled BA.1 and BA.2 because of the slow transition between the two