The Omicron variant rapidly mutated into several sublineages, from BA.1/BA.2 to BA.4/5. The currently circulating variants include BA.2.75, BQ.1, and XXB. Omicron is remarkable for the very large number of spike mutations that led to its evasion of vaccine- and infection-induced immunity.
At present, three doses of a messenger ribonucleic acid (mRNA) vaccine are recommended for protection against adverse outcomes following SARS-CoV-2 infection. However, the risk of symptomatic coronavirus disease 2019 (COVID-19) has been reduced by 67% at 2-4 weeks from the third dose but less than 50% at ten weeks or later. This means that up to 85% of people in some regions have hybrid immunity.
Earlier hybrid immunity was elicited chiefly by Alpha or earlier variants of the virus, also called ancestral variants. These individuals reacted with stronger antibody and T-cell responses to the vaccine doses than uninfected people. With Omicron, though, some scientists claim that the immune response to this VOC is strikingly muted following a breakthrough infection, especially if the patients already had a prior ancestral infection that led to immune imprinting.
To explore this hypothesis, the current paper, posted online to the preprint server medRxiv*, examined circulating and mucosal immunity in triple-vaccinated healthcare workers (HCW) before and after they were infected with BA.1 or BA.2. The HCW were first classified into those with and without a prior history of ancestral SARS-CoV-2 infection (“infected” and “infection-naïve” individuals, respectively).
