The omicron (B.1.1.529) variant, first detected in the UK on Nov 27, 2021, rapidly became the dominant strain, due in part to reduced vaccine effectiveness. An increase in sequenced cases of the omicron sub-lineage BA.2 was observed in the week beginning on Jan 3, 2022.
BA.2 has a growth advantage over BA.1 and has become the dominant strain in the UK at the time of writing. Neutralisation assays using monoclonal antibodies have suggested a small antigenic difference between BA.1 and BA.2, although sera from individuals with booster vaccinations neutralise both variants similarly.
Our analysis included all vaccines used in the UK. Vaccination status was included as an independent variable and effectiveness defined as 1 minus the odds of vaccination in cases, divided by the odds of vaccination in controls (appendix pp 1–3).
Overall, we found no reduction in vaccine effectiveness against symptomatic disease with BA.2 compared with BA.1. As previously observed, we found that vaccine effectiveness wanes over time, but there was no difference in the rate of decline between the two sub-lineages. Due to the small number of people with BA.2, we were not able to stratify by manufacturer in these analyses; however, we found little difference according to vaccine after a third dose in previous analyses of the omicron variant.
These findings are consistent with neutralisation assays, but there is discrepancy with a Danish household transmission study, which found that BA.2 was associated with increased susceptibility to infection for vaccinated individuals.
Differences between the UK and Denmark studies might be explained by different vaccination and infection histories in the respective countries, or by methodological differences, and need further exploration.
Vaccine effectiveness against severe disease is higher and retained for longer than effectiveness against mild disease.
Since the omicron variant became dominant, people are increasingly likely to have COVID-19 as an incidental finding rather than as the primary reason for admission.
Using specific definitions to identify admissions with severe respiratory disease gives higher vaccine effectiveness estimates than using broader definitions, and these estimates probably reflect the true vaccine effectiveness against hospitalisation.
Vaccine effectiveness against hospitalisation was similar for BA.1 and BA.2 after booster vaccination, although vaccine effectiveness might wane faster for BA.2 than for BA.1. The BA.2 analysis might be more susceptible to misclassification bias, because of cases incidentally hospitalised with COVID-19 due to the higher infection rates during the periods after the booster when BA.2 predominated (appendix p 4); this bias might explain the lower vaccine effectiveness estimates for BA.2 than for BA.1 in these periods.
We declare no competing interests. The UK Health Security Agency has legal permission, provided by Regulation 3 of The Health Service (Control of Patient Information) Regulations 2002, to process patient confidential information for national surveillance of communicable diseases and, as such, individual patient consent is not required to access records.
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