BACKGROUND: We estimated vaccine effectiveness (VE) against COVID-19 and long COVID during pre-Omicron and Omicron periods, by number of doses and prior infection history.
METHODS: We combined survey information from a cohort of healthcare workers in Quebec, Canada, with immunization registry and laboratory administrative data. We defined COVID-19 cases as symptomatic laboratory-confirmed infections and long COVID as self-reported symptoms persisting ≥12 weeks. We assessed VE against COVID-19 and long COVID, stratified by infection history, using a test-negative design where vaccinated participants were compared to unvaccinated participants during the pre-Omicron period or to those twice vaccinated ≥6 months before laboratory testing during the Omicron period.
RESULTS: Analyses included 8230 COVID-19 participants and 43361 tested specimens. During the pre-Omicron period, one- and two-dose VE was 75% (95%CI:64-83) and 95% (95%CI:84-98) against COVID-19, respectively, and 91% (95%CI:79-96) and 87% (95%CI:22-98) against long COVID, respectively. During the Omicron period, booster dose VE was 41% (95%CI:34-47) against COVID-19 and 57% (95%CI:46-66) against long COVID, waning by 6 months. Hybrid VE in vaccinated and previously infected individuals ranged 81% (95%CI:38-94) to 92% (95%CI:87-95) regardless of number of doses, prior infecting variant or median time since last immunological event up to 9 months.
CONCLUSION: COVID-19 vaccination prevented long COVID during the pre-Omicron period and reduced the risk by more than half post-Omicron. With most of the population by now both vaccinated and infected, repeated booster doses may add little incremental value against long COVID, an observation with important public health, immunization program and cost implications.
