COVID-19 Vaccine Effectiveness Declines Over Time, Says New Study

Strategic Insights -

A new study conducted by researchers at Laval University in Quebec, Canada, has found that the effectiveness of COVID-19 vaccines in preventing long COVID and COVID-19 itself declines over time. According to the study, which was published in the Clinical Infectious Diseases journal, the one- and two-dose vaccine effectiveness against COVID-19 was 75% and 95%, respectively, during the pre-Omicron period. However, during the Omicron period, the booster dose vaccine effectiveness was only 41% against COVID-19. The study also found that COVID-19 vaccination prevented long COVID during the pre-Omicron period and reduced the risk by more than half post-Omicron.

Key Takeaways:

  • The one- and two-dose vaccine effectiveness against COVID-19 was 75% and 95%, respectively, during the pre-Omicron period.
  • The booster dose vaccine effectiveness was only 41% against COVID-19 during the Omicron period.
  • COVID-19 vaccination prevented long COVID during the pre-Omicron period and reduced the risk by more than half post-Omicron.
  • Hybrid vaccine effectiveness in vaccinated and previously infected individuals ranged from 81% to 92%, regardless of the number of doses, prior infecting variant, or median time since last immunological event up to 9 months.
  • The study found that repeated booster doses may add little incremental value against long COVID, an observation with important public health, immunization program, and cost implications.
  • The study concluded that with most of the population now both vaccinated and infected, repeated booster doses may add little incremental value against long COVID.

Statistics:

  • 8230 COVID-19 participants were included in the study.
  • 43361 tested specimens were analyzed in the study.
  • 41% of vaccinated individuals were found to experience COVID-19 during the Omicron period (95% CI: 34-47).
  • 57% of vaccinated individuals were found to experience long COVID during the Omicron period (95% CI: 46-66).
  • 75% (95% CI: 64-83) of unvaccinated individuals experienced COVID-19 during the pre-Omicron period.
  • 91% (95% CI: 79-96) of unvaccinated individuals experienced long COVID during the pre-Omicron period.

Sources:

  • Effectiveness of COVID-19 vaccination and prior infections to reduce long COVID risk during the pre-Omicron and Omicron periods. Clinical Infectious Diseases, 2025.
  • Oxford Univ Press Inc, Journals Dept, 2001 Evans Rd, Cary, NC 27513, USA. (Oxford University Press - www.oup.com/; Clinical Infectious Diseases - cid.oxfordjournals.org)
  • NewsRx. Studies from Laval University Yield New Data on COVID-19 (Effectiveness of COVID-19 vaccination and prior infections to reduce long COVID risk during the pre-Omicron and Omicron periods). Vaccine Weekly. October 15, 2025; p 192.