Effectiveness of second dose of COVID-19 vaccines over a 6 month period

1. The effectiveness of COVID-19 vaccines consistently waned over a 26-week period, although COVID-19 hospitalization and COVID-19 mortality were consistently and significantly higher amongst unvaccinated individuals.

Evidence Rating Level: 2 (Good)

Although antibody titres diminish over time following vaccination, studies examining the waning effectiveness of COVID-19 vaccines have led to varying conclusions on the extent of the waning. Additionally, studies evaluating this waning generally use a test negative case-control design, taking into account only those who seek out testing for COVID-19. Therefore, this study compared COVID-19 related hospital admission, COVID-19 mortality, and COVID-19 infection, amongst those fully vaccinated with the Pfizer-BioNTech mRNA vaccine (BNT162b), the Oxford-AstraZeneca (ChAdOx1) vaccine, and those who are unvaccinated. This study used the OpenSAFELY-TPP database, containing health data on 44% of the English population (24 million people). The study population consisted of fully vaccinated individuals who received their second dose during the 4-week period with the greatest number of second doses distributed, or individuals with no vaccin received at the start of this period. This included 1 951 866 individuals vaccinated with BNT162b2, 3 219 349 vaccinated with ChAdOx1, and 2 422 980 whom were unvaccinated. Overall, the study found that the effectiveness of vaccines was consistently waning for every 4-week period that passed, up to 26 weeks, as the ratios of adjusted hazards ratios were consistent over time. As well, there were consistently higher rates of COVID-19 hospital admission, COVID-19 mortality, and positive COVID-19 test, amongst those who were unvaccinated. The exception was that at 23-26 weeks, rates of positive COVID-19 tests for the fully vaccinated were similar to or greater than those who were unvaccinated, including adjusted HRs of 1.72 (95% CI 1.11-2.68) and 1.86 (95% CI 1.79-1.93) for BNT162b2 and ChAdOx1 respectively. At 23-26 weeks, adjusted HRs for COVID-19 hospital admission and COVID-19 death were ≤0.20 for the BNT162b2 group and ≤0.25 for the ChAdOx1 group. In conclusion, this study found that while vaccine effectiveness consistently waned over 26 weeks, rates of COVID-19 hospital admission and mortality were significantly lower amongst the fully vaccinated, despite similar rates of positive COVID-19 tests between 23 and 26 weeks.

Click to read the study in BMJ

Image: PD

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