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Effectiveness of COVID-19 Vaccines Against Delta Variant
Quick Takes
- This study estimates the effectiveness of the BNT162b2 and ChAdOx1 nCoV-19 vaccines against the SARS-CoV-2 delta variant using data from England.
- Effectiveness of both vaccines was 10-15% lower against the SARS-CoV-2 delta variant compared to the alpha variant.
- Policies delaying a second vaccination dose should be revisited in light of these new data.
Study Questions:
What is the effectiveness of BNT162b2 (Pfizer-BioNTech) and ChAdOx1 nCoV-19 (AstraZeneca) vaccines against symptomatic disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) delta variant (B.1.617.2)?
Methods:
Using data on all persons in England who have been vaccinated against coronavirus 2019 (COVID-19) up to May 16, 2021, the investigators sought to estimate the effectiveness of vaccination against symptomatic disease caused by the delta variant or the predominant strain (B.1.1.7, or alpha variant) using two approaches. They first compared vaccination status in persons with symptomatic COVID-19 with vaccination status in persons who reported symptoms but had a negative test. Second, they estimated the proportion of persons with cases caused by the delta variant relative to the main circulating virus (the alpha variant) according to vaccination status. This analysis assumes that a similar proportion of cases of either variant would be expected in vaccinated and unvaccinated individuals should the vaccine be equally effective against both variants. Persons who had previously tested positive before the analysis period were excluded. Vaccine effectiveness was estimated among persons with a symptom-onset date that was 21 days post-first vaccine dose, and 14 days post-second vaccine dose.
Results:
COVID-19 cases in vaccinated individuals at least 16 years of age numbered 19,109; with the alpha variant detected in 14,837 samples and the delta variant in 4,272 samples. Effectiveness after one dose of vaccine (BNT162b2 or ChAdOx1 nCoV-19) was lower among persons with the delta variant (30.7%; 95% confidence interval [CI], 25.2-35.7) than among those with the alpha variant (48.7%; 95% CI, 45.5-51.7); with no significant differences between vaccines. A small difference was seen in effectiveness after the second dose: The BNT162b2 had a 93.7% (95% CI, 91.6-95.3) effectiveness among persons with the alpha variant and 88.0% (95% CI, 85.3-90.1) among those with the delta variant. The ChAdOx1 nCoV-19 vaccine had an effectiveness of two doses of 74.5% (95% CI, 68.4-79.4) among persons with the alpha variant and 67.0% (95% CI, 61.3-71.8) among those with the delta variant.
Conclusions:
Overall effectiveness of the vaccines after a single dose was lower for the delta variant compared to the alpha variant. The BNT162b2 vaccine exhibited slightly better effectiveness after the second dose compared to ChAdOx1 nCoV-19.
Perspective:
The B.1.617.2 (delta) variant of SARS-CoV-2, which was the culprit of a massive surge in India, is now detected across the globe and has threatened the success of the recent progress against COVID-19. While there have been earlier data on reduced neutralization of vaccines with other variants (notably the beta B.1.351 variant), this study shows clear differences in effectiveness of the vaccines between the alpha and delta variant, notably with regard to the number of doses. Effectiveness of current vaccines after the first dose appeared lower against the delta variant, which has major health policy implications regarding the vaccine administration schedule. Several nations have adopted a delayed administration schedule to maximize the proportion of the population with at least a single dose. Shortening of the interval between doses may have to be considered, as the delta variant is becoming rampant, and cases increase across the world. Thankfully, a two-dose vaccination regimen appears to be effective against the delta variant. Implementing policies to encourage vaccination is more important than ever.
Clinical Topics: COVID-19 Hub, Prevention
Keywords: Coronavirus, Coronavirus Infections, COVID-19, Health Policy, Polymerase Chain Reaction, Primary Prevention, SARS-CoV-2, Treatment Outcome, Vaccination, Vaccines
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