Journal Club

Estimating the Effectiveness of COVID-19 Vaccines Against the Delta Variant

By Daniel Mendoza, MD

The effectiveness of current vaccines against symptomatic disease caused by the new delta variant of SARS-CoV-2 is unclear. Lopez et al. conducted a case-control study in England to estimate the effectiveness of the BNT162b2 (Pfizer) and ChAdOx1 (AstraZeneca) vaccines against the delta variant vs the alpha variant. The investigators compared vaccination status in persons with symptomatic COVID-19 (cases) with vaccination status in persons who reported symptoms but had a negative COVID-19 test (controls). Their results were recently published in the New England Journal of Medicine.

Effectiveness after one dose of BNT162b2 or ChAdOx1 vaccines 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); the results were similar for both vaccines. With the BNT162b2 vaccine, the effectiveness of two doses was 93.7% (95% CI, 91.6-95.3) among persons with the alpha variant and 88.0% (95% CI, 85.3-90.1) among those with the delta variant. With the ChAdOx1 vaccine, the effectiveness of two doses was 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.

In conclusion, although two doses of the BNT162b2 or ChAdOx1 vaccines provided protection against the delta variant, their effectiveness decreased against this new variant. This study supports interventions to maximize vaccine uptake with two doses. It also highlights the importance of reducing viral transmission worldwide to avoid the emergence of new variants, which could be associated with further reduction of vaccine effectiveness.

(Bernal et al. New Engl J Med. 2021;385:585-594.)