COVID-19 Vaccine Effectiveness for Delta Variant Differs by Dose Number

THURSDAY, July 22, 2021 (HealthDay News) — Following two doses, the effectiveness of the severe acute respiratory syndrome coronavirus 2 vaccine is modestly lower against the delta variant versus the alpha variant, but the difference widens with only one dose, according to a study published online July 21 in the New England Journal of Medicine.

Jamie Lopez Bernal, Ph.D., from Public Health England in the United Kingdom, and colleagues used a test-negative case-control design to estimate the effectiveness of vaccination against symptomatic disease caused by the delta variant (B.1.617.2) or the alpha variant (B.1.1.7) during the period that the delta variant began circulating. Data were included for all symptomatic sequenced cases of COVID-19 in England.

The researchers found that among those with the delta variant versus the alpha variant, the effectiveness after one dose of vaccine (BNT162b2 or ChAdOx1 nCoV-19) was notably lower (30.7 versus 48.7 percent); the results were similar for both vaccines. The effectiveness of two doses of the BNT162b2 vaccine was 93.7 and 88.0 percent among those with the alpha and delta variant, respectively, while effectiveness with the ChAdOx1 nCoV-19 vaccine was 74.5 and 67.0 percent, respectively.

“We found high levels of vaccine effectiveness against symptomatic disease with the delta variant after the receipt of two doses. These estimates were only modestly lower than the estimate of vaccine effectiveness against the alpha variant,” the authors write. “Our finding of reduced effectiveness after the first dose would support efforts to maximize vaccine uptake with two doses among vulnerable groups in the context of circulation of the delta variant.”

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