The Delta variant of the SARS-CoV-2 virus that causes COVID-19 has contributed to a significant surge in cases around the globe, including India and the U.K. While more and more people continue to get vaccinated worldwide, the effectiveness of these vaccines against the delta variant remains unclear.
In this study, researchers use a negative-case control design to estimate the effectiveness of vaccination against symptomatic disease caused by the delta variant or the predominant alpha strain over the period that the delta variant has been around. Data on cases of COVID-19 in England were used to estimate the proportion of cases with either variant according to the vaccination status of the patients.
Findings show that effectiveness after one dose of the Pfizer COVID-19 vaccine was lower among people with the delta variant compared to those with the alpha variant. However, results were similar for both the Pfizer vaccine and the AstraZeneca vaccine. The effectiveness of two doses of the Pfizer vaccine was 93.7% among people with the alpha variant and 88% among those with the Delta variant. Effectiveness of the AstraZeneca vaccine after two doses was 74.5% among patients with alpha variant and 67% among those with the Delta variant.
Overall, these findings show very modest differences in vaccine effectiveness with the delta and the alpha variant after two doses. Absolute differences in effectiveness were more marked after the first dose. Hence, these findings support efforts to maximise vaccine uptake with two doses.
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