How Well Does COVID-19 Vaccination Protect Against the Delta Variant?

As the Delta (B.1.617.2) SARS-CoV-2 variant fuels a global surge in COVID-19, an urgent clinical question concerns how effective the available vaccines are against this variant. Using data from the U.K. National Health Service, investigators addressed this question for the Pfizer-BioNTech (BNT162b2) and Oxford/AstraZeneca (ChAdOx1) vaccines. In a test-negative case-control study, they compared vaccination status in patients reporting COVID-19 symptoms who tested positive (cases) or negative (controls) for COVID-19. In a secondary analysis, the relative incidence of infections with the Alpha (B.1.1.7) and Delta variants was estimated in both vaccinated and unvaccinated individuals.

Between November 20, 2020, and May 16, 2021, a total of 19,109 cases were infected with either the Alpha (14,837 patients) or Delta (4272 patients) variant and had known vaccination status. In the test-negative case-control analysis, the effectiveness of a single dose of either the BNT162b2 or ChAdOx1 vaccine was approximately 48% against the Alpha variant but only 30%–36% against the Delta variant. However, after two doses, protective effectiveness rose against each variant (BNT162b2: 94% [Alpha], 88% [Delta]; ChAdOx1: 75% [Alpha], 67% [Delta]). The secondary analysis found similar effectiveness after the first and second vaccine doses.