The Pfizer–BioNTech and Oxford–AstraZeneca COVID-19 vaccines are effective against the highly infectious Delta variant of SARS-CoV-2 — but their protection drops away over time, a study of infections in the United Kingdom has concluded.
Researchers at the University of Oxford, UK, and the country’s Office for National Statistics analysed a vast data set comprising the results of 2,580,021 PCR tests to check for SARS-CoV-2 from 384,543 UK adults between 1 December 2020 and 16 May 2021 — when the Alpha variant was dominant — and 811,624 test results from 358,983 people between 17 May and 1 August 2021, when the Delta variant was more prevalent.
The results, published in a preprint on 19 August1, suggest that both vaccines are effective against Delta after two doses, but that the protection they offer wanes with time. The vaccine made by Pfizer in New York City and BioNTech in Mainz, Germany, was 92% effective at keeping people from developing a high viral load — a high concentration of the virus in their test samples — 14 days after the second dose. But the vaccine’s effectiveness fell to 90%, 85% and 78% after 30, 60 and 90 days, respectively.