Two doses of COVID-19 vaccines effective against Delta variant: study

A large real-world analysis published in the NEJM found "only modest differences" in the effectiveness of COVID-19 vaccines against the Delta variant, as compared with the Alpha variant, after the full course of two doses is administered. However, data showed that one dose of vaccine provided "notably lower" protection against the Delta strain than against Alpha.

The analysis included 19,109 sequenced cases from people in the UK who had symptomatic COVID-19 caused by the Alpha or Delta variants and who had received at least one dose of either AstraZeneca's Vaxzevria or Pfizer and BioNTech's Comirnaty. The researchers noted that the Alpha variant was detected in 14,837 samples and the Delta variant in 4272 samples.

At least 67% effective against Delta

Results showed that the effectiveness of two doses of Vaxzevria was 74.5% among people with the Alpha variant and 67% among those with the Delta variant. Meanwhile, two doses of Comirnaty demonstrated effectiveness of 93.7% among people with the Alpha variant and 88% among those with the Delta variant. The research confirms earlier data released by Public Health England suggesting two doses of the vaccines offered similar levels of protection against Delta.

However, the NEJM report said the effectiveness of one dose of COVID-19 vaccine was 30.7% among people with the Delta variant, down from 48.7% in those with the Alpha variant. Specifically, a single dose of Vaxzevria had an effectiveness of 30% against the Delta strain, versus 48.7% against Alpha, while the respective figures for Comirnaty were 35.6% and 47.5%.

"Our finding of reduced effectiveness after the first dose would support efforts to maximise vaccine uptake with two doses among vulnerable groups in the context of circulation of the Delta variant," the study authors concluded. The findings come shortly after results from a laboratory study suggested that Johnson & Johnson's single-dose COVID-19 vaccine is much less effective against the Delta and Lambda variants than the original virus.

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