July 22, 2021 -- The Pfizer and AstraZeneca vaccines — the primary vaccine options in the United Kingdom — were less effective against Delta variant of the coronavirus compared with the Alpha variant (known as B.1.1.7), according to a study published online Wednesday in the New England Journal of Medicine.

Researchers, led by Jamie Lopez Bernal, PhD, from Public Health England, write that the lower effectiveness against the Delta variant is particularly evident after a single dose, but even after two doses they were less effective. Still, the Pfizer vaccine was at least 88% effective against Delta after two doses; the AstraZeneca vaccine was 67% effective.

The differences were particularly notable after only one dose of either of the vaccines. With one dose of Pfizer, the effectiveness against the Alpha variant was 48.7% compared with 30.7% against the Delta variant. Results after one dose of the AstraZeneca vaccine were similar.

The Delta variant first emerged in India, but rapidly spread globally and is now fueling an increase in COVID cases in countries including the United Kingdom. In the United States, the Delta variant now accounts for at least 83% of cases.

"The fact [that] there is still notable effectiveness is encouraging. However, complacency is not warranted, since the virus will continue to cause at least mild disease even in some [of] those vaccinated with two doses," Stephen J. W. Evans, an epidemiologist with the London School of Hygiene & Tropical Medicine ( in the United Kingdom, says.