Mar. 18, 2021 -- Researchers say the two-dose Astra Zeneca vaccine proved ineffective against mild-to-moderate infections caused by the coronavirus variant first found in South Africa.

Scientists at the South African Medical Research Council Vaccines and Infectious Diseases Analytics Research Unit found the vaccine was overall 10.4% effective against the variant known as B.1.351, according to the study published in the New England Journal of Medicine.

The Astra Zeneca vaccine was given to 750 participants, with 19 developing mild to moderate COVID-19. A placebo was given to 717 participants, and 23 developed symptoms. Of those 42 who developed symptoms, 39 cases were caused by B.1.351, the NEJM said.

"In this trial, we found that two doses of the ChAdOx1 nCoV-19 vaccine had no efficacy against the B.1.351 variant in preventing mild-to-moderate COVID-19," the study said.

None of those 42 had to be hospitalized. The researchers said they could not conclude whether the vaccine was effective against severe cases because none occurred. That might be because of the relative youthfulness of the participants, who had a median age of 30.

South Africa stopped using the Astra Zeneca vaccine in February amid reports it offered little protection against B.1.351. South Africa switched to the Johnson & Johnson single-dose vaccine to immunize health care workers.

Research has shown the Pfizer/BioNTech and Moderna vaccines don’t work as well against the coronavirus variant first discovered in South Africa as they do against the dominant virus strain first seen in the United Kingdom.

The variant first detected last fall in South Africa has now been reported in several countries. The CDC says that in the United States, 142 cases have been found in 25 states or U.S. jurisdictions as of Wednesday.

In comparison, 4,686 cases of the variant first found in the United Kingdom and 27 cases of the variant first found in Brazil have been reported in the United States.

Show Sources

NEJM. "Efficacy of the ChAdOx1 nCoV-19 Covid-19 Vaccine against the B.1.351 Variant"

CDC: U.S. COVID-19 Cases Caused by Variants

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