Novavax Shot More Effective in U.K. Than in South Africa

A new Covid-19 vaccine fromNovavax Inc. was effective in big trials in both the U.K. and South Africa, but the effectiveness appeared to be reduced in South Africa where a worrisome mutation is prevalent.

The results indicated that a another highly potent vaccine could soon be available to help ameliorate the pandemic, in addition to existing vaccines fromPfizer Inc.,Moderna Inc., andAstraZeneca Plc that are authorized in various countries. But the South Africa results also suggested that the virus is starting to mutate in ways that could make vaccines less effective over time.

Shares of Novavax were down 9% in late trading in New York on Thursday.

Novavax said its vaccine met its primary endpoint in a final-stage study on more than 15,000 people in the U.K., proving to be 89.3% effective in preventing symptomatic Covid-19. The result was based on an interim analysis of 62 cases, of which 56 were in the placebo group and only 6 in people who had got the vaccine. The trial found the vaccine remained effective against the more transmissible B.1.1.7 strain that is common there. There was a low level of significant adverse events, the company said.

The vaccine was somewhat less effective in a second trial in South Africa, where a worrisome variant called B.1.351 is prevalent. This variant contains mutations in spike protein that change its shape and may make it harder for antibodies to recognize it.

In the South Africa trial of over 4,400 people, the vaccine was 60% effective in the vast majority of people that were HIV negative, Novavax said in a statement. It was 49.4% effective overall, when HIV positive patients were also counted.

Most cases of the virus seen in the trial had the new South Africa mutation, the company said.

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