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Biotech firm Novavax said Thursday that its coronavirus vaccine was more than 89% effective in protecting against Covid-19 in its phase three clinical trial conducted in the United Kingdom.

The results were based on 62 confirmed Covid-19 infections among the trial’s 15,000 participants. The company said 56 cases were observed in the placebo group versus six cases observed in the group that received its vaccine. That resulted in an estimated vaccine efficacy of 89.3%, it said.

Shares of the company were up more than 23% in after-hours trading.

With the results, the company “has the potential to play an important role in solving this global public health crisis,” Novavax CEO Stanley Erck said in a statement. “We look forward to continuing to work with our partners, collaborators, investigators and regulators around the world to make the vaccine available as quickly as possible.”

The study also found that the vaccine appeared to be 85.6% effective against the U.K. variant, also known as B.1.1.7. A separate phase two study in South Africa showed that the vaccine isn’t nearly as effective against a new strain ravaging that country.

The shot was still considered effective in protecting against the virus, but at an efficacy rate of just 49.4% among 44 Covid-19 cases in South Africa, where 90% of the cases contain the troubling new variant, the company said.

As a result of the lower effectiveness against the strain in South Africa, Novavax said it plans to pick a modified version of the vaccine to better guard against the new strain “in the coming days.” It plans to test the modified vaccine in the second quarter of this year.