CHICAGO (Reuters) — Novavax Inc said on Thursday its coronavirus vaccine was 89.3% effective in preventing COVID-19 in a trial conducted in the United Kingdom, and was nearly as effective in protecting against the more highly contagious variant first discovered in the UK, according to a preliminary analysis.

A mid-stage trial of the vaccine in South Africa, where a potent variant of the virus is common, showed 60% effectiveness among people who did not have HIV.

Novavax shares rose 20% in after hours trading following the report, which comes on the day the United States reported its first cases of the South African variant.

Novavax said the UK trial, which enrolled 15,000 people aged 18 to 84, is expected to be used to apply for regulatory review in Britain, the European Union and other countries. Some 27% of people in the trial are over age 65.

The study took place as the more highly transmissible UK variant was circulating, and the preliminary analysis suggests the vaccine was 85.6% effective against this mutation, the U.S. company announced in a news release. It did not provide the study data.

In the UK trial, the effectiveness of the vaccine was close to that of the two authorized vaccines from Pfizer Inc /BioNTech SE and Moderna Inc, whose two-dose regimens were around 95% effective at preventing COVID-19 in clinical trials.

John Moore, a professor of microbiology and immunology at Weill Cornell Medical College in New York, said the Novavax UK data are essentially the same as results for the vaccines from Pfizer and Moderna.

“It’s not statistically different. The vaccine basically works well in the predominant strain circulating in the UK, which means it’s like to be equally effective in the United States,” he said.

Dr. Amesh Adalja, an infectious disease expert at the Johns Hopkins Center for Health Security, said the results were in line with hopes, and that he was concerned that people would focus too much on the weaker effectiveness shown in South Africa.

“We’ve gotten spoiled because we’ve seen the Moderna and Pfizer numbers. I know people are going to be alarmed, but 60% efficacy against the new variant is acceptable,” he said, noting that the U.S. Food and Drug Administration initially said it would approve a vaccine that was at least 50% effective.

The South African variant has been shown to evade antibody protection in lab studies by Moderna and Pfizer and BioNTech.

“The 60% reduced risk against COVID-19 illness in vaccinated individuals in South Africans underscores the value of this vaccine to prevent illness from the highly worrisome variant currently circulating in South Africa, and which is spreading globally,” Professor Shabir Maddi, lead investigator of the Novavax vaccine trial in South Africa.

Novavax said it started making new versions of its vaccine to protect against emerging virus variants in early January and expects to select ideal candidates for a booster in the coming days. The company said it plans to initiate clinical testing of these new vaccines in the second quarter of this year.

Novavax is also running a 30,000-person trial in the United States and Mexico that began in December after at least two delays it said were due to manufacturing scaling issues that prevented clearance by the FDA.

The company has received $1.6 billion from the U.S. government in funding for the vaccine trial and for 100 million doses.

Reporting by Caroline Humer and Julie Steenhuysen.

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