Novavax Inc said on Thursday its coronavirus vaccine was 89.3 percent 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 percent effectiveness among people who did not have HIV.

Novavax shares rose 20 percent 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...