Novavax says its coronavirus vaccine was highly effective in a UK trial, but it didn’t work nearly as well against the variant first found in South Africa

• Novavax on Thursday said that its vaccine was 89% effective at preventing cases of COVID-19, based on results from a UK trial.
• In a trial in South Africa, the vaccine was far less effective, likely because of a coronavirus variant that has emerged there.
• Novavax is still signing up volunteers for a trial of its experimental shot in the US and Mexico.
• Visit Business Insider’s homepage for more stories.

An experimental coronavirus shot developed by the biotech company Novavax was highly effective at preventing COVID-19 in a trial enrolling 15,000 British volunteers, a critical validation of a leading COVID-19 vaccine candidate, the company said Thursday. 

Despite the success in the UK, weaker results from a smaller study in South Africa raise a warning about the potential for virus variants to evade efforts to defeat them.

Novavax’s vaccine was 49% effective at preventing disease in the South Africa trial, where more than 9 in 10 illnesses were caused by a troubling viral strain called B.1.351. On Thursday, the US confirmed the first cases of that strain. 

The company published the trial results in a press release, and said it plans to submit them to a scientific journal for review. 

Novavax’s stock jumped 30% in after-market trading. Overall, the clinical success caps an arduous, three-decade-long history of vaccine research for the company, which has never developed a commercial medicine.

The UK and South Africa studies are both ongoing, with Thursday’s announcement detailing interim results from both trials. That means the actual efficacy of the vaccine is likely to change, as more infections are recorded in the studies. Both studies randomized volunteers to receive either two doses of Novavax’s shot or a placebo.

South Africa study results is a troubling, yet caveated, finding

The variant, B.1.351, was first discovered in December in South Africa and has since spread to at least 30 countries. 

By the time Novavax’s study began counting COVID-19 cases among volunteers, B.1.351 was the dominant strain in South Africa, making up roughly 80% of cases, according to data from Nextstrain.org presented by Novavax. 

The study’s key finding was that the vaccine was 49% effective in South Africa. The trial tallied 44 COVID-19 cases, with 15 among volunteers getting Novavax’s shots and 29 from the placebo group.

That’s far smaller than the final-stage trials ran by Pfizer and Moderna, which each respectively tallied more than 150 cases. 

Still, the 4,406-person study provides the most solid evidence to date that COVID-19 vaccines can be less effective against certain variants of the virus, particularly B.1.351. 

Given the small number of cases, the actual efficacy of the vaccine in South Africa could vary wildly. The interim analysis determined a 95% chance that the shots are between 6% and 73% effective. 

The concern is evident in a change in Novavax’s strategy. The company will start researching a new version of its vaccine that specifically includes B.1.351, aiming to start human testing in the second quarter of 2021. 

Some preliminary research raised concerns this week that other coronavirus vaccines may not work as well against this variant. Moderna’s vaccine, for instance, showed more than a six-fold reduction in neutralizing antibodies, the virus-killing proteins at the heart of the immune response, when exposed to B.1.351.

Both Moderna and Pfizer have still said they expect that their shots will work against the new variant. But both vaccine developers have also started developing variant-specific vaccines out of caution. 

Even with success, Novavax is still likely months away from starting to vaccinate the public

An effective vaccine from Novavax would be a welcome addition to the world’s arsenal of tools to fight the pandemic, which has infected more than 100 million people. Western regulators have so far authorized vaccines from Pfizer-BioNTech, AstraZeneca-Oxford, and Moderna, and several others have gained approval as well, but supplies remain severely limited.

Novavax has said that by the middle of 2021, it aims be able to make shots at a rate of 2 billion doses per year, though it’s not clear how many shots the company could actually make this year.

Despite the clinical success in the UK, Novavax CEO Stanley Erck said it would likely take two or three months for the company to actually file for an emergency OK with that country’s regulators.

The US government struck a $1.6 billion funding deal with Novavax last year, prioritizing its vaccine candidate for large-scale testing in the US and Mexico. That 30,000-person study has so far enrolled more than 16,000 volunteers. The company expects to finish enrollment in the middle of February. 

It’s not clear whether US regulators would require results from the US trial before making a decision on whether to allow the vaccine to be given more widely. Erck said the company was in discussions with the Food and Drug Administration.