Drugmaker Novavax said Thursday its COVID-19 vaccine showed nearly 90% efficacy at preventing disease in patients in a large clinical trial in the U.K. But worryingly, the Novavax vaccine showed only a 50% efficacy rate in a smaller trial in South Africa, where a new variant of coronavirus has been spreading rapidly. The Novavax immunization is a protein-based vaccine that trains people’s immune systems to recognize the “spike proteins” that surround the coronavirus. Several mutations in the South African variant led to changes in the spike protein that can help it evade immune responses.

At the World Health Organization, emergencies director Dr. Mike Ryan said vaccine makers may have to tweak their formulas as the coronavirus continues to evolve. 

Dr. Mike Ryan: “Look at influenza. We change the vaccine composition twice a year, for the Northern and Southern Hemisphere, and we’re able to issue vaccines very, very quickly to combat the predominant flu strains every year. So there’s no reason, even down the line, if this virus evolves to a point where our vaccines begin to lose effectiveness — we can adapt those vaccines. And I believe we can adapt those vaccines quickly.”