There are a lot of COVID-19 vaccines to keep track of lately. So far, the ones from Moderna and Pfizer-BioNTech have been authorized for use in the U.S., but there are plenty of others coming down the pipeline that show promise. One of them is the Novavax vaccine.

In late January, Novavax shared positive results from its phase 3 clinical trials in the U.K. The vaccine development company announced in a press release that its two-dose vaccine is almost 90% effective against preventing symptomatic forms of COVID-19. Worth noting: The vaccine was tested when the highly infectious U.K. variant (B.1.1.7) was widely circulating.

The Novavax vaccine wasn’t highly effective against every coronavirus variant, though. The company announced that its vaccine was 60% effective at preventing mild, moderate, and severe COVID-19 in study participants in South Africa. The reason is likely due to the circulation of the region’s highly infectious variant (B.1.351), says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security.