But, they say, data in recent weeks on new variants from South Africa and Brazil has undercut that optimism. And, scientists say there is still much to be learned about the immune system's ability to combat the virus. The shift in thinking among scientists has influenced more cautious government statements about when the pandemic will end. Around the same time, researchers learned of the impact of the faster-spreading variants in South Africa and in Brazil. Novavax's clinical trial data showed its vaccine was 89% effective in a UK trial, but just 50% effective at preventing COVID-19 in South Africa.
