In a United Kingdom trial, where the B.1.1.7 variant has become dominant, the vaccine was 89 percent effective, and about half the infections were with the variant. In a smaller and less definitive South African trial where nearly all the participants were infected with the variant, the vaccine was 49 percent effective, although the company underscored that when looking only at people not infected with HIV, the efficacy was 60 percent.

Laboratory tests had suggested that the immune response elicited by vaccines would be diminished against the variant first identified in South Africa, and the Novavax results bear that out. Novavax is one of the vaccines supported by the American government, including $1.6 billion for clinical development and a pre-purchase of 100 million doses. A large phase 3 trial is ongoing in North America.