US biotech firm Novavax said Thursday its two-shot COVID-19 vaccine candidate showed 89.3 percent efficacy in a major Phase 3 clinical trial in Britain.
But the positive news was offset by results that showed it was much less effective against a highly transmissible variant of the coronavirus first identified in South Africa that is spreading quickly around the world.
The company said it would immediately begin developing a new vaccine targeting this variant.
"NVX-CoV2373 has the potential to play an important role in solving this global public health crisis," said the company's president and CEO Stanley Erck, using Novavax's name for the vaccine.
The British trial involved 15,000 people age 18 to 84, including 27 percent who were over the age of 65.
The first interim analysis was based on 62 cases, of which 56 cases of COVID-19 were observed in the placebo group versus six cases among people who received NVX-CoV2373.
The company's preliminary analysis indicated the variant first identified in Britain, B.1.1.7, was detected in more than 50 percent of the confirmed cases.
Efficacy by strain was calculated at 95.6 percent against the original COVID strain, and 85.6 percent against B.1.1.7.
"These are spectacular results, and we are very pleased to have helped Novavax with the development of this vaccine," said Clive Dix, chair of the UK Vaccine Taskforce, according to a statement.
But the efficacy was much lower in a small trial conducted in South Africa.
This study enrolled just over 4,400 patients from September to mid-January, during which time the B.1.351 variant, which contains critical mutations along the virus' spike protein, was spreading rapidly through the country.
Overall efficacy was 49.4 percent in this trial, but the figure rose to 60 percent among the 94 percent of trial participants who were HIV-negative.
Novavax said it began working on new vaccines against emerging strains in early January and expects to select ideal candidates in the coming days, then begin clinical testing in the second quarter of the year.
Unlike the Pfizer and Moderna vaccines, which use mRNA technology, the Novavax shot injects pieces of the coronavirus into humans in order to evoke an immune response.