AstraZeneca Covid-19 Vaccine Effective Against U.K. Variant in Trial

Preliminary studies from

Pfizer Inc.

and

Moderna Inc.

found their Covid-19 shots continued to offer protection against new virus variants that have contributed to a fresh surge in cases in the U.K., Europe, South Africa and elsewhere.

Vaccine makers are nevertheless readying new shots that zero in on the new variants more precisely, underlining how mutations in the virus risk morphing the year-old pandemic into a long-running cat-and-mouse game between scientists and a shifting enemy. The virus behind Covid-19 has so far been linked to almost 2.3 million deaths worldwide and more than 100 million cases.

The study published Friday looked at the AstraZeneca vaccine’s effectiveness against a new variant of coronavirus first identified in the U.K. last year.

The variant has now displaced older strains to become the dominant version of the coronavirus in Britain and is spreading in many other countries, including the U.S., where public-health officials have said it could become the dominant version of the virus.

Preliminary estimates suggest the variant from the U.K. is 50%–70% more transmissible than earlier versions of the virus. U.K. scientists said recently that early data suggested it could also be deadlier.

Researchers examined blood samples from around 256 participants in an ongoing clinical trial of the vaccine in the U.K. who tested positive for Covid-19.

Genetic sequencing allowed them to identify which participants were infected with the new variant and which had an older version. A little under a third had the new variant.

By testing antibody levels and other markers of immune system activity against the virus, the researchers found the vaccine triggered an effective immune response against the new variant in 75% of cases that showed symptoms of infection, and in around two-thirds of cases if those that didn’t show symptoms were also included.

The small-scale study showed the vaccine works slightly better against older, more established versions of the virus. For those with the older strain, the vaccine was effective in 84% of symptomatic cases and 81% of all cases.

The researchers reported sharply differing antibody responses among the two groups, saying certain types of antibodies induced by the vaccine were up to nine times less effective at neutralizing the new variant than the old. Overall protection was similar, however, suggesting other parts of the immune system are playing a key role.

Andrew Pollard,

director of the Oxford Vaccine Group at the University of Oxford, said it isn’t entirely clear which biological mechanisms are most important. It might be infection-fighting T-cells or other types of antibodies, he said.

“We don’t know the answer,” he said.

Almost 120 million doses of vaccine have been administered worldwide, according to figures compiled by the University of Oxford’s Our World in Data project. Roll-outs have been patchy, with some countries such as Israel and the U.K. moving rapidly to inoculate their most at-risk citizens and others, including in Europe, lagging behind due to supply and other issues. The U.S. has so far given at least one dose of vaccine to 35 million people, around 10% of its population.

Vaccine makers say the technology behind Covid-19 vaccines should allow them to swiftly retool their production lines to produce shots targeted more precisely at new and emerging variants.

Some studies have suggested a variant first identified in South Africa might be less susceptible to existing vaccines than the U.K. variant. Companies including Moderna, Pfizer and its partner

BioNTech

SE,

Johnson & Johnson

and

Novavax Inc.

are designing new vaccines to specifically target the South African variant.

Babak Javid,

associate professor of infectious diseases at the University of California, San Francisco, said small differences in how vaccines perform against new variants compared with established versions isn’t a major concern provided those vaccinated are protected against severe illness and hospitalization. That will be critical to determining when countries relax lockdowns and other public health restrictions, he said.

Write to Jason Douglas at jason.douglas@wsj.com