A single dose of Pfizer's coronavirus vaccine may not generate a sufficient immune response to protect against dominant new variants, except in people who have already been infected with COVID-19, a UK study has found.

The Imperial College-led study, which looked at immune responses in British healthcare workers after their first dose of the Pfizer shot, found that people who had previously had mild or asymptomatic infection had enhanced protection against more infectious mutated variants that emerged in Britain and South Africa. But the immune response after a first dose of the shot was weaker in people who had not previously been infected, potentially leaving them at risk from such variants, researchers leading the work said on Friday.