About 7 million Americans have received the updated versions of the coronavirus vaccines, compared to 56.5 million people who received last year's version.
New Covid variants are currently circulating in the UK, and a person s immune response may depend on their previous exposure to the virus, according to new research.
The ZOE health study, which estimates figures for UK Covid infections, suggests there were 97,904 new daily cases of symptomatic infection on September 16.