The Cov-Boost study monitored the effect of top-up doses of the Pfizer and Moderna vaccines in 166 people.
Scientists found their immune response waned in the months after third shots but climbed to greater levels after a fourth.
Participants had 12 to 16 times higher levels of antibodies in the blood a fortnight afterwards, compared with the day it was delivered.
Trial lead Professor Saul Faust, director of the NIHR Southampton Clinical Research Facility, said: “These underline the benefits of the most vulnerable people receiving current spring boosters and gives confidence for any prospective autumn booster programme in the UK.”
More than three million people have received a fourth dose. They are being offered to all over-75s, care home residents and over-12s with weakened immune systems.
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