All over-40s in the UK will be offered a third dose of a Covid vaccine, after advice from the government scientists.
The move would top up protection and help limit the spread of the virus over winter.
The Joint Committee on Vaccination and Immunisation also said 16- and 17-year-olds, initially offered only a single dose, should now receive a second.
So far, 12.6 million people have had a booster dose.
They have been given to the over-50s, front-line medical staff and people with health conditions that put them at greater risk.
Prime Minister Boris Johnson said it would be an "utter tragedy" if double vaccinated people became seriously ill or died "because they allowed their immunity to wane".
Cases have started to tick up in the UK after the lull caused by the half-term holidays - the latest daily figure for number of people testing positive is 39,705.
This should be offered six months after the second dose.