The British government says booster vaccines will be offered to people aged 50 and over, those in care homes and frontline health and social care workers.
Experts said the Pfizer/BioNTech vaccine should be used as the booster dose for more than 30 million people, and that it was safe to be given alongside the usual winter flu jab.
All those who are clinically extremely vulnerable and anyone aged 16 to 65 in an at-risk group group for Covid will also be eligible for a jab.
Three vaccines have been approved as safe and effective as boosters - AstraZeneca, Pfizer and Moderna - but experts have decided to opt for Pfizer as a preference after studies showed it is well tolerated and works well as a booster.
The Pfizer jab as a booster can be given to people who had two doses of AstraZeneca previously.
If necessary, Moderna may be used as an alternative, but as a half-dose booster shot after studies showed it was effective with few side-effects.
People should receive their third booster dose at least six months after they received their second dose of a Covid vaccine.
When there is more data, experts plan to look at whether boosters should also be offered to healthy people under the age of 50.