A second booster dose of Covid vaccine will be offered to all adults over-75 and the most vulnerable over-12s in the UK this spring.
The UK's vaccines advisers said an extra dose would help top up their protection against severe Covid-19, BBC reported.
Many of the oldest received their latest shot back in September and immunity may now be waning, it said.
Until now, only people with severely weakened immune systems had been eligible for a fourth dose.
But now adults aged 75 years and over, residents in a care home for older adults, individuals aged 12 years and over who are immunosuppressed, or have weakened immune systems are eligible for the fourth shot.
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While adults will be offered a Pfizer or Moderna vaccine, children aged 12-18 will receive Pfizer.
This will include about 7.2 million people in the UK aged over 75, who have already had their primary course of Covid vaccines, followed by another half a million people who are immunosuppressed.
These are people with diseases such as blood cancer or who've had an organ transplant or are receiving chemotherapy.
The second booster will be administered six months after the third dose, the report said.
The Joint Committee on Vaccination and Immunisation (JCVI) said vaccinating these groups was a "precautionary" move and did not mean there was any current danger to their health.
But because people in these groups are at higher risk from Covid-19, have weaker immune systems than other people and were last vaccinated in September or October 2021, their protection from vaccines might be waning more quickly.
Data suggests that older people who have had two doses of a Covid vaccine have about 45 per cent protection against hospital admission with Omicron, rising to 90 per cent straight after their first booster.
Ten weeks later, that protection has dropped to about 88 per cent.
"Last year's booster vaccination programme has so far provided excellent protection against severe Covid-19," Prof Wei Shen Lim, chair of Covid vaccination at the JCVI, was quoted as saying.
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"To maintain high levels of protection for the most vulnerable individuals in the population, an extra spring dose of vaccine is advised ahead of an expected autumn booster programme later this year."
Other countries who have rolled out fourth Covid booster for immunocompromised include Israel, the US, Chile and South Korea.
Israel now also plans to roll out the fourth jab for all adults.
