US drug regulators have given approval for immunocompromised Americans to get an additional Covid jab as a booster to help stave off infection and illness.
The order issued by the Food and Drug Administration (FDA) affects around 10 million people, including transplant recipients and cancer patients.
It marks the first time that US health officials have indicated that booster shots may be necessary to fight Covid.
Several others countries have begun providing booster jabs to some groups.
A Centers for Disease Control and Prevention (CDC) vaccines panel on Friday unanimously voted in favour of boosters for vulnerable groups. The agency is expected to adopt the recommendation soon, and boosters could become available by this weekend.
Experts say the two-dose Moderna and Pfizer vaccines or the single-dose Johnson & Johnson may not sufficiently protect some people, especially those who have a weakened immune system.
All three vaccines are currently approved in the US under an emergency use authorisation. Pfizer, which has applied for full authorisation, has lobbied US and European regulators to approve a third booster dose.
It comes as evidence grows that antibody protection from vaccines may wear off over time, and as some people seek their third dose on the black market.
On Thursday, US disease expert Dr Anthony Fauci said boosters may only be needed for those whose immune systems are low.
"We don't feel at this particular point that apart from the immune-compromised, we don't feel we need to give boosters right now," the advisor to President Joe Biden told CBS.
Meanwhile, the World Health Organization, has called for a moratorium on booster shots until at least the end of September to allow every country to vaccinate at least 10% of their population.
Israel has already provided booster shots to people over 60 who were vaccinated at least five months earlier.
The UK, France and Germany had plans to begin distributing them starting in September.
In Chile, which began giving booster shots on Wednesday to people vaccinated with the Chinese Sinovac vaccine, queues formed outside clinics as the elderly scrambled to increase their immunity.
Other countries planning a booster rollout include Sweden, the UAE, Indonesia, Thailand and South Korea.
By BBC Health News