The US Centers for Disease Control and Prevention has endorsed the administration of a booster shot of the Pfizer-BioNTech COVID-19 vaccine for certain demographics.
CDC Director Rochelle Walensky announced the recommendation on Friday.
The health authority says booster shots should be administered at least six months after the second Pfizer dose.
It recommends the additional jab for people 65 years and older and people aged 50 to 64 with underlying medical conditions.
It also says people aged 18 to 49 who have underlying medical conditions, and people aged 18 to 64 who have jobs that put them at greater risk of being infected, may receive the booster.
An advisory panel for the CDC had excluded people who perform such high-risk jobs, citing insufficient evidence to support an extra shot, but Walensky overruled the recommendation.
She said in a statement that "in a pandemic, even with uncertainty," the CDC "must take actions that will do the greatest good."
It is highly unusual that the CDC's recommendations differ from those of the panel.
The health authority said it will urgently evaluate available data so it can make decisions on booster shots for the Moderna and Johnson & Johnson vaccines.