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.
Several others countries have begun providing booster jabs to some groups.
The issue of booster vaccines will be discussed by the Centers for Disease Control and Prevention (CDC) vaccines panel at a meeting on Friday. If approved by the group, 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.
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.
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.
The UK, France and Germany had plans to begin distributing them starting in September.