US health authorities on Thursday approved updated versions of Pfizer and Moderna's Covid-19 vaccines, with the companies saying the jabs will be available "in the coming days."

The latest vaccines, developed using the KP.2 variant of the virus, are designed to target variants currently in circulation and provide better protection against the risks of hospitalization or death, the US Food and Drug Administration (FDA) said.

"We strongly encourage those who are eligible to consider receiving an updated Covid-19 vaccine," senior FDA official Peter Marks said in a statement.

The vaccines will be available for people six months of age and up. A booster dose must be administered at least two months after the previous shot.

The FDA's announcement came as Covid cases have been on the rise this summer in the United States, which is already bracing for the winter epidemic season.

Pfizer and Moderna both said their vaccines will be available "in the coming days" in pharmacies, hospitals and clinics.

"In the 2023-24 season, we saw more than 600,000 hospitalizations due to Covid in the US," Moderna CEO Stephane Bancel said in a statement.

"Staying up to date with your Covid-19 vaccine remains one of the best ways for people to be protected and prevent severe illness.

US health officials have been promoting annual Covid jabs, much like the flu shot.

But only 22.5 percent of adults and 14 percent of children have received an updated vaccine dose for the 2023-2024 season, according to data from the Centers for Disease Control and Prevention.