US government officials also said that they expect many Americans to get an approved vaccine to prevent COVID-19 at no charge once it begins distribution, potentially in January.

“For any American who is vulnerable, who cannot afford the vaccines, and desires the vaccine, we will provide it for free,” a US government official said during a call with reporters, requesting anonymity.

The official also said that based on conversations with commercial health insurers, it expects a vaccine to be covered with no out-of-pocket costs, similar to coverage policies insurers have put in place for COVID-19 related medical services.

"We will be working with insurers," the official said.

The Trump Administration last month launched "Operation Warp Speed" - a joint HHS and Department of Defense program - to accelerate development of coronavirus vaccines, treatments and diagnostics.

The effort has "maximized the possibility" of having a vaccine before year end, the government official said.

The Trump Administration previously announced it has put more than $2 billion behind three vaccines in development, one being tested by AstraZeneca Plc along with the University of Oxford, a Moderna Inc candidate and one from Johnson & Johnson. It has also given more than $30 million each to Merck & Co and Sanofi SA for their separate vaccine efforts.

A government official said it was still deliberating which companies would ultimately be among those it will support, based on the success of closely watched trials and manufacturing capability.

The New York Times reported earlier this month that the White House had selected five companies: Moderna, AstraZeneca, Pfizer Inc, J&J and Merck.