A panel of US experts has voted unanimously to recommend the Pfizer and Moderna Covid-19 vaccines over Johnson & Johnson's, which has been linked to deadly side effects.

According to the US Centers for Disease Control and Prevention (CDC), a total of 57 J&J patients have developed a rare blood clot disorder.

Nine people are known so far to have died: seven women and two men.

A total of 16 million US residents have received the single-dose J&J vaccine.

On Thursday, the Advisory Committee on Immunization Practices (ACIP) recommended that the J&J vaccine be reserved for cases where patients are unable or unwilling to receive the more widely available Pfizer and Moderna jabs.

The CDC must still decide whether to accept the panel's recommendation.

The blood-clotting side effect remains rare, with federal health officials estimating that it has been reported at a rate of 3.8 per million doses given.

Of the 57 confirmed cases, 36 were admitted to intensive care units. The dead ranged in ages from 28 to 62, with obesity the primary underlying medical condition.

All the cases so far have taken place within a month of the jab, the vast majority after nine days.

"I don't feel comfortable… not having the ACIP make a clear statement that reflects the recognition that we have of the concerns about these rare but oftentimes fatal side effects," ACIP member Dr Beth Bell said on Thursday.

A number of panellists warned against removing J&J as an option entirely, particularly at a time when US public health officials are trying to vaccinate the country's remaining unvaccinated residents.

"It's really important, though, that we not completely eliminate this vaccine," said Dr Jason Goldman, an assistant professor of clinical biomedical science at Florida Atlantic University.

The use of the J&J vaccine was temporarily halted in April due to concerns over blood clots.

The CDC has noted that the use of the vaccine prevented thousands of Covid-19-related hospitalisations, compared to a relatively small number of people who experienced the side effects.

The latest CDC statistics show that over 76% of US residents above the age of five have received at least one dose of the Covid-19 vaccines.