NEW YORK - In a nod to the ongoing risk the coronavirus poses to millions of Americans, the Centres for Disease Control and Prevention (CDC) recommended on Wednesday that adults aged 65 and older and those with weakened immune systems receive another dose of the reformulated booster that debuted last autumn.

The endorsement followed a day-long discussion by the CDC’s expert advisers.

The Food and Drug Administration (FDA) authorised the booster plan on Tuesday, and the CDC’s recommendation was the final administrative step.

Eligible Americans will be able to receive booster doses immediately.

Federal health officials are also phasing out the original vaccine formulas created by Pfizer-BioNTech and Moderna, revoking their authorisations in the United States.

And instead of needing an initial series of two shots, unvaccinated people will now require just a single dose of the reformulated, or “bivalent”, Covid-19 shot to be considered vaccinated.

Until now, federal officials had required two doses of the older vaccine before recipients could begin to receive the bivalent boosters, a process some experts felt was confusing.

Limited data on the reformulated vaccines indicates that in older adults, the shots offer additional protection against severe disease and death from Covid-19, although the protection wanes rapidly in the weeks after inoculation.

There are about 53 million adults aged 65 and older in the US, accounting for about 16 per cent of the population, according to the Census Bureau. And seven million Americans have weak immune systems due to illness or medication.

Roughly 250 people in the US are still dying from Covid-19-related causes each day, a vast majority of whom are over 70 or have impaired immune systems. The median age of those hospitalised is 75, according to the CDC.

Yet only about 43 per cent of adults 65 and older have received a bivalent booster shot.

By this point, most Americans have built up some immunity against the virus, whether through prior infections, vaccinations or both. The new guidelines acknowledge as much, but allow for those still at high risk from the virus to protect themselves, and to do so free of charge.

“The one-size-fits-all policy was simple but not optimal,” said Dr Jeremy Faust, an emergency medicine physician and health policy expert at Brigham and Women’s Hospital in Boston. “The new regimen acknowledges that there’s now an extraordinary spectrum of Covid-19 risk, from mild to massive, depending on who you are.”

People who are severely immunocompromised, such as organ transplant recipients, may want to opt for booster shots every six months or even more frequently, Dr Faust said.

The new guidelines come weeks after Britain and Canada recommended additional shots for older adults and immunocompromised people. Britain recommended the shots for those 75 and older, and Canada only for those 80 and older.

The CDC now says that adults aged 65 and older may opt for another dose of the bivalent vaccine at least four months after their first shot.

Those with weakened immune systems may do so two months after their previous bivalent dose, and may choose to receive more doses in consultation with their healthcare provider.

In the meeting of CDC advisers on Wednesday, Dr Camille Kotton, a physician at Massachusetts General Hospital, noted that the new recommendations did not include immunocompromised children aged six months through four years.

That leaves those medically frail children – including recipients of organ transplants – unprotected against the virus, she said.

“As a mum and doctor, this seems like we are leaving them so vulnerable,” she said in an interview.

For most Americans, the FDA plans to encourage annual Covid-19 shots in autumn, much as it does with flu shots. Health officials will decide on the exact composition of the shot in June, based on the version of the virus circulating at the time.

The bivalent vaccine targets the original variant of the coronavirus as well as variants BA.4 and BA.5, which were dominant last summer. But BA.4 and BA.5 were quickly supplanted by newer versions.

The most recent Omicron sub-variant, XBB.1.5, now accounts for about 78 per cent of cases in the US, and another version, XBB.1.6, for about 7 per cent. So far, the reformulated vaccines seem to prevent severe illness and hospitalisation after infection with these variants.

Federal health officials also are making changes to the process for those receiving the initial shots.

Unvaccinated people will receive a single dose of the bivalent vaccine, rather than multiple doses of the original monovalent vaccine. The rationale is that most unvaccinated Americans now presumably have some measure of immunity from a prior infection and may not need two doses at the beginning, the FDA said.

Some experts were sharply critical of the advice. Reams of data suggest that the vaccines are most protective when given in two doses and followed by one or more boosters to reinforce the shield, said Dr John Moore, a virus expert at Weill Cornell Medical College.

“FDA has consistently over-interpreted the performance of the bivalent formulation when given as a booster,” Dr Moore said. “Now it seems to have gone beyond the science and decided it has some kind of magic power as a first dose.”

The FDA said it had “carefully reviewed the available epidemiologic evidence, scientific publications and data provided by sponsors indicating that a single bivalent vaccine dose provided to individuals previously infected with Covid-19 provides an immune response equal to, or superior to, two doses of the original vaccine”.

“The agency believes that this approach will help encourage future vaccination, particularly among those who have not chosen to be vaccinated to date,” the FDA said. NYTIMES