Pfizer's COVID-19 pill appears to provide little or no benefit for younger adults, while still reducing the risk of hospitalisation and death for high-risk older users, a study has found.

In a 109,000-patient study in Israel, researchers found Paxlovid reduced hospitalisations among people 65 and older by roughly 75 per cent when given shortly after infection.

That is consistent with earlier results used to authorise the drug in the Australia, the US and other nations.

But people between the ages of 40 and 65 saw no measurable benefit, according to the analysis of medical records.

The study has limitations due to its design, which compiled data from a large Israeli health system rather than enrolling patients in a randomised study with a control group, the gold-standard for medical research.

The findings reflect the changing nature of the pandemic, in which the vast majority of people already have some protection against the virus due to vaccination or prior infection.

For younger adults, in particular, that greatly reduces their risks of severe COVID-19 complications.

The US Centers for Disease Control and Prevention recently estimated 95 per cent of Americans aged 16 and older have acquired some level of immunity against the virus.

"Paxlovid will remain important for people at the highest risk of severe COVID-19, such as seniors and those with compromised immune systems," Dr David Boulware, a University of Minnesota researcher and physician, said.

"But for the vast majority of Americans who are now eligible, this really doesn't have a lot of benefit."

A spokesman for Pfizer declined to comment on the results, which were published in the New England Journal of Medicine.

The US Food and Drug Administration (FDA) authorised Paxlovid late last year for adults and children 12 and older who are considered high risk due to conditions like obesity, diabetes and heart disease.

It was provisionally approved for use in Australia by the Therapeutic Goods Administration in January for adults 18 and over.

Access to the drugs via the Pharmaceutical Benefits Scheme was expanded in July to include Australians over 70 who test positive to the virus, those over 50 with two or more risk factors for severe disease, and Aboriginal or Torres Strait Islander people over 30 with two or more risk factors.

People aged 18 or over who are immunocompromised may also be eligible.

At the time of the FDA decision there were no options for treating COVID-19 at home, and Paxlovid was considered critical to curbing hospitalisations and deaths during the pandemic's second winter surge.

The FDA made its decision based on a Pfizer study in high-risk patients who had not been vaccinated or treated for prior COVID-19 infection.

"Those people do exist but they're relatively rare because most people now have either gotten vaccinated or they've gotten infected," Dr Boulware said.

Pfizer reported earlier this summer that a separate study of Paxlovid in healthy adults - vaccinated and unvaccinated - failed to show a significant benefit. Those results have not yet been published in a medical journal.

A White House spokesman on Wednesday pointed to several recent papers suggesting Paxlovid helps reduce hospitalisations among people 50 and older.

The studies have not been published in peer-reviewed journals.

"Risk for severe outcomes from COVID is along a gradient, and the growing body of evidence is showing that individuals between the ages of 50 and 64 can also benefit from Paxlovid," Kevin Munoz said in an emailed statement.