WASHINGTON (NYTIMES) - Biden administration health officials increasingly think that vulnerable populations will need booster shots even as research continues into how long the coronavirus vaccines remain effective.

Senior officials now say they expect that people who are 65 and older or who have compromised immune systems will most likely need a third shot from Pfizer-BioNTech or Moderna, two vaccines based on the same technology that have been used to inoculate the vast majority of Americans thus far.

That is a sharp shift from just a few weeks ago, when the administration said it thought there was not enough evidence to back boosters yet.

On Thursday (July 22), a key official at the Centres for Disease Control and Prevention said the agency is exploring options to give patients with compromised immune systems third doses even before regulators broaden the emergency-use authorisation for coronavirus vaccines, a step that could come soon for the Pfizer vaccine.

Dr Amanda Cohn, chief medical officer of the CDC's immunisations division, told an advisory committee to the agency that officials were "actively looking into ways" to provide certain people access to booster shots "earlier than any potential change in regulatory decisions".

"So stay tuned," she added.

The growing consensus within the administration that at least some Americans will need a booster is tied in part to research suggesting that the Pfizer vaccine is less effective against the coronavirus after about six months. More than half of those fully vaccinated in the United States so far have received Pfizer's vaccine, in two doses administered three weeks apart.

Pfizer's continuing global study of its clinical-trial participants shows that four to six months after the second dose, the vaccine's effectiveness against symptomatic infection drops from a high of 95 per cent to 84 per cent, according to the company.

Data from the Israeli government, which has fully vaccinated more than half of its population with Pfizer doses since January, also points to a downward trend in effectiveness over time, although administration officials are viewing that data cautiously because of wide margins for error.

The most-recent figures from the Israeli Ministry of Health, released late this week, suggested that Pfizer's vaccine was just 39 per cent effective in preventing infection in that country in late June and early July, compared with 95 per cent from January to April.

The vaccine remained more than 90 per cent effective in preventing severe disease and nearly as effective in preventing hospitalisation. Israel began offering a third Pfizer dose to citizens with severely weakened immune systems on July 12.

In the US, Dr Anthony Fauci, who heads the infectious-disease division of the National Institutes of Health, said he was surprised by the apparent steep falloff in the Pfizer vaccine's effectiveness that the Israeli data seems to suggest. He said he wanted to compare it with data that the CDC had been gathering from cohorts of thousands of people across the United States.

"People are sort of raising their eyebrows a bit," he said.

While other questions abound, senior administration officials said it appeared increasingly clear that the vaccines would not grant indefinite immunity against the virus and that boosters might be necessary for at least some people perhaps nine months after their first shot.

The administration has already purchased more than enough vaccine to deliver third doses of both Pfizer and Moderna, and has been quietly preparing to expand the distribution effort, should it become necessary.

With so little data yet public, many health officials and experts have spoken cautiously about booster shots. Dr Paul Offit, a member of the Food and Drug Administration's outside advisory committee of vaccine experts, said a rise in mild or moderate cases of Covid-19 among vaccinated people did not necessarily mean a booster was required.

"The goal of this vaccine is not to prevent mild or low, moderate infectious disease," he said. "The goal is to prevent hospitalisation to death. Right now, this vaccine has held up to that."

Prematurely dangling the prospect of a third dose could also work as a deterrent against vaccination, other health experts warn. If Americans think that immunity from the vaccines is short-lived, they said, they may be less likely to get their initial shot.

"We don't want people to believe that when you're talking about boosters, that means that the vaccines are not effective," Dr Fauci testified at a congressional hearing Tuesday. "They are highly effective."

Among the vaccine manufacturers, Pfizer has been especially proactive in sharing its data with the government. But the administration was taken aback by the company's public announcement this month that it planned to seek emergency authorisation from the FDA for a booster shot.

The company said early data from its booster study showed the level of neutralising antibodies among clinical trial participants who received a third dose six months after the second was 5 to 10 times as high as among two-dose recipients.

At the CDC advisory panel's meeting on Thursday, Dr Cohn, the medical officer for the vaccine division, suggested it might be possible to offer booster shots to those with weakened immune systems through an investigational study or other avenues, without waiting for the FDA.

Pfizer is expected to soon publicise its clinical-trial research about waning immunity and the benefits of a booster shoot in articles in a peer-reviewed journal.

Moderna has yet to release data on any booster studies, officials said.

Johnson & Johnson's one-shot vaccine has so far played a minor role in the nation's vaccination campaign. Clinical-trial data on how that vaccine works with two shots is expected next month.