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, a key official at the Centers 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 authorization for coronavirus vaccines, a step that could come soon for the Pfizer vaccine.
Dr. Amanda Cohn, chief medical officer of the CDC's immunizations 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 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 to 84%, 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 point 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% effective in preventing infection in that country in late June and early July, compared with 95% from January to April.
The vaccine remained more than 90% effective in preventing severe disease and nearly as effective in preventing hospitalization. Israel began offering a third Pfizer dose to citizens with severely weakened immune systems on July 12.
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 needed for at least some people perhaps nine months after their first shot. The administration has already purchased more than enough vaccine for third doses of both Pfizer and Moderna, and has been quietly preparing to expand the distribution effort, should it become necessary.
Pfizer and BioNTech will supply the U.S. with another 200 million doses of their COVID-19 shot, setting up a stream of vaccine deliveries through next April in a push to protect kids and potentially provide boosters, Bloomberg News reported.
Of the new doses, 65 million will be tailored for the younger pediatric population, should the vaccine be cleared for kids younger than 12, according to the official, who spoke under the condition of anonymity as the contract is not public.
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 hospitalization 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.