As research continues into how long coronavirus vaccines remain effective, Biden administration health officials increasingly think that vulnerable populations will need booster shots.
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.
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.
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 percent to 84 percent, 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 against infection over time, though not against severe disease. Administration officials are viewing that data cautiously because of wide margins for error.
Dr. Anthony S. Fauci, who heads the infectious disease division of the National Institutes of Health, said the apparent steep falloff in the Pfizer vaccine’s effectiveness against infection in the Israeli data had epidemiologists “raising their eyebrows a bit.”
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.
Dr. Paul A. 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, making people less likely to get their initial shots, other health experts said.
“We don’t want people to believe that when…
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