WASHINGTON — A new study by Pfizer and BioNTech suggests that their updated coronavirus booster released in September is nearly four times as good as its predecessor at boosting antibodies against the currently dominant version of the virus for people over age 55.

Federal officials are hoping that the encouraging results will bolster what has so far been a dismal public response to the retooled shots. Only about 8 percent of Americans ages 5 and up have received the new boosters from Pfizer and Moderna, which are recommended for people in that age group who have had an initial round of vaccination.

Pfizer and BioNTech announced the study results in a news release. The companies said that one month after getting the new booster, clinical trial participants over 55 had antibody levels that were 3.8 times as high as those who received the original booster. The number of participants in the study was small, with 36 people receiving the new booster and 40 receiving the old one.

The control group included only older adults, and the findings so far are limited to one month after the shot. Results from a similar clinical trial by Moderna are expected soon.

Biden administration officials cast the results as good news in the battle against Covid-19, but whether many Americans will care much remains to be seen. As the pandemic nears the three-year mark, the public seems deeply weary of Covid vaccinations. In a Kaiser Family Foundation poll conducted in September, one in five adults said they had not even heard about the new boosters.

“Folks have just tuned out,” said Michael Fraser, the chief executive of the Association of State and Territorial Health Officials. “Which is totally unfortunate and not the place where you want to be.”

Administration officials said the results of the study offered a new reason for Americans to seek out the updated shots before what experts fear could be a winter surge of the virus that leads to tens of thousands of unnecessary deaths.

“You can see the train coming down the tracks,” said Dr. Peter Marks, the top vaccine regulator for the Food and Drug Administration. “The time to get off the tracks is now, not when you can smell the oil from the train engine.”

Pfizer said study participants in the control group had been boosted about six months earlier, while those who received the updated shot had had their last booster 11 months earlier. Both groups had similar antibody levels before receiving the new booster.

The updated shot targets the version of the virus that now looms largest in the United States, a subvariant of Omicron known as BA.5. But by next month, a new Omicron subvariant is expected to become dominant.

Administration officials say those subvariants are structurally alike enough that the updated shots should continue to provide potent protection. Dr. Marks said that he expected some decrease in the boosters’ ability to neutralize the virus but their overall benefits to hold.

While urging Americans to get the updated shots, Biden administration officials are not casting them as a final answer to the pandemic. For two years, the administration has been engaged in a continual game of catch-up. By the time scientists have revised the vaccine to protect against the virus’s newest incarnation, it has already shifted to another form.

That reality, some experts said, could be part of the reason Americans are showing diminished enthusiasm for each added shot. So far, the administration has not persuaded Congress to allot more money to develop the next generation of vaccines that could end the behind-the-curve cycle.

Asked whether positive clinical trial results would help pique interest in the boosters, Dr. Philip Huang, the health director for Dallas County in Texas, said, “Anything that provides some more reinforcement for people who are on the fence or delaying always helps.” But, he said, “if there is a bump-up in hospitalizations and serious illness, that’s when people start to take it seriously.”

“It’s a shame that’s what it takes to get motivation up,” he added.

Some public health experts say President Biden himself undermined any sense of urgency when he declared in a television interview in September that “the pandemic is over.” Last week, he tried to draw attention to the new shots by getting his own dose on camera.