A Food and Drug Administration committee on Friday recommended coronavirus booster shots to recipients of the Pfizer-BioNTech vaccine who are 65 and older or are particularly vulnerable to COVID-19.

The committee declined to recommend the shots for those younger than 65, citing a lack of evidence that they are currently necessary, despite President Joe Biden’s hopes that all Americans who got the Pfizer-BioNTech vaccine would be able to get booster shots eight months after their second dose.

Biden had originally promised booster shots for all Americans who received a Pfizer-BioNTech or Moderna vaccine, then tightened that group to only those who had received a Pfizer-BioNTech vaccine, before the FDA on Friday narrowed eligibility even further.

In Connecticut, hospital officials say they are ready to distribute booster shots once they receive authorization from the FDA and Centers for Disease Control and Prevention, which reportedly could come in the next week.

Here is what Friday’s decision means.

Who will be eligible for booster shots?

If the FDA and the CDC accept the committee’s recommendations, which reportedly could happen in the next week, booster shots will be available for anyone 65 and older or who is at risk of severe COVID-19 symptoms — but only those who originally received the Pfizer-BioNTech vaccine.

Those eligible for booster shots will be able to receive them six months after their second shot.

People who received the Moderna or Johnson & Johnson vaccines are not yet cleared for booster shots. Moderna has submitted an application for a booster dose to the FDA and hopes to gain authorization soon. Johnson & Johnson has not applied to distribute booster doses.

When will they be able to get them?

Though the timeline is not yet fully clear, Gov. Ned Lamont said Thursday nursing home residents in Connecticut could expect to receive booster shots within the next week to 10 days.

“Hurry up,” Lamont urged federal regulators. “These folks were vaccinated in many cases eight months ago, and they are the most vulnerable — and they are in a congregate setting. I’ve got to follow the rules, but I would hurry up.”

A small number of Americans — those who are moderately or severely immunocompromised — are already eligible to receive booster shots. In Connecticut that means about 100,000 people, more than 20,000 of whom have already taken advantage.

Why are booster shots important?

Since the COVID-19 vaccines were first introduced, experts warned that the immunity they confer may wane over time. Now, some research suggests that has begun to happen, at least to some extent.

“The studies have shown that immunity is waning,” said Dr. Ulysses Wu, an infectious disease specialist at Hartford HealthCare. “But it’s not waning to the point where everybody thinks we’re falling off a cliff. ... What was probably 90% immunity when you got the vaccine is probably about 80%, which is still pretty good.”

Booster shots, Wu said, will provide an extra level of protection, particularly to those who are most vulnerable.

The booster shot debate has become particularly pressing since the emergence of the delta variant, which has caused COVID-19 spikes nationwide, including in Connecticut. Some experts expect cases to further increase this fall, as weather in the Northeast cools and most activity moves indoors.

“As we move into the fall, we see SARS-CoV-2 still circulating,” said Dr. David Banach, an epidemiologist at UConn Health. “If boosters can prevent hospitalizations and severe illness and also potentially slow the spread of the virus, there’s a lot of importance there.”

Why weren’t booster shots authorized for everyone?

The true necessity of booster shots at this time has been a subject of debate among scientists. While some studies have shown that immunity against COVID-19 wanes over time, others have been less decisive.

On Wednesday, FDA scientists reported that evidence is mixed and that the overall data suggests vaccines “still afford protection against severe COVID-19 disease and death in the United States.” And on Friday, the FDA committee opted against recommending booster shots for all Pfizer-BioNTech recipients due to lack of evidence that they are necessary.

“It’s unclear that everyone needs to be boosted, other than a subset of the population that clearly would be at high risk for serious disease,” said Dr. Michael G. Kurilla, a committee member and official at the National Institutes of Health, according to the New York Times.

There’s also an ethical concern: At a time when dozens of countries, particularly in the Global South, have vaccinated less than 10% of their populations, is it fair for Americans to seek third doses? The head of the World Health Organization last week urged nations to hold off on booster shots until at least the end of this year, “to prioritize vaccinating the most at risk people around the world who are yet to receive their first dose.”

Distributing vaccines more widely worldwide isn’t just a humanitarian project. The longer COVID-19 is able to flourish in parts of the world, experts say, the more likely it is to mutate in ways that threaten the entire globe.

“If the rest of the world isn’t vaccinated, we’re going to come in contact with them,” Wu said. “That’s the issue.”

According to the New York Times, the Biden administration is negotiating a deal to buy an additional 500 million COVID-19 vaccine doses and distribute them overseas.

What will happen next?

Connecticut hospital officials say they have the vaccine supply and the necessary infrastructure to distribute more booster shots. All they’re waiting for is the go-ahead.

The FDA, which is not required to accept recommendations from advisory committees but typically does, will likely decide by next week, per the Times. The CDC will then weigh in on how the vaccine doses should be used.

Though it does not seem likely that booster shots will be widely available by Sept. 20, as Biden initially promised, Pfizer-BioNTech recipients age 65 and older or who are at serious risk of COVID-19 may be able to get them not long after that.

Alex Putterman can be reached at aputterman@courant.com.