Jennifer Boral badly wants a Covid-19 vaccine booster shot.

An asthmatic with a history of getting colds that develop into serious lung infections, the 41-year-old unemployed real-estate agent in Briarcliff Manor, N.Y., is terrified that getting Covid-19 might land her in the hospital or worse.

Though her doctor believes she is high risk...

Jennifer Boral badly wants a Covid-19 vaccine booster shot.

An asthmatic with a history of getting colds that develop into serious lung infections, the 41-year-old unemployed real-estate agent in Briarcliff Manor, N.Y., is terrified that getting Covid-19 might land her in the hospital or worse.

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Though her doctor believes she is high risk and wrote a note to help her get vaccinated early in the Covid-19 vaccine rollout in February, Ms. Boral isn’t technically eligible for a booster yet. The only patients cleared for boosters for now are those with seriously compromised immune systems such as people who have recently received an organ transplant, blood-cancer patients and those taking immunosuppressant drugs.

“I would go out and get one immediately, if I could,” Ms. Boral said. “I don’t understand why our country is dragging its feet.”

The Biden administration’s plan to make boosters available to more people in the U.S. is in flux. A Food and Drug Administration advisory panel on Friday endorsed booster shots of the Covid-19 vaccine from Pfizer Inc. and BioNTech SE for adults 65 and older and those at high risk of severe disease. Some officials and researchers support additional vaccines for most people to bolster immunity against the virus. Others say boosters may only be necessary for now for certain groups like older and immunocompromised people.

The uncertainty is frustrating patients and leaving physicians and pharmacists to reach different conclusions about whether people in their care qualify for Covid-19 booster shots.

An FDA advisory panel has endorsed booster shots from Pfizer and BioNTech for adults 65 and older and those at high risk of severe disease. A cancer patient received his third vaccine dose on Friday.

Some doctors say current guidelines exclude high-risk patients who might benefit from a booster, such as cancer survivors who recently completed a chemotherapy treatment or patients with a history of respiratory problems, like Ms. Boral.

“There’s a lot of gray area around how to define someone as moderately or severely immunocompromised,” said David Cohn, an oncologist and chief medical officer at the James Cancer Hospital in Columbus, Ohio. “You kind of have to set a bar somewhere.”

Some hospitals, including James Cancer, have begun contacting cancer patients and other people with compromised immune systems to schedule booster shot appointments.

“Our goal is to make it as easy as possible” to get a booster, Dr. Cohn said.

Other doctors and some public-health officials have said patients shouldn’t seek booster shots until authorities can administer as many vaccines as possible to patients who have been hesitant to get vaccinated and to residents of developing nations where vaccine supply is limited.

The Biden administration announced that Americans who have been fully vaccinated with a two-dose regimen against Covid-19 should receive a booster, citing the threat from the highly contagious Delta variant. WSJ breaks down what you need to know. Photo: Hannah Beier/Reuters The Wall Street Journal Interactive Edition

World Health Organization Director-General Tedros Adhanom Ghebreyesus said this month that the body lacks sufficient data to recommend booster shots for healthy patients. He asked wealthy nations on Wednesday to hold off on administering boosters until the end of the year.

“Starting with boosters, especially giving it to healthy populations, is really not right,” Dr. Tedros said.

Booster shots have been available to immunocompromised people in the U.S. since mid-August, when the FDA authorized a third shot for patients over the age of 18 who had received a second dose at least 28 days before. Studies suggest that the effectiveness of vaccines has held up well against the highly-contagious Delta variant of the SARS-CoV-2 virus, which became the dominant strain in the U.S. over the summer, but that vaccine efficacy wanes as time passes after the shot is given.

Some doctors have said boosters should only be given when they are most effective, after immunity has waned, and in the most efficient dosages possible.

Pharmacy chains like CVS Health Corp. and Walgreens Boots Alliance Inc. said they are offering booster shots to people who sign a form stating that they qualify for the shot because they are immunocompromised, based on a list of criteria from the Centers for Disease Control and Prevention.

The CDC’s list of qualifying patients also includes people with advanced or untreated HIV infection, patients in active cancer treatment and recent stem-cell transplant recipients. Both chains said they aren’t requiring documentation of immunocompromised status from patients.

Some doctors have said boosters should only be given when they are most effective, after immunity has waned, and in the most efficient dosages possible, both points that haven’t been well-defined by data yet. Most people should hold off on getting boosters until the data is more conclusive, said Megan Ranney, a professor of emergency medicine at the Alpert School of Medicine at Brown University.

“Most likely we are going to all need boosters at some point, because that’s just how vaccines work. Immunity wanes. Viral mutations happen. But the question to me is, at what point?” Dr. Ranney said.

Lucy McBride, an internist in Washington, D.C., said she is generally following CDC and FDA guidelines on booster shots, but that some patients’ situations don’t easily fit the government’s recommendations. She cited examples of people who live with severely immunocompromised family members, or older people who have accepted work assignments in areas of high community transmission. She said she would advise seeking a booster to people in both situations.

“Because the messaging has been so confusing and because the data is still evolving, we’re basically asking patients and pharmacies to make their own decisions,” said Dr. McBride. Ultimately, she said, the decision falls to her and her peers.

“At the end of the day, it comes down to the person in front of me, and because the CDC can’t possibly speak to every American, they have to draw these somewhat arbitrary lines in the sand,” she said.

There are some physicians who say current guidelines exclude high-risk patients who might benefit from a booster.

Some patients, especially older Americans, are getting impatient. Richard Graus, a 78-year-old retired geologist from Front Royal, Va., said he has been wearing a mask indoors and monitoring the news to find out when he can get a third shot. The recommendation of the FDA panel on Friday means he is likely to have access to another shot soon.

“I’m of an age where the immunity that I have is probably not as good as it was when I was younger,” Mr. Graus said. “I’m not desperate for it at this point, and I can wait another month if that’s what it takes, but I don’t want to have to be overly cautious for the rest of my life.”

Write to Robbie Whelan at robbie.whelan@wsj.com