By Jamie Gumbrecht and Elizabeth Cohen, CNN

(CNN) — The US Centers for Disease Control and Prevention’s vaccine advisers will meet on Thursday to revisit the benefits and risks for the Johnson & Johnson Covid-19 vaccine.

The Advisory Committee on Immunization Practices is scheduled to vote on “updated recommendations for use,” according to an agenda for the meeting posted online Wednesday.

The single-dose Johnson & Johnson vaccine is authorized for use in people age 18 and older, and can be used as a booster shot for adults fully vaccinated with the J&J, Pfizer or Moderna vaccines. People who received the J&J vaccine are advised to get a booster dose of any of the authorized vaccines two months after their first shot.

According to the agenda, the CDC’s vaccine advisers will hear presentations about thrombosis and thrombocytopenia syndrome, known as TTS, a rare, serious type of blood clotting condition that has been linked to the J&J vaccine, but not to the Pfizer/BioNTech or Moderna vaccines.

During a White House Covid-19 briefing on Wednesday, CDC Director Dr. Rochelle Walensky did not say why the committee was meeting about Johnson & Johnson’s Janssen vaccine, or whether she believes the benefits of the vaccine still outweigh the risks.

“ACIP meets intermittently to review the safety data of all of their vaccines and I will look forward to their discussions tomorrow to review of those data and to any recommendations that come thereafter,” Walensky said in response to questions from CNN’s Jeremy Diamond.

A source close to the situation told CNN the CDC has been “coy” about what the vote will be about, but it may involve limiting who’s recommended to get the vaccine.

CDC’s vaccine advisers are scheduled to meet from noon to 4 p.m. ET Thursday, and a vote is set for 2:30 p.m.

The source said it’s possible the CDC could recommend against the use of Johnson & Johnson’s vaccine in women under age 50. TTS is very rare. Women 30 to 49 have seen the highest rates of this type of blood clot but it has sometimes occurred in older women and in men.

“I think a fair amount of the discussion will be about J&J — do we need to use this one at all since we have so much of the others, or, if it’s used, should it be focused on certain populations?” the person said.

The source said because of TTS, and because the vaccine is not as effective against Covid-19 as Pfizer or Moderna vaccines, “J&J hasn’t lived up to its billing as ‘one and done.’”

“We could just take J&J off the table and do fine in this country,” the source added. About 16 million people have been fully vaccinated with the J&J vaccine in the United States, compared with about 73 million with Moderna and 113 million with Pfizer.

J&J vaccine previously linked to rare cases of blood clots

The US Food and Drug Administration and CDC have said for months that the J&J vaccine’s known and potential benefits outweigh its known and potential risks. However, the CDC says that women younger than 50 should be made aware of the increased risk of TTS, and that there are other vaccine options available.

Thrombosis and thrombocytopenia syndrome has been linked to both the Johnson & Johnson vaccine and AstraZeneca vaccine, which is not available in the United States, but is widely used around the world and uses similar technology.

The agencies say people who receive the J&J vaccine — and particularly women younger than 50 — should watch for symptoms for several weeks after vaccination, including severe or persistent headaches or blurred vision; shortness of breath; chest pain; leg swelling; persistent abdominal pain; or easy bruising or tiny blood spots under the skin beyond the injection site. If any of those symptoms develop, people should seek medical care immediately.

On Tuesday, the FDA said people with a history of TTS should not get the Johnson & Johnson Covid-19 vaccine.

“You should not get the Janssen COVID-19 Vaccine if you had a severe allergic reaction after a previous dose of this vaccine; had a severe allergic reaction to any ingredient of this vaccine; had a blood clot along with a low level of platelets (blood cells that help your body stop bleeding) following Janssen COVID-19 Vaccine or following AstraZeneca’s COVID-19 vaccine (not authorized or approved in the United States),” reads the fact sheet for vaccine recipients and caregivers.

In a statement in response the FDA’s update, Johnson & Johnson said “the safety and well-being of the people who use our products is our number one priority” and that it “strongly support raising awareness of the signs and symptoms of this rare event.”

Early reports of TTS, which involves blood clots with low platelets, drove a short pause in the use of the Johnson & Johnson vaccine earlier in the year, but use of the vaccine quickly resumed. Safety monitoring has been ongoing since then.

The CDC said that as of November 24, more than 16.4 million doses of the J&J vaccine had been given in the United States, and the CDC and FDA have 54 confirmed reports of people who later developed TTS. Symptoms began one to two weeks following vaccination, the FDA says.

The FDA said Tuesday that cases of TTS after the Johnson & Johnson vaccine “have been reported in males and females 18 years of age and older, with the highest reporting rate of approximately 1 (one) case per 100,000 doses administered in females 30-49 years of age; overall, approximately 1 out of 7 cases has been fatal,” the agency added.

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