"We will find a time to reconvene," ACIP executive secretary Dr. Amanda Cohn told the meeting. "We will try to identify what that date is by Friday of this week so that people have a little bit more time to get it on the calendars."

Federal regulators are keen to get more information about the blood clots, called cerebral venous sinus thromboses (CVST), that are found in combination with a shortage of blood-clotting cells called platelets. The combination makes the condition extra dangerous, because using the blood thinners typically employed to treat blood clots can cause hemorrhaging in these patients.

At least four of the six cases were treated with the blood thinner heparin when they first developed symptoms, Dr. Aran Maree, chief medical officer for pharmaceuticals at J&J's Janssen vaccine arm, told the meeting.

He gave details of what's known about the cases, which include: a 45-year-old woman who died; a 38-year-old woman who has not recovered; an 18-year-old woman who has not recovered; a 48-year-old woman who has not recovered; a 26-year-old woman who has recovered and a 28-year-old woman whose status is unknown.

They also include a possible seventh case -- a 59-year-old woman who has not yet recovered and whose blood clots might not fit the same profile as the others.

Maree said the company still feels the vaccine is safe enough to use.

"I'd like to reiterate that based on the current data, Janssen believes the overall benefit-risk profile for our vaccine is positive across the population for which it's authorized," Maree told the meeting.

"We strongly support ensuring awareness of the signs and symptoms of this very rare event, as well as recommendations to ensure the correct diagnosis, treatment and reporting by health care professionals."

ACIP staff said the committee could consider recommending changes in who gets the vaccine -- for instance, only men, as most cases of the clots are among women, or only people over 50, as most of the cases were in people under 50.

But members said they may not know enough yet to make such recommendations.

"I do not want to vote on this issue today. I do not want to vote not to recommend the vaccine -- I think that is not really something I necessarily believe," Dr. Beth Bell, a clinical professor at the University of Washington in Seattle, told the meeting.

"I just don't feel there's enough information to make an evidence-based decision. We won't have all the information, but I think there are some things that we can gather relatively quickly, which all have to do with the benefit/risk balance," Bell said.

"We do need to better understand the risk, which we know is going to be very rare, very low, but we really don't know exactly how low and how to correctly characterize it," Bell added.

"I think we need to not vote and gather the necessary information so we can make an evidence-based decision."

Bell, who chairs the ACIP's working group, said earlier it was also unclear if the blood clots seen in people who got the Janssen vaccine were the same as those seen in people in the UK and Europe who got the AstraZeneca vaccine, which is not yet authorized in the US.

Both vaccines use adenoviruses, a type of common cold virus, to carry genetic material from the coronavirus into the body to elicit immunity. One theory is that these adenoviruses somehow prompt an immune response that causes the low platelet count and the blood clots.

"The extent to which the cases seen after each of these adenoviral vector vaccines represent exactly the same syndrome is not entirely clear at this time," Bell told the meeting.

The group has a scheduled meeting in three weeks -- on May 5 -- or could schedule another emergency meeting.

But Dr. Nirav Shah, director of Maine's Center for Disease Control and Prevention and representative for the Association of State and Territorial Health Officials, said waiting was equally bad.

"We are in a situation where not making a decision is tantamount to making a decision," Shah said, noting that people who could benefit from getting a one-dose vaccine would go unvaccinated as a result of further delay.

Dr. Camille Kotton of Massachusetts General Hospital and Harvard Medical School, said the country needs the one-shot vaccine.

"Putting this vaccine on pause for those of us that are frontline health care workers has really been devastating," Kotton told the meeting.

"I agree in general that we don't have data to make a decision at this time. But we were planning on using this vaccine in the state of Massachusetts for people who are homebound and otherwise not able to get a vaccine.

"We were planning on using it for our vulnerable in-patient population often with many comorbidities and at high risk for disease but who haven't been able to get vaccinated otherwise. And then it certainly was going to be used in what may be otherwise underserved populations or populations that aren't able to get mRNA vaccines," Kotton added.

The mRNA vaccines made by Moderna and Pfizer/BioNTech require special refrigeration and are given in two-dose regimens.

"So I definitely want us to be cautious and very careful with our decision making, but I also just want to emphasize that this one-and-done vaccine that didn't require the cold chain that the mRNA vaccines do, it's a significant loss," Kotton said.

Dr. Paul Offit, a member of the US Food and Drug Administration's vaccine advisory committee, called it "an unfortunate non-decision."

"One thing they could have said was, 'Just explain to people that there is this very rare, but very real side effect, remembering that of every million people that get Covid, 1,850 will die,'" said Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia. "There are no risk-free choices -- just choices to take different risks."

Earlier, CDC Director Dr. Rochelle Walensky said it was not clear her agency had heard about every blood clot that might be linked to the vaccine. "Right now, we believe these events to be extremely rare, but we are also not yet certain we have heard about all possible cases, as this syndrome may not be easily recognized as one associated with the vaccine," she told a media briefing.

She said while people need to be aware of the potential symptoms, the vaccine surveillance system is working. "Jointly, CDC and FDA were able to identify these rare events and act quickly to alert healthcare providers and the public. This demonstrates that the safety systems we have in place are working," she said.

Symptoms people should be on the lookout for after getting the Janssen vaccine include severe headache, abdominal or leg pain or shortnessof breath, Walensky said.