Beset with unanswered questions, a federal advisory committee agreed Wednesday that use of Johnson & Johnson’s COVID-19 vaccine should not resume in the United States until they get more information about a possible link between the shot and rare but severe blood clots in women under 50.
After a four-hour emergency meeting, the panel of independent experts who advise the Centers for Disease Control and Prevention said they needed more time to assess the potential risks of the shot, including whether it posed a danger to men and older people.
The CDC and the Food and Drug Administration called for an immediate pause in use of the one-dose vaccine on Tuesday after regulators said six women recently developed unusual clots, including one who died.
Nearly 7.5 million people in the United States have received the vaccine, which uses technology that Johnson & Johnson licensed from Beth Israel Deaconess Medical Center. Given that the rare clots occurred within two weeks of the women getting the shot, the committee wants to see if more cases are reported among the nearly 3.8 million people who have received the vaccine since March 30.
The panel needs to “figure out if this is a needle in a hay stack or the top of an iceberg,” Dr. Sandra Fryhofer, an adjunct associate professor of medicine at Emory University School of Medicine who serves as a liaison representative on the committee.
Several members of the panel, officially called the Advisory Committee on Immunization Practices, agreed on the need for more information and hoped to reconvene in a week to 10 days to decide what limits, if any, should be placed on use of the vaccine.
“I don’t want to send a message that there’s something fundamentally wrong with this vaccine,” said Dr. Beth Bell, a clinical professor at the University of Washington’s School of Public Health who chaired a working group that has been investigating the blood clots.
But she supported the continued pause, she said, because she wants to “be able to defend the decision” if the panel ultimately recommends that vaccinations resume.
The pause was a setback in the effort to help tame the pandemic and is likely to have global repercussions. Although the Johnson & Johnson vaccine was the third authorized for emergency use, on Feb. 27, many public health experts had considered it a potential workhorse for vulnerable communities, including the homeless and homebound.
The vaccine has also been viewed as a better option for developing countries. Unlike the first two vaccines cleared for emergency use in December from Pfizer-BioNTech and Cambridge-based Moderna, the Johnson & Johnson requires one shot instead of two. It also doesn’t have the cold-storage requirements of those vaccines.
So far, the Johnson & Johnson vaccine has accounted for less than 4 percent of the more than 193 million doses of COVID-19 vaccines that have been administered in the United States.
At a briefing at Boston’s Hynes Convention Center, where federal and state officials are jointly running a mass vaccination center, Governor Charlie Baker said he foresees “minimal disruptions” in the Massachusetts vaccine drive from the Johnson & Johnson suspension.
After a one-time surge of about 100,000 doses of Johnson & Johnson last week, the state was set to administer just 11,600 shots this week — a plan that is now on hold — compared with more than 350,000 doses of the Pfizer and Moderna vaccines.
The temporary loss of the Johnson & Johnson vaccine “is a bump,” said Marylou Sudders, the Massachusetts secretary of health and human services. “We’re looking forward to getting guidance from the FDA and the CDC. And we’ll pivot accordingly.”
The six women who developed blood clots were between the ages of 18 and 48. A Virginia woman died and a second woman in Nebraska has been hospitalized in critical condition. All developed symptoms that included headaches, abdominal pain, nausea, and neurological problems.
All were diagnosed with blood clots in vessels that drain blood from the brain, a rare condition called cerebral venous sinus thrombosis, or CVST, in combination with low levels blood platelets. Two also had clots in vessels that drain from the intestines.
Dr. Tom Shimabukuro, deputy director of the CDC’s Immunization Safety Office, told the committee that the combination of symptoms was highly unusual. Although CVST was identified in only six people who received the vaccine, that appeared to be at least three times the expected rate of occurrence.
Similar blood clot cases have occurred in Europe in a small number of recipients of AstraZeneca’s COVID-19 vaccine, which is not authorized in the United States and relies on similar technology. That prompted some European countries to restrict use of the vaccine to certain age groups.
As serious as the cases of blood clots were, medical experts pointed out on Tuesday that they were far rarer than clots experienced by people who take other prescribed medicines or who get sick from COVID-19 itself.
Dr. Helen W. Boucher, chief of the Division of Geographic Medicine and Infectious Diseases at Tufts Medical Center, noted that 500 to 1,200 blood clots occur per 1 million people who take birth control pills, 1,700 clots per 1 million smokers, and 165,000 clots per 1 million people with COVID-19.
Some medical experts said in interviews Wednesday that regulators had little choice but to pause the rollout of the Johnson & Johnson vaccine.
Dr. Paul Offit, a member of a separate committee that advised the FDA to clear all three vaccines for emergency use, told the Globe that regulators need to make sure the vaccine is safe. But he feared that the interruption will make people leery about getting all three vaccines.
“On a more rational planet, I think that this would strengthen trust in a system that is constantly looking out for even rare serious adverse events,” said Offit, who directs the Vaccine Education Center at Children’s Hospital of Philadelphia and has often drawn the ire of anti-vaccine activists. “But we don’t live on that planet.”
Another member of the FDA advisory committee, Dr. Eric Rubin, an immunologist at the Harvard T.H. Chan School of Public Health, agreed that the pause might increase vaccine hesitancy even if regulators conclude the vaccine is safe.
“I hope that people see that these issues are being approached thoughtfully and that safety is being taken seriously,” said Rubin, the editor-in-chief of the New England Journal of Medicine.
Johnson & Johnson, the New Brunswick, N.J., health care giant, said in a statement Tuesday that it is “working closely with medical experts and health authorities” to investigate the blood clots and decided to delay rollout of the vaccine in Europe.
Federal officials downplayed the impact of the Johnson & Johnson suspension on their broader vaccination program, stressing there will be more than enough doses from Pfizer-BioNTech and Moderna to vaccinate every American adult in the coming months.
But they acknowledged the daily rate of vaccinations, which averaged 3.3 million shots during the past seven days, could dip as providers scramble to find new appointments for people who’d scheduled Johnson & Johnson injections.
“In the short term, we expect some impact on the daily averages as we transition,” Jeffrey Zients, the White House virus response coordinator, said at a press briefing. But overall, he said, “There is plenty of supply in the system of Moderna and Pfizer, including to replace J&J appointments with Pfizer and Moderna vaccine.”
Robert Weisman of the Globe staff contributed to this report.
Jonathan Saltzman can be reached at firstname.lastname@example.org.