Here’s what to know about Johnson & Johnson COVID vaccines being paused

Johnson & Johnson Janssen Covid-19 vaccines at a vaccination center at the Miami Dade College North Campus in North Miami, Fla.Eva Marie Uzcategui/Bloomberg/file

Federal health officials are calling for a pause in the administration of the Johnson & Johnson coronavirus vaccine, while at the same time asking people who have recently gotten the vaccine, and health care providers, to be on the lookout for a possible rare and severe side effect of the shots.

Here’s what you need to know:

What’s causing the concern?

Federal officials are reviewing data involving six reported cases of a “rare and severe type of blood clot” in individuals who received the vaccine, according to the US Centers for Disease Control and Prevention and the US Food and Drug Administration.

The six cases involved women between the ages of 18 and 48, and symptoms occurred 6 to 13 days after vaccination, CDC and FDA officials said. One woman died and one is in critical condition.

How many people have gotten the Johnson & Johnson vaccine?

The vaccine has been administered so far to 6.8 million people nationwide. Officials have noted that the vaccines only accounts for a small fraction of the 121 million people who have received at least a first dose of vaccine nationwide, and no major problems have been observed with the two other vaccines, from Moderna and Pfizer.

More than 192,000 people in Massachusetts had been given Johnson & Johnson shots as of Tuesday, according to the state Department of Public Health. That includes nearly 120,000 people who, the state reported, got the vaccine in the last three weeks.

What exactly should people who have gotten a Johnson & Johnson shot look out for?

If you received the Johnson & Johnson vaccination within the past three weeks, and you develop severe headache, abdominal pain, leg pain, or shortness of breath, CDC and FDA officials are urging you to contact your health care provider.

CDC principal deputy director Dr. Anne Schuchat said in a Tuesday morning news conference that the symptoms would be different from the mild, flu-like symptoms many people experience a few days after getting vaccinated.

Asked for his advice to people who have gotten a Johnson & Johnson shot recently, Dr. Anthony Fauci, President Biden’s lead adviser on the pandemic, noted the tiny number of cases reported relative to the number of shots and suggested people should remain calm but vigilant.

“Don’t get an anxiety reaction because, remember, it’s less than one in a million,” he said. “However, having said that, pay attention.”

A Boston expert said the symptoms would not be subtle.

”The blood clots of concern have predominantly involved the abdominal organs or the brain,” said Dr. Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital. Thus abdominal pain or headache would be a sign. But the symptoms would be sudden and severe.

If you have a severe headache or stomach ache, especially if you don’t normally experience such symptoms, call your doctor, Sax advised.

”The side effect is extremely rare but also very serious,” Sax said. “Overall, I think it’s reassuring that without safety surveillance strategies, we’re able to pick up such a rare side effect.”

Dr. David Hamer, a professor at Boston University School of Public Health and School of Medicine and attending physician at Boston Medical Center, said people who got shots more than two weeks ago have “no need to worry.”

“If they’re two weeks out from the vaccine, they’re very unlikely to have this,” he said in a telephone interview.

Hamer noted that the blood clotting disorder is “a very rare event as of right now.”

“All of these events have happened 6 to 13 days after vaccination to women under the age of 48,” he said. “It may be that not everybody is at risk.”

What is the message to doctors?

The normal treatment for the blood clots might be dangerous for people who have this side effect, officials are telling doctors.

In the six cases found so far, the type of blood clot known as cerebral venous sinus thrombosis was seen in combination with low levels of blood platelets, said Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research.

”Treatment of this specific type of blood clot is different from the treatment that might typically be administered,” Marks said in a joint statement with the CDC’s Schuchat. “Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given.”

That means it’s important for doctors to ask patients who come in with those symptoms if they have been vaccinated recently, officials said.

Officials are also asking health care providers to report any cases to the Vaccine Adverse Event Reporting System at https://vaers.hhs.gov/reportevent.html. Reports about more cases could help scientists learn more about what’s causing the reaction, officials said.

What are the next steps?

The cases will be further reviewed by the CDC and FDA. The CDC’s Advisory Committee on Immunization Practices is expected to meet Wednesday to review the data on the initial cases.

“I think this is an unusual occurrence of a serious adverse event that you want to make sure before you go forward, you investigate it thoroughly and that’s exactly what they’re doing,” said Fauci.

Hamer noted that the same blood clotting disorder has also been seen in some recipients of the AstraZeneca Covid-19 vaccine in Europe.

As the CDC gathers more data, Hamer said, “there could be additional reports.”

If there are risk factors in terms of age, sex, or existing medical conditions, then it’s possible the vaccine could be restricted to certain groups, Hamer said.

Fauci said it was “premature” to say whether the vaccine could be removed completely from the market, but he suggested that it’s possible the vaccine could return after the pause with with restrictions.

Is this a big setback for the race to vaccinate the American population and end the pandemic?

The Biden administration says no.

White House COVID-19 response coordinator Jeff Zients said at the White House news conference that the United States will still have enough supply of the two other vaccines to meet the administration’s goal of getting 200 million shots administered in 100 days and to make vaccine available to everybody who wants a shot by the end of May.

“We have enough supply from Moderna and Pfizer to hit the targets that we’ve set, the 200 million shots in 100 days, and to head toward the Fourth of July that we’ve talked about as a country, a more normal Fourth of July,” he said.

“The bottom line is the vaccines from Moderna and Pfizer are clearly safe and saving lives and every American should get vaccinated when it’s their turn,” he said.

Ryan Huddle of the Globe staff contributed to this report.