The Food and Drug Administration and the Centers for Disease Control and Prevention have put the Johnson & Johnson COVID-19 vaccine on “pause” because of reports of a rare, life-threatening blood clotting syndrome.

A vaccine advisory panel met Wednesday but asked to reconvene in a few weeks. The advisors said they need more data before they recommend continuing, discontinuing, or restricting the vaccine based on age and gender.

At the meeting on Wednesday, health officials said they have received reports of seven cases out of more than 7.2 million Johnson & Johnson doses administered in the United States. All seven cases were women between the ages of 18 and 48, one of whom died. J&J has received reports of two additional cases, including a 25-year-old man.

Symptoms occurred six to 13 days after vaccination, health officials said.

The Philadelphia Department of Public Health and all partner clinics have paused the use of the single-dose J&J vaccine until further notice, city officials said Tuesday. Pennsylvania and New Jersey health officials have done the same.

» READ MORE: What to do if you were scheduled for a Johnson & Johnson COVID-19 vaccine appointment

Here’s what you need to know about this rare side effect:

What is this blood clotting syndrome?

Experts theorize that a component of the J&J vaccine — the adenovirus that carries genetic instructions into cells to activate the immune response — also triggers an autoimmune response. Basically, this abnormal reaction causes blood clots to form in veins in various organs, including the brain. The increased clotting depletes the body’s store of platelets, the disc-like blood cells the are vital to forming clots.

This unusual combination of clotting with low platelet count is ultra-rare in the general population — less than one in a million per year, federal health officials estimate. That’s why they are concerned that the cluster of cases following the debut of the J&J vaccine in early March may be related to the shot.

The J&J vaccine accounts for about 5% percent of vaccines administered so far in the U.S.

Is this rare side effect the same as those reported in the AstraZeneca vaccine?

The clotting syndrome in the J&J cases is similar to the reaction seen abroad in about 160 recipients of the AstraZeneca vaccine, which has not yet been authorized for use in the United States. Some of the AstraZeneca cases were fatal, though investigators say the risk of clots remains extremely low.

Like the J&J vaccine, AstraZeneca’s version uses adenovirus technology.

Are there any known risk factors for the clotting syndrome?

In general, the risk of most clots is higher with pregnancy, oral contraceptive use, cancer treatment, and certain infections.

None of those has emerged as a clear factor in this clotting syndrome. Premenopausal women make up most of the cases seen worldwide, but young men have also developed the syndrome. Of the first six U.S. cases, none of the women were pregnant or post-partum, none had COVID-19 before vaccination, and none had a history of coagulation problems. Three were obese, one had asthma, one had high blood pressure, and one was taking a contraceptive pill.

Are the Pfizer and Moderna vaccines producing the same reactions?

None have been reported in the Pfizer and Moderna vaccines, which are made using mRNA technology.

» READ MORE: RNA vaccine 101: This is what happens when the COVID-19 shot goes into your arm

If you had the J&J vaccine, what signs and symptoms should you look for?

Vaccine recipients who develop severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after vaccination should contact their doctor, federal officials said. Blood clots of any kind are potentially serious; if you have these symptoms, get help. Don’t wait for them to go away.

I have an appointment for the Johnson & Johnson vaccine. What do I do?

Check before you go. Some local clinics can substitute Pfizer or Moderna vaccines Other sites have had to close temporarily.

I was on the fence about getting vaccinated even before this happened. Why should I trust the Pfizer or Moderna vaccines?

COVID-19 has so far killed more than 560,000 Americans, and experts say immunization is essential to curbing, and hopefully ending, the pandemic. The blood clotting disorder has not been associated with the Pfizer and Moderna vaccines, which have been given to more than 190 million people.

The very fact that an ultra-rare potential side effect was quickly identified through federal vaccine surveillance shows that the regulatory system is working as it should, experts say.

Pennsylvania’s acting secretary of health, Alison Beam, said at a news conference Tuesday that “the CDC and the FDA are closely monitoring vaccines for safety and effectiveness, and if there is a cause for concern, they will take action because safety is paramount.” She stressed that individuals should “proceed with getting vaccinated as soon as possible to fight the virus.”