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Federal health officials warned doctors and patients Tuesday to watch for symptoms that could indicate an extremely rare but serious form of blood clot in the brain suffered by six women who received Johnson & Johnson’s coronavirus vaccine. So what should you look for if you are one of the more than 7 million people who have received the company’s one-dose vaccine or are scheduled to receive it soon?

Frequently Asked Questions

  • What are the symptoms of a blood clot?
  • What is cerebral venous sinus thrombosis?
  • Why did the FDA and CDC urge a pause in the use of the Johnson and Johnson vaccine?
  • Is there a link between the vaccine causing blood clots and birth control?

What are the symptoms of a blood clot?

The symptoms include headaches, leg pain, abdominal pain and shortness of breath that begins six to 13 days after receipt of the vaccine, officials said. Those symptoms are different from the much more common flu-like aches and lethargy that can occur a day after receiving the shot, which are generally harmless side-effects of inoculation against the coronavirus.

Vaccine recipients with the symptoms cited by officials should seek medical attention to determine whether they have a low count of blood-clotting platelets in their blood.

What is cerebral venous sinus thrombosis?

All six of the women known to have suffered cerebral venous sinus thrombosis had low platelet counts, leading experts to speculate that the vaccine somehow, activates platelets and leads to blood clots in the channels that help route blood away from the brain. The clots can cause strokes or similar damage to the brain.

“If someone ends up with a very severe headache or any kind of shortness of breath, pain in their legs, pain in their abdomen … they would want to seek medical attention, and if there are low platelets at that time,” further discussion with a health-care provider is necessary, Peter Marks, director of the Center for Biologics Evaluation and Research (CBER) at the Food and Drug Administration, said at a news briefing Tuesday morning.

In guidance sent to health-care providers later Tuesday, the Centers for Disease Control and Prevention also included backache, new neurological symptoms, leg swelling and new or easy bruising among possible symptoms. And the agency warned physicians not to treat the condition with heparin, a common anticoagulant, which in this case could do more harm.

Why did the FDA and CDC urge a pause in the use of the Johnson and Johnson vaccine?

All of the people known to be affected are women between the ages of 18 and 48. One died, and one is in critical condition, officials said. The development led states across the nation to pause immunization with the Johnson & Johnson vaccine while a federal expert panel examines the problem. Authorities said they hoped the development would not lead to fewer people accepting vaccines for the coronavirus, which has caused more than 562,000 deaths in the United States so far.

Is there a link between the vaccine causing blood clots and birth control?

Though the known cases are all among women of childbearing age, “at this time it’s not clear that there is any association with the oral contraceptive pill,” Marks said. Of course, it’s difficult to make any generalizations based on six cases. One woman who suffered a clot was using oral birth control medication, according to Anne Schuchat, principal deputy director of the CDC.

Gowthami Arepally, associate professor of pathology at Duke University School of Medicine, said that cases of cerebral venous sinus thrombosis are something she is used to seeing, often among women using hormonal forms of birth control that raises the risk of such clots.

What makes the cases so unusual is the pairing with low platelets, the blood cells involved in clotting. Blood clots are thick collections of blood cells that can be caused by injuries, illness and long periods of sitting, among other reasons.

“It happens in very sick people, hospitalized people, not in healthy people,” Arepally said. What is being seen in the otherwise healthy patients resembles a rare immune reaction to heparin, she said.

Krishna Upadhya, a physician and vice president of quality care and health equity for Planned Parenthood Federation of America, said women who take hormonal birth control pills and have received the Johnson & Johnson vaccine should not stop using that form of contraception.

“While both things are reported to have, maybe, an association with blood clots, the blood clots being investigated related to the vaccine have a really different potential cause,” she said. “Birth control pills are very safe and very important for people who want to prevent pregnancy and for a lot of other health conditions.”

Upadhya said any woman concerned about possible symptoms related to the vaccine and contraception should consult her doctor.

Who is most at risk after having received the vaccine?

It is unclear who is at the most risk after receiving the Johnson and Johnson vaccine. Among people with normal platelet counts, the disease affects 2 to 14 per million people per year. The Johnson & Johnson vaccine has been distributed for six weeks. The blood clots also appear to be similar to problems caused by the AstraZeneca vaccine in Europe.

European regulators said it was “plausible” that the clots were linked to that vaccination. Johnson & Johnson disclosed Tuesday that the cases were being reviewed with European regulators and said the company was delaying its rollout there.

The clots have not been associated with the Pfizer-BioNTech or Moderna vaccines. They use different technologies than the Johnson & Johnson and AstraZeneca vaccines.

What are experts saying?

Angela Rasmussen, a virologist affiliated with Georgetown University, urged people not to overreact and wait for additional information.

“I am a woman between the ages of 18 and 48 who got the Johnson & Johnson vaccine eight days ago, and I recommend that people do what I’m doing, which is not panic,” she said. “First of all, we don’t know if these six cases are actually associated with the vaccine. We don’t know if those six cases had some other conditions that would have predisposed them to having blood clots like this. We don’t know a lot about it, which is why there’s a pause.”

Schuchat said at the news conference there may have been underlying conditions among some of the six women who suffered these blood clots.

Lisa Maragakis, an associate professor of medicine, infectious-disease and epidemiology at Johns Hopkins University School of Medicine, added that “any time a message like this comes out, particularly for those who are within that two-week window, there’s a concern about ‘What does this mean for me?’” Maragakis said, “I feel like overwhelmingly the message should be one of reassurance, that we don’t yet know if these events are even associated with a vaccine. But even if they are, we know that serious events can happen with vaccines. These events are serious but appear to be, at least at this point, very rare.”

Rasmussen encouraged people to contextualize the available data. Six reports of this condition out of nearly 7 million shots delivered, Rasmussen said, is “1 in 1,000,000, basically."

She added: “I’m not worried that I’m going to have a blood clot. ”

How does the risk of a clot compare to the risk from the pandemic?

Acting FDA commissioner Janet Woodcock stressed that the risk of clots is tiny in comparison with the danger posed by the pandemic. Vaccines are the best protection against the coronavirus, she said.

“We’re recommending a pause out of an abundance of caution, but on an individual basis, a provider and a patient can make a decision on whether to receive the vaccine,” she said.

With millions of Americans still hesitant to receive the available coronavirus vaccines or rejecting them altogether, Maragakis said, “it is very unfortunate that this pause is necessary right now.” She added that the move probably will cause additional concern about the Johnson & Johnson vaccine.

“I hope it doesn’t spill over more generally to the other types of vaccines because they are lifesaving, and we’re seeing overwhelming data that they’re preventing serious disease from covid-19,” she said.

The recommendation to pause should not be an indication that the vaccines are unsafe or unregulated, Rasmussen said. She added that she is already seeing some vaccine opponents citing the Johnson & Johnson development as a reason to not take the shots.

“This is exactly how we know that these vaccines are safe — because when we get any kind of indication, even if it’s rare, even if it’s a tiny little blip like six cases, they will investigate to make sure that these vaccines are safe,” she said. “People should feel confident in this process.”

Carolyn Y. Johnson contributed to this report.