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How serious is this Johnson & Johnson scare? Here’s what we know

Vials containing doses of the Johnson & Johnson vaccine.
Vials containing doses of the Johnson & Johnson vaccine.PHILL MAGAKOE/Bloomberg

Many Americans awoke Tuesday to unsettling news about the Johnson & Johnson coronavirus vaccine and its possible link to a rare but dangerous blood-clotting disorder. But doctors emphasize that this possible side effect is extremely rare. Here’s what you need to know.

How scary is this?

Only six people out of 6.8 million experienced clots after taking the Johnson & Johnson vaccine. That’s fewer than one in a million.

To put that in context, Dr. Helen W. Boucher, chief of the Division of Geographic Medicine and Infectious Diseases at Tufts Medical Center, provided these statistics: 500 to 1,200 blood clots occur per one million people who take birth control pills, 1,700 clots per million smokers, and 165,000 clots per million people with COVID-19.

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Clearly, the risks of COVID-19 far outweigh the risks of this or any other approved vaccine.

Should people who received the vaccine be on the lookout for symptoms of blood clots?

The rare blood clots occurred about a week after vaccination, and none longer than three weeks after. “If you’re more than a month out from the vaccine, you shouldn’t worry,” said Dr. Jean Marie Connors, a hematologist at Brigham and Women’s Hospital.

If you received the Johnson & Johnson vaccine within three weeks, rest assured that the symptoms of a blood clot are not subtle. If you develop one, you will feel severe pain in the leg, abdomen, or head, or shortness of breath, depending on where the clot has lodged, Boucher said.

“I’m speaking of a very bad headache or very bad abdominal pain. Not a little bit of nausea,” she said.

The symptoms are “different from the mild flu-like symptoms that many people experience a couple of days after receiving the vaccine,” Dr. Anne Schuchat, the CDC’s principal deputy director, said at a press briefing Tuesday.

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What should you do if you have severe leg, abdomen, or head pain, or shortness of breath?

Call your doctor, who will assess whether you need to come in for testing. If a vaccine reaction is suspected, your doctor will test platelet levels in your blood and scan for a clot using an imaging technology, such as an ultrasound, a CT scan, or an MRI, depending on the location of the suspected clot, Boucher said.

For treatment, the key message is not to use the blood thinner heparin, even though it is a standard treatment for a blood clot. In this particular condition, heparin could “cause tremendous harm or the outcome can be fatal,” Dr. Peter Marks, director of the FDA Center for Biologics Evaluation and Research, said at the briefing.

Is the issue with the Johnson & Johnson vaccine similar to the problems that arose with the AstraZeneca vaccine?

Yes. “It’s plainly obvious to us already,” Marks said, “that what we’re seeing with the [Johnson & Johnson] vaccine looks very similar to what was being seen with the AstraZeneca vaccine.” In Europe, more than 200 cases of the rare blood-clotting problem were reported among the 34 million people who had received the AstraZeneca vaccine, which has not been approved for use in the United States.

Both vaccines use a similar technology: a deactivated adenovirus carries DNA into human cells to trigger an immune response. And with both vaccines, several people suffered from blood clots while also having low platelets, an extremely rare combination.

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What causes this reaction?

In normal circumstances, blood clots will form after major surgery, injury, or prolonged immobilization, when slow blood flow and inflammation can conspire to create blockages, explained Connors, the hematologist.

The type of clot seen in these rare vaccine-related events follows a different process. Based on a study of cases linked to the AstraZeneca vaccine, it appears the immune system activates the platelets, causing them to form clots for no reason. The platelets get used up in these clots, leading to low platelet levels.

“It appears that the platelets are being activated even when they’re not needed,” Connors said. “Platelets should be running around quietly minding their own business.”

Heparin, the blood thinner most often used to break up clots, can make matters worse by further activating the platelets, leading to more clots. “You add fuel to the fire,” Connors said.

That’s why it’s critical for doctors to ask about vaccines if they encounter a patient who may have blood clots. A better treatment for them would be non-heparin anticoagulants to break up the clots and immunoglobulin to tamp down the immune response that causes the clot formation, Connors said.

Who is most at risk for developing clots after vaccination?

The six people who got blood clots after taking the Johnson & Johnson vaccine were all women age 18 to 48 — one died and another is in critical condition. But that doesn’t mean that men are not susceptible. Indeed the number of people affected is so small it’s impossible to make any generalizations, the CDC’s Schuchat said.

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Won’t this incident contribute to vaccine hesitancy?

Public health officials argue it should do the opposite.

“Overall, I think it’s reassuring that with our safety surveillance strategies, we’re able to pick up such a rare side effect,” said Dr. Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital.

Boucher, the Tufts infectious disease specialist, said she hopes that pausing the Johnson & Johnson vaccinations will reassure people that the safety measures are working. “People are paying attention, doing their jobs protecting the public health,” she said.

“It’s really important to come back to the risk of COVID disease,” Boucher added. “We have vaccines that are effective and safe, to protect you from a disease with a measurable and significant risk of dying. All the scientific expertise we can bring to bear in our country is working to keep the public safe.”


Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer.