What's known about COVID vaccines and rare clots

In this file photo dated March 31, 2021, on Tuesday April 13, a nurse fills a syringe with Johnson & Johnson’s one-off COVID-19 vaccine at the Vaxmobile at the Uniondale Hempstead Senior Citizens’ Center in Uniondale, NYUS recommending a “break” in using the Vaccine to check for reports of potentially dangerous blood clots. (AP Photo / Mary Altaffer, File)

A rare, rogue immune response is the prime suspect as authorities investigate highly unusual blood clots after using two similar COVID-19 vaccines from Johnson & Johnson and AstraZeneca.

The U.S. recommended that states suspend administration of the J&J vaccine on Tuesday while authorities investigate six reports of unusual blood clots, including one fatality, in more than 6.8 million Americans who have received the single-dose vaccine to date .

However, the small number of cases was a cause for concern, as European authorities only said last week that similar blood clots may be related to the AstraZeneca vaccine, which is not yet okay in the US. As a result, some countries restricted its use to certain age groups. Also on Tuesday, J&J delayed its upcoming launch in Europe.

These are not typical blood clots. They’re weird in two ways.

First, they occur in unusual parts of the body, such as veins that drain blood from the brain. Second, these patients also have abnormally low levels of platelets – cells that help form clots – a condition usually associated with bleeding, not clotting.

Scientists in Norway and Germany first addressed the possibility that some people might have an abnormal immune system response to the AstraZeneca vaccine, making antibodies that attack their own platelets. That’s how the U.S. is now studying clots in J&J vaccine recipients, said Dr. Peter Marks, the Food and Drug Administration’s vaccine chief, on Tuesday.

First note: a widely used blood thinner called heparin sometimes causes a very similar side effect. Very rarely, heparin recipients make antibodies that both attack and overstimulate platelets, said Dr. Geoffrey Barnes, a clot expert at the University of Michigan.

“It can cause both sides of the blood clotting spectrum,” Barnes said. Because heparin is so widely used in hospitals, this response is “something that every hospital in America knows how to diagnose and treat”.

EXPLAINER: What is Known About COVID Vaccines and Rare Clots?

Sharis Carr, a nurse at the Aaron E. Henry Community Health Service Center in Clarksdale, Miss., Holds a box of doses of the Johnson & Johnson COVID-19 vaccine on Wednesday, April 7, 2021. The US recommends a “break” in using the Johnson & Johnson COVID-19 single-dose vaccine to investigate reports of potentially dangerous blood clots. (AP Photo / Rogelio V. Solis)

There are also incredibly rare reports of this strange combination of blood clots and platelets in people who have never taken heparin, for example after an infection. These unexplained cases didn’t receive much attention, Barnes said, until the first clot reports surfaced in some AstraZeneca vaccine recipients.

Health officials said one reason for the J&J hiatus is to make sure doctors know how to treat patients suspected of having these blood clots, including avoiding giving them heparin.

The Centers for Disease Control and Prevention offered advice on how to detect and treat unusual blood clots later Tuesday.

In two studies in the New England Journal of Medicine last week, teams of researchers from Norway and Germany found platelet-attacking antibodies in the blood of some AstraZeneca vaccine recipients who had the strange blood clots. The antibodies were similar to those found with the heparin side effect, although the patients had never used this blood thinner.

It is not yet clear whether there is a similar relationship with the J&J vaccine. However, the vaccines from J&J and AstraZeneca, as well as a Russian COVID-19 vaccine and a vaccine from China, are made using the same technology. They train the immune system to recognize the spike protein that envelops the coronavirus. To do this, they use a cold virus called adenovirus to transport the spike gene into the body.

The FDA markings wouldn’t tell if the weird clots are common with these so-called adenovirus vector vaccines. In addition to the AstraZeneca data, J&J is making an Ebola vaccine the same way, and he said authorities would “examine the entirety of the evidence”.

The most commonly used COVID-19 vaccines in the US – from Pfizer and Moderna – are made using a completely different technology, and the FDA said there is no evidence of a similar clot problem with these vaccines.

What about people who worry because they got the J&J vaccination? Marks said it was important not to confuse the rare risk of clotting with normal flu-like symptoms that people often experience a day or two after getting a COVID-19 vaccination. He said that symptoms such as a severe headache or severe abdominal pain would appear a week to three weeks after the J&J vaccine.

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