A new study hints at a reason the J.&J. and AstraZeneca vaccines may cause blood clots in rare cases.

An advisory group from the Centers for Disease Control and Prevention has recommended that the Johnson & Johnson Covid-19 vaccine hiatus be lifted for all adults while also putting up a warning sign about a rare but dangerous bleeding disorder. However, a central mystery remains: how could a vaccine given to nearly eight million people cause the side effect in just a few of them?

There’s no clear answer yet, but Dr. Andreas Greinacher, a researcher at the University Medical Center Greifswald in Germany, leads an attempt to find out. Speaking at a news conference on Tuesday, he said he had an agreement with Johnson & Johnson to study the components of the vaccine to see if it could interfere with normal blood clotting under certain rare conditions.

“We just agreed that we’d like to work together,” he said.

It is possible, said Dr. Greinacher that the Johnson & Johnson vaccine can cause rare side effects through the same process that he suspects is responsible for similar side effects of the AstraZeneca vaccine. The main component of both vaccines are harmless viruses called adenoviruses, which invade human cells and release a coronavirus gene that later triggers an immune response.

On Tuesday, Dr. Greinacher and his colleagues published a report on how the AstraZeneca vaccines might trigger the side effect. The study has not yet been published in a scientific journal.

The scientists found that components of the AstraZeneca vaccine can adhere to a protein that releases platelets when blood clots form. These lumps of molecules could be viewed by the body as foreign invaders, the scientists speculated, triggering a cascade of reactions that turn platelets into dangerous clots.

Dr. Paul A. Offit, a vaccines expert at Philadelphia Children’s Hospital who was not involved in the study, found Dr. Greinacher fascinating, but far from the final word. “It throws a lot of opportunities,” he said.

Dr. Offit said it was not clear which of the many factors the researchers looked at could explain the rare blood clots in people vaccinated with AstraZeneca’s doses. “It’s like taking a sip from a fire hose,” he said.

At a press conference on Tuesday, Dr. Greinacher said the research could reveal ways the AstraZeneca vaccine can lower the risk of blood clots or treat the side effects. However, he stressed that the small risk of these side effects was outweighed by the protection that vaccines like AstraZenecas offer against Covid-19.

“Not being vaccinated is far more dangerous than being vaccinated and at risk for this adverse drug reaction,” he said.

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