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


By Carl Zimmer

As a panel of experts advising the Centers for Disease Control and Prevention discusses a handful of rare blood clots that health officials have investigated in Johnson & Johnson coronavirus vaccine recipients, a central mystery looms: How might a vaccine that has been 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 University Medicine Greifswald in Germany is leading one effort to find out. At a news conference on Tuesday, he said that he had reached an agreement with Johnson & Johnson to inspect the components of the vaccine to see if it can disrupt the normal blood clotting process under certain rare conditions.

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

It’s possible, Dr. Greinacher said, that the Johnson & Johnson vaccine can cause rare side effects by the same process that he suspects is responsible for similar side effects from the AstraZeneca vaccine. The main ingredient in both vaccines are harmless viruses known as adenoviruses, which slip into human cells and deliver a coronavirus gene that will later trigger an immune response.

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

The scientists found that components in the AstraZeneca vaccine can stick to a protein that platelets release during the formation of blood clots. These clumps of molecules could be seen by the body as foreign invaders, the scientists speculated, triggering a cascade of reactions that cause platelets to turn into dangerous clots.

Dr. Paul A. Offit, a vaccine expert at Children’s Hospital of Philadelphia who was not involved in the study, found Dr. Greinacher’s study intriguing but far from the final word. “He throws out a lot of possibilities,” he said.

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

At a news conference on Tuesday, Dr. Greinacher said that the research might point to ways, in the AstraZeneca vaccine, of lowering the risk of the clots or of treating the side effects. But he emphasized that the small risk of those side effects was strongly outweighed by the protection that vaccines such as AstraZeneca provide 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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