As a panel of experts advising the Centers for Disease Control and Prevention discusses a handful of rare blood clots that health officials have studied in recipients of the Johnson & Johnson coronavirus vaccine, a central mystery looms: How could a vaccine that has been given to nearly eight million people cause the side effect in only some of them?
There is still no clear answer, but Dr. Andreas Greinacher, a researcher at the Medical University of Greifswald in Germany, is leading an effort to find out. In a news conference on Tuesday, he said he had reached an agreement with Johnson & Johnson to inspect the vaccine components to see if it can disrupt the normal blood clotting process under certain rare conditions.
“We just decided that we would like to work together,” he said.
It is possible, Dr. Greinacher said, that the Johnson & Johnson vaccine may cause rare side effects with 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 release a coronavirus gene that will later trigger an immune response.
On Tuesday, Dr. Greinacher and his colleagues released a report on how AstraZeneca vaccines can trigger the side effect. The study has not yet been published in a scientific journal.
Scientists have found that components of the AstraZeneca vaccine can attach to a protein that platelets release during blood clot formation. These clusters 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 last word. “It throws out a lot of possibilities,” he said.
Dr Offit said it was unclear which of the many factors the researchers studied could explain the rare blood clots in people vaccinated with AstraZeneca doses. “It’s like sipping from a fire hose,” he said.
In a press conference on Tuesday, Dr. Greinacher said research could point to ways, in the AstraZeneca vaccine, to reduce the risk of clots or treat side effects. But he pointed out that the small risk of these side effects was strongly offset by the protection that vaccines like AstraZeneca provide against Covid-19.
“Not being vaccinated is much more dangerous than being vaccinated and at risk for this adverse drug reaction,” he said.