A panel of advisers to the Centers for Disease Control and Prevention has recommended lifting the pause on the Johnson & Johnson Covid-19 vaccine for all adults while adding a warning label about a rare but dangerous blood clotting disorder. But a central mystery persists: 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 could 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 cause an immune response.
On Tuesday, Dr. Greinacher and his colleagues released a report on how the AstraZeneca vaccines might trigger the side effect. The study has not yet been published in a scientific journal.