A panel of experts met on April 14, 2021, to review evidence on blood clots that have been reported in seven people after they received the one-dose Johnson & Johnson COVID-19 vaccine. The panel, the Advisory Committee on Immunization Practices, or ACIP, advises the Centers for Disease Control and Prevention on immunization. It delayed voting on a recommendation to the CDC so that members can further evaluate risk and data. The clotting, which resulted in one woman’s death, led the CDC and FDA on April 13, 2021, to pause use of the J&J vaccine. Dr. William Petri, an infectious disease physician and immunologist at the University of Virginia School of Medicine, answers questions to help put this development in context.
What is this potential side effect of the J&J vaccine for COVID-19?
The potential side effect is a blood clot in the veins that drain blood from the brain. This is called central venous sinus thrombosis. In the vaccine-associated cases of this, platelets in blood, which are important for making clots, have been lower than normal. This same side effect has been seen in the AstraZeneca COVID-19 vaccine that also uses an adenovirus to deliver the coronavirus spike glycoprotein. In the case of the AstraZeneca vaccine, the clotting disorder has been linked to antibodies against platelet factor 4 (PF4) that are apparently induced by the adenovirus backbone of the vaccine. This antibody causes the clotting disorder by activating platelets to clot.
It is important to note that this disorder, called vaccine-induced immune thrombotic thrombocytopenia, is not a problem with the mRNA-based Pfizer and Moderna COVID-19 vaccines.
How many people have experienced this possible reaction?
As of April 13, 2021, about one in a million: Six cases out of the 6.8 million doses of the J&J vaccine administered in the U.S. These six cases all occurred in women ages 18-48, and from 6 to 13 days after vaccination. That’s about half as likely as getting struck by lightning in a year. A seventh case was included in the ACIP review on April 14.