The following essay is reprinted with permission from The Conversation, an online publication covering the latest research.
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.
What do I do if I got the J&J shot?
The CDC and FDA are recommending that people who have received the J&J vaccine within the last 3 weeks who develop severe headache, abdominal pain, leg pain or shortness of breath should contact their health care provider.
This type of blood clot is treatable with the use of blood thinners or anticoagulants. If a patient has low platelets, however, a doctor would not prescribe the widely used anticoagulant heparin but instead another kind of blood thinner. Untreated, these blood clots can be fatal.
CDC and FDA officials explain the reasons for halting the vaccine in a media call.
What are the CDC and FDA specifically recommending for the J&J vaccine?
Because of this rare occurrence, even though it has not been shown to be due to the vaccine, the CDC and FDA have recommended a pause in use of the J&J vaccine until these cases can be further reviewed.
What are the next steps?
The CDC convened a meeting of the Advisory Committee on Immunization Practices on April 14, 2021. The ACIP is an independent board of 15 scientific and medical experts selected by the health and human services secretary that advises the CDC on vaccines for children and adults. People with ties to vaccine manufacturers are excluded from the ACIP membership because of potential conflict of interest.
On April 14, ACIP reviewed the available evidence but did not vote on recommendations because panel members expressed concern that the panel needs more time to evaluate data and risks. The vaccine has been given to 3.8 million people in the past two weeks. Therefore, not enough time has passed to see whether other people might also experience these serious clots. The panel is expected to meet again within a week to 10 days.
Is this similar to what happened with the AstraZeneca vaccine in Europe?
A similar rare problem of blood clotting with low platelets in the cerebral venous sinus and also in the abdominal veins and arteries has been seen in connection with the use of the AstraZeneca COVID-19 vaccine used in Europe. There, 182 cases were reported in 190 million doses – again, roughly 1 in 1 million people vaccinated. The European Medicines Agency investigated this and concluded that central venous sinus thrombosis with low platelets should be listed as a possible “very rare side effect” of the AstraZeneca vaccine.
On April 13, 2021, Johnson & Johnson announced it was delaying the rollout of its vaccine in Europe in response to the U.S. review.
What is the take-home message?
The U.S. has a total of three vaccines authorized under emergency use authorization for COVID-19, and this side effect has not been observed in the other two vaccines, developed by Moderna and Pfizer. The Moderna and Pfizer vaccines do not use the same technology used in the J&J and AstraZeneca vaccines. So vaccination against COVID-19 can continue, while efforts are made to determine if the clotting disorder is related by chance or a true, but extremely rare, side effect of the J&J vaccine.
I believe it is a testament to the emphasis by the CDC and FDA on vaccine safety that J&J vaccinations have been paused while this is studied by independent scientists and medical experts.
Editor’s Note (4/15/21): This article was updated to add additional research and the description of the Advisory Committee on Immunization Practices meeting.