This week, federal health agencies paused the administration of the Johnson & Johnson COVID-19 vaccine while the U.S. Food and Drug Administration and the Centers for Disease Control investigate reports of a rare disorder involving blood clots after vaccination.

The University of Virginia’s Dr. William Petri examined the evidence in a piece for The Conversation, re-posted below. Petri is a chaired professor of infectious diseases and international health and vice chair for research in the Department of Medicine. His lab is studying the effects of COVID-19 on the immune system and possible treatments, including vaccines.

Here’s what he had to say in The Conversation. 

What is the 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. It is called central venous sinus thrombosis. In these vaccine-associated cases of this, platelets in blood, which are important for making clots, have been lower than normal, causing an unusual combination of clotting and bleeding. While researchers do not know for certain why this is so, platelet counts could be lower, perhaps because they have been used up making these clots.

How many people have experienced this possible reaction?

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 to 48, and from six to 13 days after vaccination. That is about half as likely as getting struck by lightning in a year. What is being determined now is what is the normal background number of cases we might see in the general population without the vaccine as a factor. This will make it possible to determine if the clotting problem is a vaccine side effect or not.

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 three weeks who develop severe headache, abdominal pain, leg pain or shortness of breath should contact their health care provider.

Fortunately, 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.

What are the CDC and FDA recommending for the J&J vaccine?

Because of this rare occurrence, even though it is not 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 will convene a meeting of the Advisory Committee on Immunization Practices on Wednesday. The ACIP is an independent board of 15 scientific and medical experts selected by the Secretary of Health and Human Services 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.

The ACIP will review the available evidence and make recommendations to the CDC, based to a great extent on the likelihood that the complication is vaccine-related.

Is this similar to what happened with the AstraZeneca vaccine in Europe?

A similar and rare problem of blood clotting with low platelets in the cerebral venous sinus and also in the abdominal veins and arteries has been seen with the use of the AstraZeneca COVID-19 vaccine used in Europe. There, 182 cases in 190 million doses – again, roughly one 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 Tuesday, Johnson & Johnson announced it is 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 for 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 is 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.