Should You Worry If You Got the J&J Vaccine? What Doctors Say

The Johnson & Johnson vaccine for Covid-19 is on pause over rare, severe blood clots in six women. If you got the vaccine, here's what to know about your risk and how to watch for symptoms.

Rare blood clots tied to a Covid-19 vaccine

Six women in the United States have developed a rare type of blood clot after receiving the Johnson & Johnson (Janssen) Covid-19 vaccine.

On Tuesday, April 13 the Centers for Disease Control and Prevention (CDC) and Food and Drug Administration (FDA)  put a pause on administering the vaccine while they investigate the rare cases, which included one death.

So far, more than 6.8 million people in the United States have received the Johnson & Johnson vaccine, which requires only one dose to be effective.

Other Covid-19 vaccines—like those made by Moderna and Pfizer—require two shots and aren’t linked to any blood clots.

Even though the clots may have been caused by the Johnson & Johnson shot, there’s reason to not worry too much if you received the vaccine, says William Schaffner, MD, an infectious diseases specialist at Vanderbilt University Medical Center in Nashville. Here’s why.

The blood clots are rare

With six cases out of almost 7 million doses administered, you have less than a one in a million chance of developing these blood clots after receiving the Johnson & Johnson vaccine.

Reports of the same thing happening with the AstraZeneca Covid vaccine emerged recently in Europe but, again, only rarely. (The AstraZeneca vaccine is not available in the United States.)

The two are probably connected as both the AstraZeneca and Johnson & Johnson vaccines employ similar technology: They use an inactivated virus to tell your cells how to produce the spike protein on the surface of SARS-Cov-2, the virus that causes Covid-19.

The blood clots themselves, called cerebral venous sinus thrombosis (CVST), affect blood flowing out of the brain which, paradoxically, occurs in conjunction with low platelet counts (thrombocytopenia). They’ve also been seen in the abdomen.

Typically, low platelet levels would protect against clots, but this particular combination has also been seen in people receiving the blood thinner heparin (called heparin-induced thrombocytopenia).

It happens when antibodies produced by the immune system bind to certain platelet factors.

In this case, it’s the vaccine that sets the immune response in motion, says Robert Glatter, MD, an emergency physician with Lenox Hill Hospital in New York City.

Only certain groups of people have been affected

The six U.S. cases were only in women ages 18 to 48. Most of the cases in Europe were also in younger women.

“If you don’t fall into that category, take two deep breaths,” says Dr. Schaffner.

It’s unclear why mostly younger women are being affected but it may have to do with the involvement of antibodies.

“This is immune-mediated and younger women just generally are more apt to have immune-mediated disease,” says Dr. Schaffner. “We don’t really know why that is.”

(Learn why women may have more vaccine side effects than men.)

Nor can anyone say why it’s associated with the vector vaccines and not the Pfizer and Moderna vaccines, both of which use a different technology.

They both use messenger RNA or mRNA, which doesn’t use any active or inactivated virus at all.

Instead, the messenger RNA delivers instructions so the cells themselves can create the spike protein targeted by the vaccine.

“We don’t have a definitive answer yet, but it’s likely similar to what we have seen with the AstraZeneca vaccine, involving an immune response related to the vaccine that adversely affects the function of the platelets, which in turn promotes the formation of these clots,” says Dr. Glatter.

box of janssen johnson and johnson covid vaccineTom Williams/Getty Images

The blood clots occurred soon after the vaccine

Specifically, the clots started six to 13 days after vaccination, according to the CDC.

“As far as what we know now, if you’re beyond that, you’re out of the vulnerable window,” says Dr. Schaffner. The clots associated with the AstraZeneca vaccine also happened within two weeks of the shot.

This time window is true of vaccines in general: Most side effects occur soon after receiving the vaccine, most often within 24 hours.

The Moderna and Pfizer vaccines have been linked to anaphylactic (allergic) reactions but these have typically occurred within minutes to 24 hours of getting the shot.

The blood clots are treatable

The clots are treatable, though not with heparin, which is the usual medication used to treat blood clots.

Instead, alternative treatments have to be administered instead of heparin, including antibodies that attack the “bad” antibodies.

Blood clots also may be accompanied by warning signs.

“If a person experiences headaches, dizziness, visual changes, or abdominal pain within two weeks of receiving the vaccine, they should seek immediate attention in the nearest emergency department,” says Dr. Glatter.

According to a CDC statement on April 13, people who “received the J&J/Janssen Covid-19 vaccine within the past three weeks  who develop severe headache, abdominal pain, leg pain, or shortness of breath should contact their health care provider.”

The FDA’s safety system is working

The FDA has an extensive surveillance system in place to detect such side effects from vaccines and it’s working.

“Our monitoring system is quite effective in picking up these rare events—almost like a needle in a haystack,” says Dr. Glatter.

Health agencies are asking providers to continue reporting adverse events and the CDC’s Advisory Committee on Immunization Practices has had a meeting to further discuss the issue.

“I agree with the pause in administering the vaccine at this time so that the FDA can investigate these clotting events which can be deadly,” says Dr. Glatter. “If it turns out there is a causative link similar to what we have seen with the AstraZeneca vaccine, it’s likely that the Johnson & Johnson vaccine won’t be withdrawn, but reserved for lower-risk groups.”

In many European countries, the vaccine is being given to older people only.

There are other vaccines out there

The Johnson & Johnson vaccine has become increasingly popular precisely because it is “one and done.”

That said, there are two other highly effective vaccines now widely available in the United States: one from Pfizer and one from Moderna, each of which is about 95 percent effective in preventing asymptomatic disease.

The main side effects from all three vaccines are not serious: a sore arm and flu-like symptoms like a mild fever and aches.

These go away on their own leaving you far more protected from Covid-19 than you were before.

Here’s more information on the Covid vaccines’ side effects.


Amanda Gardner
Amanda Gardner is a freelance health reporter whose stories have appeared in,,, WebMD, HealthDay, Self Magazine, the New York Daily News, Teachers & Writers Magazine, the Foreign Service Journal, AmeriQuests (Vanderbilt University) and others. In 2009, she served as writer-in-residence at the University of Wisconsin School of Medicine and Public Health. She is also a community artist and recipient or partner in five National Endowment for the Arts grants.