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This article has been reviewed and updated as of April 24, 2021.

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Credit: Andrii Shyp/iStock

This story includes additional reporting from Leah Rocketto. 

A previous recommendation to pause the use of the Johnson & Johnson COVID-19 vaccine was lifted Friday, April 23, following a thorough safety review conducted by the U.S. Food and Drug Administration (FDA) and the U.S. Centers for Disease Control and Prevention (CDC) determined that “the vaccine’s known and potential benefits outweigh its known and potential risks in individuals 18 years of age and older.”

During the pause, which was issued on April 13, 2021, medical and scientific teams from both organizations examined available data to assess the risk of cerebral venous sinus thrombosis (CVST) — blood clots in the brain — with thrombocytopenia (low platelet levels in the blood).

The CDC and FDA stated that use of the Johnson & Johnson COVID-19 vaccine should be resumed, as it is safe and effective in preventing COVID-19. That said, the FDA and CDC said they will, “remain vigilant in continuing to investigate this risk” and assured there are surveillance systems in place to monitor the safety of all COVID-19 vaccines.

Additionally, the organizations have conducted extensive outreach to health care providers to ensure they are aware of the potential but very rare blood clot risk and know how to properly manage and report it.

The FDA and CDC revised the Janssen COVID-19 Vaccine Fact Sheet for Healthcare Providers Administering Vaccine (Vaccination Providers) and Fact Sheet for Recipients and Caregivers to include information about the risk of CVST and thrombosis with thrombocytopenia syndrome (TTC). The organizations encourage health care providers, vaccine recipients, and caregivers to review the sheets closely.

“Safety is our top priority. This pause was an example of our extensive safety monitoring working as they were designed to work — identifying even these small number of cases,”  Janet Woodcock, MD, Acting FDA Commissioner, said during a meeting of the CDC’s Advisory Committee on Immunization Practices on April 23. “We are confident that this vaccine continues to meet our standards for safety, effectiveness and quality. We recommend people with questions about which vaccine is right for them have those discussions with their health care provider.”

Why the Johnson & Johnson COVID-19 Was Paused

 Experts were already aware of a similar issue with blood clots linked to the AstraZeneca COVID-19 vaccine because of reports in Europe that led to pauses and restrictions in use of the vaccine there. The Johnson & Johnson vaccine is made similarly to the AstraZeneca vaccine.

Then they observed a “safety signal” in the U.S. reporting system for adverse events following vaccination: six cases of very rare and severe blood clots in the brain that affected women after they received the Johnson & Johnson vaccine. All women were between the ages of 18 and 48; one person died. Some people also had blood clots in other parts of their body, including their lungs and limbs. As the CDC advisory panel reviewed more data, they discovered a couple of additional cases.

As of April 23, the FDA and CDC confirmed a total of 15 cases of TTC were reported out of more than 7 million people who received the vaccine. Of the 15 cases, three resulted in death. All but one of these cases occurred in women between the ages of 18 and 59.

Although blood clots are generally common — affecting 900,000 Americans a year, according to the CDC — this particular type of blood clotting issue is very, very rare.

It is called cerebral venous sinus thrombosis (CVST) — blood clots in the brain — with thrombocytopenia (low platelet levels in the blood). This is a highly unusual combination of symptoms because normally platelets cause the blood to clot. You wouldn’t expect to see low levels of platelets along with blood clotting.

This can cause both clotting and bleeding in the brain, which can lead to serious complications.

What Experts Know and Don’t Know

Based on the current evidence, the CDC says a “causal relationship between thrombosis with thrombocytopenia and the Janssen [Johnson & Johnson] COVID-19 vaccine is plausible,” but there is still not enough information to determine if the vaccine is related to or caused this health issue.

In an effort to determine if the vaccine is related to or caused this health issue, the American Society of Hematology has published information on the diagnosis and treatment of TTC  following the Johnson & Johnson vaccine. In this, they listed a diagnosis criterion, which includes the following:

  1. COVID vaccine (Johnson & Johnson/AstraZeneca onlyto date) 4 to 30 days previously
  2. Thrombosis (often cerebral or abdominal)
  3. Low blood platelet count
  4. Positive PF4 “HIT” (heparin-induced thrombocytopenia) test

All four criteria must be met for a person to be considered to have vaccine-related TTS.  Experts could use this criteria going forward as they continue to monitor this issue. As of now, we still do not know for sure whether or how the vaccine could be affecting this very rare blood clotting issue.

A key question is understanding the “background rate” for this condition, or the number of events that would be expected to occur naturally in people who didn’t get the vaccine. That number itself, however, is hard to determine because, as experts are stressing, CVST with thrombocytopenia is so uncommon. CDC safety expert Tom Shimabukuro, MD, has estimated that the clotting disorder in women ages 20 to 50 who received the Johnson & Johnson vaccine occurred at least three times more often than would be expected, reported the New York Times.

Keep in mind, however, that if it were just the vaccine causing this problem, many, many more people would have it. “Some quirk of biology very likely predisposes certain people to have a bad reaction to the vaccine,” according to the New York Times. If the vaccine alone were responsible, there would be many more cases, some researchers say.”

There is still a lot that experts do not know about this condition, including:

Risk Factors

For example, there don’t seem to be any obvious common risk factors among those affected. As MedPage Today noted, none of the original patients were pregnant or postpartum, and only one was using oral contraceptives. (Being pregnant or postpartum and taking birth control can raise your risk of blood clots.) No one had any pre-existing blood clotting disorders.

Occurrence in Men and Older Adults

We also don’t know whether the condition may affect older people or men more than is currently thought (it could be underreported).

Rates Among Those with COVID

We know blood clotting commonly affects people infected with COVID-19 and plays a big role in complications, hospitalization, and organ damage. It’s also important to understand whether this rare disorder also affects people who have COVID-19, because that could affect how we perceive the risks and benefits of the vaccine in relation to COVID.

How Could the Vaccine Cause This Blood Clotting Disorder?

 Both the AstraZeneca and Johnson & Johnson COVID-19 vaccine use adenoviruses to carry DNA instructions into our cells that teach our immune system how to recognize the coronavirus in order to make antibodies against it.

Because there is no connection with this rare blood clotting disorder and the vaccines from Pfizer and Moderna, which use different technology, researchers suspect that there could be something about the adenovirus that is playing a role in this rare disorder.

Recent reports published in the New England Journal of Medicine provide a possible clue. Researchers examined data from a small group of European patients who developed rare blood clots after they received the AstraZeneca vaccine. They found that the patients had developed rare antibodies — an immune system response — to proteins involved in blood clot formation.

“However, it’s not yet clear whether people who develop reactions to the vaccines already have some platelet factor 4 antibodies before they are vaccinated, or whether the vaccines somehow spur the body to make these antibodies, which then launch a kind of autoimmune attack,” Medscape explained.

This is just one possible explanation.

If a causal link between the Johnson & Johnson vaccine (and AstraZeneca vaccine) and CVST with thrombocytopenia is established, more research will be needed to understand what is driving it.

What to Look for If You’ve Had the J&J Vaccine Recently

All of the people who experienced this blood-clotting disorder developed symptoms within about two weeks of receiving the vaccine. The CDC says that if you got the vaccine more than three weeks ago, your risk of developing a blood clot is very low.

If you got the vaccine within the last three weeks, remember that your risk of developing this kind of blood clot is also extremely rare. But it’s wise to be on the lookout for the following symptoms of a blood clot:

  • Severe headache
  • Blurred vision
  • Fainting
  • Seizures
  • Pain in your abdomen (chest or stomach)
  • Leg pain or swelling
  • Shortness of breath

If you experience any of these, seek medical attention and tell your provider that you recently received the Johnson & Johnson vaccine.

The overwhelming odds are that you do not have cerebral venous sinus thrombosis with thrombocytopenia.

But it’s important for your provider to know because the blood thinner medication heparin, which is a common treatment for clots, can harm people with this particular type of blood clotting and should not be used.

What to Know If You Have Risk Factors for or a History of Blood Clots

This is an area where experts are hesitant to make any definite recommendations, because so little is known about this rare blood clotting disorder. Just because you may be at increased risk for other kind of clots DOES NOT MEAN you are at increased risk for CVST with thrombocytopenia.

Many health conditions and medications can increase your risk for more common kind of blood clots, known as deep vein thrombosis (DVT). They can be more common in some autoimmune and rheumatic conditions, including rheumatoid arthritis, lupus, and inflammatory bowel disease.

According to the Mayo Clinic, other risk factors for DVT blood clots — not for CVST with thrombocytopenia — include:

  • Injury or surgery
  • Birth control or hormone replacement therapy
  • Pregnancy or postpartum
  • Being overweight or obese
  • Cancer
  • Heart failure
  • Personal or family history of DVT
  • Smoking
  • Sitting for long periods
  • Prolonged bed rest
  • Genetic predisposition

Rheumatologists that we spoke with said that, most people with rheumatic conditions could still get the Johnson & Johnson vaccine and emphasized how rare the blood clotting side effect is. However, people who have a history of blood clotting disorders could getting the Pfizer or Moderna COVID-19 vaccine if they have a choice. Everyone emphasized the importance of getting vaccinated.

“For the general population with rheumatic conditions, I would not restrict use of the J&J vaccine whatsoever,” says Vinicius Domingues, MD, a rheumatologist in Daytona Beach, Florida and medical advisor to CreakyJoints. “For younger women with lupus and prior blood clots or with a history of blood clots, we would likely encourage Moderna or Pfizer out of an abundance of caution. But the most important message remains: Get vaccinated.

“I would be cautious with patients at a higher risk for thrombo-embolic [blood clothing] events, especially women under 50,” says Nilanjana Bose, MD, a board-certified rheumatologist with the Rheumatology Center of Houston. She urged patients to keep listening to recommendations from the discuss concerns with their health care provider.

“If you already have a vaccine appointment scheduled for Pfizer or Moderna, go get it,”  says Michael Putman, MD, an Assistant Professor at the Medical College of Wisconsin. “For most people, the benefits of the J&J vaccine outweigh the potential risks. If you have concerns about your situation, you should talk to your doctor.”

Remember that public health experts have repeatedly stated that this clotting disorder is exceedingly rare and that the benefits of getting the Johnson & Johnson (or the AstraZeneca vaccine) far outweigh their risks.

ACOG Statement on the Johnson & Johnson COVID-19 Vaccine. The American College of Obstetricians and Gynecologists. April 13, 2021. https://www.acog.org/news/news-releases/2021/04/acog-statement-on-johnson-johnson-covid-19-vaccine.

Are You At Risk for Clots? U.S. Centers for Disease Control and Prevention. March 8, 2021. https://www.cdc.gov/ncbddd/dvt/features/blood-clot-risk.html.

Bussel J, et al. Vaccine-induced Immune Thrombotic Thrombocytopenia: Frequently Asked Questions. American Society of Hematology. April 19, 2021. https://www.hematology.org/covid-19/vaccine-induced-immune-thrombotic-thrombocytopenia.

Deep vein thrombosis (DVT). The Mayo Clinic. December 22, 2020. https://www.mayoclinic.org/diseases-conditions/deep-vein-thrombosis/symptoms-causes/syc-20352557.

FDA and CDC Lift Recommended Pause on Johnson & Johnson (Janssen) COVID-19 Vaccine Use Following Thorough Safety Review. U.S. Food and Drug Administration. April 23, 2021. https://www.fda.gov/news-events/press-announcements/fda-and-cdc-lift-recommended-pause-johnson-johnson-janssen-covid-19-vaccine-use-following-thorough.

Goodman B. How Some COVID-19 Vaccines Could Cause. Medscape. April 14, 2021. https://www.medscape.com/viewarticle/949262#vp_3.

Grady D. C.D.C. Panel Keeps Pause on Use of J&J Vaccine, Citing Need to Assess Potential Risks. The New York Times. April 14, 2021. https://www.nytimes.com/2021/04/14/health/cdc-johnson-vaccine-pause.html.

Greinacher A, et al. Thrombotic Thrombocytopenia after ChAdOx1 nCov-19 Vaccination. The New England Journal of Medicine. April 9, 2021. doi: https://doi.org/10.1056/NEJMoa2104840.

Interview with Michael Putman, MD, an Assistant Professor at the Medical College of Wisconsin

Interview with Nilanjana Bose, MD, a board-certified rheumatologist with the Rheumatology Center of Houston

Interview with Vinicius Domingues, MD, a rheumatologist in Daytona Beach, Florida

Mammen M, et al. Advisory Committee on Immunization Practices Meeting. April 23, 2021. https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2021-04-23/04-COVID-Mammen-508.pdf.

Walker M. ACIP Punts on J&J COVID Vaccine. MedPage Today. April 14, 2021. https://www.medpagetoday.com/meetingcoverage/acip/92101.

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