Editor’s note: This article was published on April 30, 2021. COVID-19 information and recommendations are subject to change. For the most up-to-date information, visit the Centers for Disease Control and Prevention (CDC) website or view our most recent COVID-19 blog posts.

On April 13, the Centers for Disease Control and Prevention (CDC) and the Federal Drug Administration (FDA) recommended the U.S. pause the use of the Janssen (Johnson & Johnson) COVID-19 vaccine to investigate six reported U.S. cases of rare and severe blood clots.

The CDC and FDA have since recommended resuming administration of the Janssen COVID-19 vaccine following a thorough review of benefits and risks.

A CDC advisory committee reviewed the evidence and concluded that the benefits greatly outweigh the risks of the Janssen vaccine. They found resuming use of the Janssen vaccine would be expected to prevent thousands of ICU admissions and hundreds of deaths due to COVID-19 over six months.

While the U.S. did not know much when the CDC and FDA paused the Janssen COVID-19 vaccine, we now know much more about this rare complication and who is at risk.

“This is a new syndrome called TTS. It is an unusual combination of a blood clot with low platelet counts,” said Dr. Edward Belongia, epidemiologist and COVID-19 researcher with Marshfield Clinic Research Institute. “It is rare, but serious and potentially life-threatening. Most cases of TTS have involved veins in the brain.”

The CDC and FDA have now found 15 U.S. cases of TTS in women between the ages of 18-59. Thirteen of the women were under 50 years old. These women experienced symptoms 6-15 days after receiving the Janssen COVID-19 vaccine. Doctors in Europe have discovered a similar syndrome as a rare complication of the AstraZeneca vaccine (the AstraZeneca vaccine is not used in the U.S.). Researchers have not found any TTS cases in people who received the mRNA COVID-19 vaccines by Pfizer/BioNTech or Moderna.

Why the CDC and FDA paused the vaccine

The typical treatment for blood clots is heparin. It helps the body prevent blood clots and breaks down existing clots. However, for patients with TTS, use of heparin can actually make the problem worse.

One of the primary reasons the CDC and FDA paused the Janssen (Johnson & Johnson) COVID-19 vaccine was to help doctors identify TTS so they can treat these patients correctly and safely.

“These cases were initially identified through the Vaccine Adverse Event Reporting System, or VAERS,” Dr. Belongia said. “VAERS reports do not prove that a vaccine causes a particular problem, but they serve as an early warning system for safety concerns that need to be investigated. In this case, the pause allowed the CDC and FDA to alert doctors and investigate the problem before making a recommendation. CDC and FDA will continue to monitor the safety of all COVID-19 vaccines through VAERS, the Vaccine Safety Datalink and other safety monitoring systems.”

What are the signs of TTS?

Seek care immediately if you develop any of these symptoms up to three weeks after receiving the Janssen COVID-19 vaccine:

• A severe headache
• Shortness of breath
• Chest pain
• Leg swelling
• Persistent abdominal pain
• Blurred vision
• Easy bruising or bleeding

This is especially important for women under age 50. These symptoms typically begin 1-2 weeks after vaccination. You should not confuse these symptoms with the common and minor side effects that occur 1-2 days after vaccination. These symptoms should resolve within a couple days.

For more information about whether you should be concerned, talk to your doctor.

Your chances of getting a blood clot

The chance of a woman between the ages of 18-49 getting a blood clot after receiving the Janssen COVID-19 vaccine is very rare – less than seven women per million vaccinated. The risk is about one case per million among women 50 and older. This risk is extremely low compared to the benefit. For every 1 million doses of the Janssen vaccine given to women 18–49 years, CDC estimates that 297 hospitalizations, 56 ICU admissions and six deaths related to COVID-19 could be prevented. Researchers expect about seven cases of TTS would occur in these women.

Every medication and vaccine comes with certain risks.

“Communicating risks is one of the primary responsibilities a doctor has every day when advising patients about medications or vaccines,” Dr. Belongia said. “It is impossible to eliminate all risk, but it is important for each person to understand the balance of benefits and risks. For the Janssen vaccine, it’s clear that the benefits outweigh the risks.”

CDC is updating patient education materials to make sure women under age 50 know about the increased risk for TTS. These women should be aware that other COVID-19 vaccines are available if they are concerned about the risk of TTS. The FDA has also added a warning about the risk of TTS in Janssen vaccine recipients.