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Though extremely rare, the warning signs of a blood clot are different from the normal side effects that may occur following vaccination. MediaNews Group/Los Angeles Daily News via Getty Images
  • Officials are warning people who received the Johnson & Johnson COVID-19 vaccine to watch for signs of an extremely rare blood clotting disorder.
  • The warning signs of a possible blood clot differ from the normal side effects that may occur after vaccination that show your immune system is doing its job.
  • The symptoms of a possible blood clot include severe headache, abdominal pain, leg pain, and shortness of breath.
  • So far, there have been six reported cases, with symptoms showing up 6 to 13 days after vaccination.
  • After a month, the risk of any side effects from the Johnson & Johnson COVID-19 vaccine is extremely low.

On April 13, federal officials warned people who received the Johnson & Johnson COVID-19 vaccine to watch for signs of an extremely rare, severe type of blood clot in the brain that may be linked to the vaccine.

This comes after six women who received the vaccine developed cerebral venous sinus thrombosis (CVST) — a clot in the area of the brain that drains blood from the brain.

All women also had low levels of blood platelets, a condition called thrombocytopenia, according to federal officials.

It’s not clear yet whether these cases are definitely caused by the vaccine.

However, the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) recommended pausing the vaccine rollout while scientists investigate the cases and their possible connection to the vaccine.

So, what warning signs should you look for if you received this one-dose vaccine, or if you receive it when the rollout resumes?

In all six reported cases, symptoms began 6 to 13 days after vaccination.

So, “for people who got the vaccine more than a month ago, the risk to them is very low at this time,” Dr. Anne Schuchat, principal deputy director of the CDC, said during a joint media briefing with the FDA on April 13.

“For people who recently got the vaccine within the last couple weeks, they should be aware to look for any symptoms,” she added.

The symptoms include:

  • severe headache
  • abdominal pain
  • leg pain
  • shortness of breath

In guidance for healthcare professionals, the CDC also included:

  • backache
  • new neurologic symptoms such as weakness in the limbs or sudden changes in vision
  • swelling in the leg
  • tiny red spots on the skin
  • new or easy bruising

Dr. Jose Mayorga, executive director of the UCI Health Family Health Centers and an assistant clinical professor of family medicine, said people who develop any of these symptoms after vaccination should contact their doctor or seek urgent medical care.

If you seek care, tell your doctor or the emergency room staff that you have been vaccinated recently.

Treatment for this condition is different than typical treatments for other types of blood clots, many of which include the blood thinner heparin.

“With cerebral venous sinus thrombosis, heparin may be dangerous,” Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said during the media briefing.

“Alternative treatments need to be given, preferably under the guidance of physicians experienced in the treatment of blood clots.”

The warning signs of a blood clot are different from the normal side effects that may occur in the first few days after vaccination, such as:

  • headache
  • fatigue
  • muscle pain
  • nausea
  • fever

These postvaccination side effects are generally harmless, and are signs that the vaccine has produced a beneficial immune response.

If you experience any severe side effects, especially 1 to 2 weeks after vaccination, seek medical care.

“If someone ends up with a very severe headache, or any type of shortness of breath, pain in their legs, or pain in their abdomen that’s severe, they would want to seek medical attention,” said Marks during the briefing.

So far, only six cases of a blood clot in the brain after the J&J vaccine have been reported. Additional cases may be identified, but at this point, the risk is extremely low.

“It is important to compare this risk with other well-known medications, substances, and COVID-19 infection,” Mayorga said, “all of which can also cause clots.”

For example, hormonal birth control can increase the risk of blood clots. These are different types of clots than those that occurred in the six women who received the J&J vaccine.

In 2012, the FDA posted data showing that if 10,000 women used birth control pills, 3 to 9 of them would develop a blood clot.

Pregnancy also increases the risk of blood clots, particularly in the legs.

The risk of developing a blood clot from COVID-19 is also higher than the apparent risk of developing one after receiving the J&J vaccine.

Again, these are different types of clots than were seen in the six women who were vaccinated with the J&J shot.

While clinical trials test the safety of vaccines, some rare side effects may not show up until millions of people have received the vaccine.

So, the FDA and CDC continually monitor the safety of vaccines after they are approved for public use. This is done through the V-safe smartphone app and the Vaccine Adverse Event Reporting System (VAERS).

The agencies paused the J&J rollout to have time to alert healthcare professionals about what warning signs to look for in people who received this vaccine, and to look for additional cases.

The stoppage also allows the CDC’s vaccine advisory committee to review data on the six blood clotting cases.

This independent group of scientists will determine whether the cases are likely caused by the vaccine — so far, we don’t know — and if any additional steps need to be taken.

The Advisory Committee on Immunization Practice met on April 14 and will meet again next week after additional information on the cases has been gathered.

Although pausing the rollout of a vaccine in the middle of a pandemic is concerning, it’s actually a sign that the FDA and CDC’s vaccine safety monitoring systems are working.

“Our public health institutions are tasked with protecting us,” Mayorga said. “This is a great example of how committed they are to upholding this duty, even with something so rare.”