Already Got the J&J Vaccine? What You Need to Know Now

If your J&J shot was more than a month ago, you most likely have nothing to worry about, Fauci said on Tuesday. If it was more recent, pay attention to new or worsening symptoms in the days and weeks that follow your vaccine, but “don't get an anxiety reaction because, remember, it's less than one in a million,” Fauci said.

A severe headache — “probably worse than someone has ever experienced” — is one warning sign for this type of adverse event, Moss said. So is abdominal pain and shortness of breath — these symptoms warrant medical attention. In expanded clinic guidance, the CDC also says health care providers should be on the lookout for leg swelling, backaches, new neurologic symptoms, tiny red spots on the skin, and new or easy bruising.

3. Treatment for this blood clot is different from others

An anticoagulant drug, or blood thinner, called heparin is commonly used to treat blood clots. But when it comes to the clots that are under review, heparin may cause more harm than good. Administering it “would be a mistake in this situation because it could be dangerous and make the situation much worse,” Fauci noted.

Instead, a CDC health alert says non-heparin anticoagulants and high-dose intravenous immune globulin (a solution made from blood plasma that contains antibodies) should be considered in treatment of patients who develop this rare blood clot after having received the J&J vaccine, although treatment should be individualized, the CDC's Principal Deputy Director Anne Schuchat said. Consulting with hematology specialists is also recommended.

"So the pause not only allows us to take a look at the cases and learn more, but it is also a signal out there to help the physicians,” Fauci said.

4. It's unclear if certain people are more at risk

Health experts don't yet know if there are certain risk factors or characteristics that may put vaccinated individuals at greater risk for this severe blood clot, but Fauci said that's something that will be studied.

"If they find some common denominators among the women who were involved that might be synergizing and, essentially, enabling this type of an adverse event, they may know that for those who don't have that, [the vaccine] may be much safer,” he explained. “There may be clues when you go down and really get granular about every single case."

Investigators will also want to study any mechanisms, or processes, at play. One theory: J&J's viral vector vaccine, which is different from Pfizer-BioNTech and Moderna's messenger RNA (mRNA) vaccines, may trigger an immune response in which antibodies affect the blood platelets, Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said in an April 13 briefing. To date, there are no reports of blood clots that have been linked to the authorized mRNA vaccines.

5. Pause indicates the ‘system is working'

While some people may find the pause concerning, Moss said it's an indication “the vaccine safety surveillance system is working."

The CDC encourages all vaccinated individuals to report any side effects through its V-safe app and any adverse reactions to the Vaccine Adverse Event Reporting System (VAERS). Health care providers should also report adverse events.

"Our FDA is internationally known for their capability of making sure that we have the safest products out there,” Fauci said. “You want to make sure that safety is the important issue here.”

Rachel Nania joined AARP as a health and medicine writer in 2019 after spending several years as a radio reporter and editor in Washington, D.C. She is the recipient of a 2018 Gracie Award and a 2019 regional Edward R. Murrow Award, and participated in a 2019 dementia fellowship with the National Press Foundation.