Federal health officials have lifted the pause on Johnson & Johnson’s one-shot Janssen COVID-19 vaccine, but women younger than age 50 years should know the risks of the rare but real side effect. Fifteen women experienced the reaction and three have died.

The reaction has not been observed in the other two vaccines approved for use in the United States.

The decision to recommend the Janssen vaccine again came after an April 23 meeting of the CDC’s Advisory Council on Immunization Practices. Previously, at an April 13 press conference, the CDC and the FDA announced the pause after six blood clot cases were reported in the United States after more than 6.8 million doses were administered.

As of April 23, more than 8 million doses of the Janssen COVID-19 vaccine had been administered in the United States. An extensive review after the pause revealed 15 cases of blood clots, including three women who died and seven who remain hospitalized. Of the latter, four patients are in intensive care. Five women survived the reaction and were discharged.

“The reports reviewed all occurred in women between 18 and 59 years old, with a median of 37 years,” the CDC stated. “These reports represent a reporting rate of seven such events per 1 million vaccinations among women 18 through 49 years old, and a rate of 0.9 per 1 million vaccinations among women 50 years and older. For all women, this is a rare adverse event. For women 50 years and older and men of all ages, the adverse event is even more rare. Reports show that symptoms of this adverse event started between six and 15 days after vaccination.”1

A review of all available data show the Janssen COVID-19 vaccine’s benefits outweigh its risks. “However, women younger than 50 years old should be aware of the rare but increased risk of this adverse event and that there are other COVID-19 vaccine options available for which this risk has not been seen,” the CDC warned.

For three weeks after receiving the vaccine, watch for possible symptoms of this reaction, the CDC advised. People should seek medical care immediately if they experience one or more of these symptoms:

  • severe or persistent headaches or blurred vision;
  • shortness of breath;
  • chest pain;
  • leg swelling;
  • persistent abdominal pain;
  • easy bruising or tiny blood spots under the skin beyond the injection site.

A key reason for the vaccination pause was to alert providers that normal anticoagulation treatment for blood clots like heparin might worsen this rare reaction.

“The issue here with these types of blood clots is that if one administers the standard treatments, one can actually cause tremendous harm, or the outcome can be fatal,” Peter Marks, MD, PhD, director of the Center for Biologics Evaluation and Research at the FDA, said at the press conference. “We want to make sure if providers see people with low blood platelets or if they see people who have blood clots, they need to inquire about the history of recent vaccination.”

In these cases, cerebral venous sinus thrombosis (CVST) was seen in combination with thrombocytopenia (low levels of blood platelets). The combination of CVST clots and low platelet counts is unusual, Marks said.

“The combination here — the real thing that is so notable — is not cerebral venous sinus thrombosis or thrombocytopenia,” he said. “Those two things can occur. It is the occurrence together that makes a pattern, and that pattern is very similar to what was seen in Europe with [AstraZeneca vaccine].”

Both the Janssen and AstraZeneca vaccines use benign adenoviruses to spur an immune reaction to SARS-CoV-2 spike proteins and prevent infection.

The CDC previously issued an alert on the issue that included these recommendations for clinicians2:

  • Be suspicious of symptoms of serious thrombotic events or thrombocytopenia in patients who have recently received the Janssen vaccine, including severe headache, backache, new neurologic symptoms, severe abdominal pain, shortness of breath, leg swelling, petechiae, or new or easy bruising. Test platelet counts and screen for evidence of immune thrombotic thrombocytopenia.
  • In patients experiencing such an event following the Janssen vaccine, use a screening PF4 enzyme-linked immunosorbent assay as would be performed for autoimmune HIT [heparin-induced thrombocytopenia]. The CDC strongly recommends consulting a hematologist.
  • Do not treat patients with CVST following receipt of the Janssen vaccine with heparin, unless HIT testing is negative.
  • If HIT testing is positive or cannot be performed, non-heparin anticoagulants and high-dose intravenous immune globulin should be strongly considered.


  1. Centers for Disease Control and Prevention. CDC recommends use of Johnson & Johnson’s Janssen COVID-19 vaccine resume. Updated April 25, 2021.
  2. CDC Health Alert Network. Cases of cerebral venous sinus thrombosis with thrombocytopenia after receipt of the Johnson & Johnson COVID-19 vaccine. April 13, 2021.