Update (April 26): On April 23, officials at the US Centers for Disease Control and Prevention and the Food and Drug Administration lifted the pause on the Johnson & Johnson vaccine. The FDA will add a warning about the risk for developing rare blood clots to fact sheets given to providers and patients, Reuters reports.

Following six cases of rare blood clots in women who had received Johnson & Johnson’s COVID-19 vaccine in the US, the Food and Drug Administration and the Centers for Disease Control and Prevention have recommended a temporary suspension of its rollout. In a statement released today (April 13), the agencies announced that they are investigating the reports of cerebral venous sinus thrombosis (CVST), the same condition observed as an extremely rare side effect of AstraZeneca’s COVID-19 vaccine.

“Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution,” the statement reads. “This is important, in part, to ensure that the health care provider community is aware of the potential for these adverse events and can plan for proper recognition and management due to the unique treatment required with this type of blood clot.” Specifically, the anticoagulant drug heparin should not be used to treat patients in these situations, the agencies recommend. Geoffrey Barnes, an anticoagulation expert at the University of Michigan, tells the Associated Press that other blood thinners and perhaps an antibody infusion would be appropriate instead.

Nearly 7 million doses of Johnson & Johnson’s one-shot COVID-19 vaccine have been administered so far in the US. The six reports of CVST were all in women ranging in age from 18 to 48 who developed the clot between 6 and 13 days after vaccination. In addition to clots, these individuals also had low platelets counts.

AstraZeneca and Johnson & Johnson both designed their vaccines to deliver a bit of SARS-CoV-2 genetic material via an adenoviral vector, although the viral ferries are different types of adenovirus. 

According to Reuters, of the 34 million people who received AstraZeneca’s COVID-19 vaccine, 169 developed CVST, and the European Medicines Agency considers clotting to be a rare side effect of the immunization. “The reported combination of blood clots and low blood platelets is very rare,” the EMA said in a statement last week, “and the overall benefits of the vaccine in preventing COVID-19 outweigh the risks of side effects.”

See “Blood Clots a Very Rare Side Effect of AstraZeneca Vaccine: EMA

It’s not clear why these blood clots might occur following vaccination. “Of course, there are hypotheses: maybe it’s something with the vector, maybe it’s an additive in the vaccine, maybe it’s something in the production process. . . . I don’t know,” Sabine Eichinger, a haematologist at the Medical University of Vienna, told Nature last week. “It could be any of these things.”

According to the AP, New York State, the city of Dallas, and mass vaccination events coordinated by the federal government have put their Johnson & Johnson vaccine programs on hold.