By Peter Loftus

U.S. health authorities recommended a pause in the use of Johnson & Johnson's Covid-19 vaccine in order to investigate rare but severe cases of blood clots.

The U.S. Food and Drug Administration and the U.S. Centers for Disease Control and Prevention announced the move Tuesday, after finding that six women between the ages of 18 and 48 years who got the vaccine had developed blood clots.

A panel of outside experts will meet Wednesday to review the matter for the CDC, while the FDA will also conduct an investigation.

The FDA said it was recommending a pause in the use of the J&J vaccine "out of an abundance of caution."

The halt comes at a dangerous time during the pandemic, as health authorities race to vaccinate as many people as possible before variants develop that can evade the shots. Among the biggest hurdles is overcoming hesitancy to get vaccinated.

The specter of blood clots could add to concerns about the safety of the shots, though vaccines from Pfizer Inc. and its partner BioNTech SE and from Moderna Inc. have proven relatively safe so far.

J&J's vaccine was the third authorized for use in the U.S. It uses the same technology as a vaccine from AstraZeneca PLC and the University of Oxford, not yet authorized in the U.S. that has also been linked to rare cases of blood clots.

In the case of J&J's shot, six women ages 18 to 48 developed blood clots after taking the J&J vaccine, the FDA and CDC said. The clots developed 6 to 13 days after vaccination. The women also had in their blood low counts of platelets, which help with clotting.

Given the nature of the side effect, doctors shouldn't use the normal course of clotting treatment, involving a drug called heparin, the FDA and CDC said. "In this setting, administration of heparin may be dangerous, and alternative treatments need to be given," they said.

Write to Peter Loftus at peter.loftus@wsj.com

(END) Dow Jones Newswires

04-13-21 0816ET