U.S. health officials called for an immediate pause in using Johnson & Johnson’s single-dose COVID-19 vaccine after six women who received it developed a rare and severe form of blood clotting.

The decision comes at a crucial juncture in the U.S. vaccination effort, which has gained speed in recent weeks at the same time a worsening outbreak driven by coronavirus variants grips the upper Midwest. And it is likely to heighten worry about vaccines around the world after similar issues caused delays in distribution of AstraZeneca Plc’s shot.

A type of blood clot in the brain called cerebral venous sinus thrombosis was seen with low levels of blood platelets in the women, who were all between the ages of 18 and 48, federal health officials said Tuesday in a statement. One of the cases was fatal and a second is in critical condition, Peter Marks, director of the Food and Drug Administration’s Center for Biologics Evaluation and Research, said in a Tuesday briefing.

More than 6.8 million doses of the single-shot J&J vaccine have been administered in the U.S. through Monday. Those vaccinated a month ago are at low risk of the complication, but people who received it in the last couple of weeks should be on the lookout for headache, leg pain, abdominal pain and shortness of breath, said Anne Schuchat, principal deputy director at the Centers for Disease Control and Prevention.

No definitive cause for the disorder has yet been determined, according to Marks, who said it was very similar with blood clotting incidents seen with the AstraZeneca Plc vaccine. It may be be tied to an immune response after vaccination that leads to an activation of platelets and clotting, he said.

Schuchat, meanwhile, emphasized that the clotting events with low platelet counts are not being seen with Pfizer Inc.-BioNTech SE vaccine and the Moderna Inc. shot.

“We are recommending a pause in the use of this vaccine out of an abundance of caution,” the CDC and the FDA said in a joint statement Tuesday. “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.”

A key reason for the pause, FDA officials said in the briefing, is that the unusual constellation of symptoms means that the normal treatment for blood clots, the blood-thinner heparin, could make the problem worse or even kill a patient getting the rare adverse reaction.

Europe Rollout

J&J said in a statement that it’s decided to delay the rollout of its vaccine in Europe. Walgreens Boots Alliance Inc. and CVS Health Corp., which have been administering the shot in drugstores, each said they would pause immediately. J&J shares fell 2.6 per cent to US$157.39 at 10:56 a.m. in New York trading.

The Biden administration has been seeking to accelerate vaccinations in recent weeks as new coronavirus variants heighten the risk that the pandemic could drag on. White House COVID-19 response coordinator Jeff Zients said that the pause “will not have a significant impact on our vaccination plan.”

“We expect it to be a matter of days for this pause,” the FDA’s Janet Woodcock said at the Tuesday briefing.

The decision could increase vaccine hesitancy, which is seen as a significant obstacle for health policy makers as they attempt to broaden immunizations. The J&J shot has been popular due to its single dose -- vaccines made by Pfizer Inc.-BioNTech SE and Moderna Inc. require two doses spaced several weeks apart.

Many of the J&J doses in the U.S. have been targeted to harder-to-reach populations through the federal government’s distribution program that targets pharmacies, federally qualified health centers and other locations.

The CDC will convene a meeting Wednesday of its Advisory Committee on Immunization Practices to review the cases and assess their potential significance. The FDA will review the CDC assessment as it also investigates these cases, according to the statement.

Global Impact

The recommended pause may also further complicate the global vaccination push, coming just weeks after a vaccine relying on a similar approach and developed by AstraZeneca and the University of Oxford raised similar concerns in Europe. Clotting fears have prompted many countries to impose age restrictions on use of the Astra vaccine, which was co-developed with the University of Oxford.

J&J has recently begun shipments to the European Union. The bloc’s drugs regulator, the European Medicines Agency, didn’t immediately respond to a request for comment.

The EMA said Friday that it is assessing blood clots in people who received the J&J vaccine, after four serious cases of unusual clots accompanied by low blood platelets, one of which was fatal, emerged.

On Friday afternoon, FDA spokesperson Abby Capobianco said in an emailed statement that the agency hadn’t found a causal relationship between the J&J vaccine and the six cases of thromboembolic events, and was continuing its assessment of the cases

“Our analysis of the data will inform the potential need for regulatory action,” she said. The agency had shared information with the EMA, which became “the basis” of the EU probe, she said.

Asked Monday if further analysis had determined a causal relationship, the FDA declined to comment.

Collecting Information

A handful of vaccination sites in Georgia, Colorado and North Carolina temporarily stopped administering the J&J vaccine last week in the wake of adverse reactions, including nausea and fainting. Such side effects are typical of vaccination, health authorities said at the time.

“Analysis by the CDC did not find any safety issues or reason for concern,” the North Carolina Department of Health and Human Services said Friday in a statement. “The CDC recommends continuing to administer the vaccine.”

That same day, J&J said in a statement that it was collecting information, assessing adverse events, and sharing reports with the FDA and other health agencies.

“There is no greater priority than the safety and well-being of the people we serve,” the company said. “This is part of the established process to inform health authorities’ comprehensive surveillance programs that monitor the overall safety of medicines, as well the vaccines authorized for use against this pandemic.”

--With assistance from Naomi Kresge, Reg Gale, Drew Armstrong and Josh Wingrove.