The military on Tuesday swiftly halted administering Johnson & Johnson's COVID-19 vaccine, hours after federal health officials announced that a handful of people who had received the vaccine experienced "rare and severe" blood clots.
The government's surprise decision could deal a blow to the military's efforts to vaccinate troops in remote locations overseas, which was dependent on increasing availability of the Johnson & Johnson vaccine.
And it has the potential to undermine some troops' confidence in COVID-19 vaccines at a time when concerns about such skepticism are on the rise.
The Centers for Disease Control and Prevention and Food and Drug Administration announced Tuesday morning that six women who had received the J&J vaccine experienced a type of blood clot called cerebral venous sinus thrombosis, along with low levels of blood platelets.
All six women, who were between the ages of 18 and 48, were in the United States and experienced symptoms six to 13 days after vaccination.
The government said these blood clot cases appear to be extremely rare. As of Monday, there had been more than 6.8 million doses of the J&J vaccine administered in the U.S.
But the health agencies recommended that administration of the vaccine be paused "out of an abundance of caution" until the six cases are further investigated.
Pentagon Press Secretary John Kirby said in a statement issued later Tuesday morning that the Defense Department had immediately paused use of the J&J vaccine.
Kirby said the Pentagon does not yet know how long the pause might last.
"The safety of our force and their families is a top priority for the department," Kirby said. "We are communicating this pause to our military health facilities and are reviewing our global vaccine distribution to address this issue and ensure we can continue to provide vaccines to our DoD population at home and abroad."
But halting the use of the Johnson & Johnson vaccine deals a particular blow to the military's efforts to swiftly vaccinate the rest of its force, civilians and family members. The J&J vaccine's more relaxed dosage and refrigeration requirements made it easier to distribute around the world, particularly to troops in remote locations.
Unlike the Pfizer and Moderna vaccines, which require two shots to take full effect, Johnson & Johnson's vaccine requires only one.
Pfizer's vaccine also must be stored in ultra-cold refrigerators, making it harder to ship overseas. In an April 8 news conference, Maj. Gen. Jill Faris, deputy surgeon general for the Army National Guard, said that the military has primarily been shipping Moderna's vaccine to Europe for that reason.
But as Johnson & Johnson's single-dose vaccine became more available, Faris said, it would be easier to vaccinate soldiers in remote locations in Europe and Africa that do not have Army medical treatment facilities nearby.
There are concerns about some troops' reluctance to get vaccinated, which could make it harder to achieve full immunity in the ranks. According to statistics released by the Marine Corps, 48,000 Marines, nearly 40% of those who have been offered the vaccine, have turned it down.