The US is pausing the Johnson & Johnson coronavirus vaccine rollout, citing concerns over rare blood clots

• The CDC and the FDA have recommended pausing the rollout of J&J's single-dose COVID-19 vaccine.
• They cited an "abundance of caution" related to six cases of blood clots in people who got the shot.
• The US has administered 6.8 million doses of the vaccine so far.

The US is putting an immediate pause on the rollout of Johnson & Johnson's single-dose COVID-19 vaccine, citing reports of blood clots in six people who received the shot.

The people who developed the clots were all women between 18 and 48 years old, and they experienced the adverse reaction six to 13 days after their vaccinations, according to the Centers for Disease Control and Prevention and the Food and Drug Administration.

The federal government has the authority to pause the vaccine rollout that it is carrying out directly but not those run by state agencies. After the announcement on Tuesday, however, US states and localities began to pause their J&J vaccinations as well.

Dr. Janet Woodcock, the FDA's acting commissioner, said in a briefing on Tuesday that she expected this pause to be temporary.

"We expect it to be a matter of days for this pause," she said.

About 6.8 million doses of the J&J vaccine have already been given in the US. J&J had said it was on track to provide close to 100 million doses to the US by the end of May.

The White House said a pause would "not have a significant impact on our vaccination plan," since the two other vaccines authorized in the US - from Pfizer-BioNTech and Moderna - are "more than enough supply to continue the current pace of vaccinations."

J&J shots have accounted for less than 5% of vaccinations so far in the US, the White House said in a statement.

US regulators are investigating the rare blood clots

The CDC and the FDA are investigating the reports of blood clots, and the CDC is convening an outside panel of experts to weigh in Wednesday.

"Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution," the agencies said in a statement.

J&J said in a statement that it's working with regulators and medical experts to look into the blood-clot reports. The company said it's also delaying the rollout of the vaccine in Europe.

"We have been working closely with medical experts and health authorities, and we strongly support the open communication of this information to healthcare professionals and the public," J&J said.

All six of the people with the reported adverse effect had a rare type of blood clot in the brain called a central venous sinus thrombosis in combination with low levels of blood platelets. Platelets help the body with blood clotting after an injury.

Dr. Peter Marks, the director of the FDA's Center for Biologics Evaluation and Research, said that one of the blood-clotting cases was fatal and that one patient was in critical condition.

Marks also noted that treatment for CVST could be different than for other blood clots, since blood thinners like heparin can be dangerous.

The European Medicines Agency announced Friday that it was also closely monitoring J&J's vaccine after four cases of blood clots were reported in people who got the shot in the US. One of those cases involved a volunteer in a clinical trial, while the other three were people who got the shot after it was authorized by the FDA. One of the four died. These cases are included in the group the FDA and the CDC are investigating.

Dr. Anne Schuchat, the CDC's principal deputy director, said people who had received the J&J vaccine in the past couple of weeks should keep an eye out for symptoms related to blood clots, which include severe headache, abdominal pain, leg pain, or shortness of breath.

For those who've received the vaccine more than a month ago, Schuchat said, the risk of developing a blood clots is "very low."

AstraZeneca's vaccine may be linked to unusual blood clots as well

Regulators in the European Union and the UK said last week after an investigation that potentially fatal blood clots could be a rare side effect of the AstraZeneca vaccine. Both regulators concluded that the benefits of AstraZeneca's vaccine outweighed the risk. Still, some countries have halted the use of that vaccine or restricted it to certain populations.

AstraZeneca's vaccine has not yet been authorized in the US.

Marks said on Tuesday that the blood clots related to AstraZeneca's shot were part of the reason the FDA and the CDC recommended a pause after seeing similar rare reactions in people who received the J&J vaccine. Both are viral-vector vaccines.

"It's their occurrence together that makes a pattern. And that pattern is very, very similar to what was seen in Europe with another vaccine," Marks said. "We have to take the time to make sure we understand this complication and we address it properly."

Read more: COVID-19 Vaccine Tracker: AstraZeneca's shot proves safe and effective, and is headed to the FDA

Other, more mild reactions to J&J's shot have popped up, too

Last week, some vaccination sites in the US paused their use of the J&J shots for reasons unrelated to blood clots. Sites in Colorado, North Carolina, and Georgia temporarily stopped administering the shot after about 45 people experienced minor adverse reactions involving nausea, dizziness, fainting, or lightheadedness within 15 minutes.

The FDA's full review of J&J's trial data, which was released in February, found the shot to be effective and safe.

"Any pause in vaccination shows that the system is working because we're not trying to sweep anything under the rug," Arnold Monto, an epidemiologist at the University of Michigan's School of Public Health, previously told Insider.