Federal health authorities recommended Tuesday that providers temporarily stop administering the Johnson & Johnson COVID-19 vaccine while they investigate a potential link to very rare blood clots.
Six women between the ages of 18 and 48 developed blood clots after receiving the Johnson & Johnson vaccine. One person died, and another is in critical condition, the Food and Drug Administration said.
"We are recommending this pause while we work together to fully understand these events, and also so we can get information out to health care providers and vaccine recipients," Dr. Janet Woodcock, acting FDA commissioner, said during a briefing Tuesday.
The review is expected to be completed quickly, lasting "a matter of days," officials said.
The clots are considered extremely rare. Overall, more than 6.8 million people in the U.S. have received the Johnson & Johnson vaccine.
No such issues have been reported with the COVID-19 vaccines made by Pfizer-BioNTech or Moderna, health officials said.
In an updated statement Tuesday, Johnson & Johnson said it was "aware of an extremely rare disorder involving people with blood clots in combination with low platelets in a small number of individuals who have received our COVID-19 vaccine."
"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," the company said.
The pause "will not have a significant impact" on the country's vaccination plan, Jeff Zients, the White House COVID-19 Response Coordinator, said in a statement, adding that there is "more than enough supply" to meet President Joe Biden's "goal of 200 million shots by his 100th day in office."
The Centers for Disease Control and Prevention will convene its vaccine advisory committee on Wednesday to look into the possible link between the Johnson & Johnson vaccine and blood clots.
"I know that the information we're providing today is going to be very concerning to Americans who have already received the Johnson & Johnson vaccine," Dr. Anne Schuchat, principal deputy director for CDC, said during the briefing. "For people who got the vaccine more than a month ago, the risk is very low."
For those who recently got the Johnson & Johnson vaccine, Schuchat advised to be aware of sudden, severe headaches, abdominal pain or shortness of breath.
The six people who experienced clots were all women whose symptoms began six to 13 days after vaccination, the FDA and CDC said. They developed a type of blood clot in the brain called a cerebral venous sinus thrombosis, or CVST, as well as low levels of blood platelets.
Usually, people with blood clots are treated with a blood thinner called heparin. But in rare cases, the immune system reacts to the drug in a phenomenon called heparin-induced thrombocytopenia, which increases the risk for clotting.
For this reason, giving heparin to those who develop CVST might actually be dangerous. Other blood thinners or medications may be more appropriate, officials said.
In an interview Tuesday, a 29-year-old woman in Nashville, Tennessee, described a severe reaction to the Johnson & Johnson vaccine.
It was not clear if the woman, who spoke to NBC News on the condition of anonymity, was one of the six people in question. In a statement, the FDA said it was unable to "confirm or share personally identifiable information," citing HIPAA rules.
The woman said she got the vaccine on March 19 at a Nashville hospital and initially experienced normal reactions, such as a fever. Then, about a week later, she went to the emergency room with uncontrollable shaking, troubling breathing and a fever.
“Since then, (I) have just had a few strange symptoms that could have been chalked up to a viral infection, mostly just a headache that persisted for two weeks, and some leg pain and swelling,” she said in a phone interview from her hospital bed, “and then also just some random bruises that would appear throughout my body."
When she went to the ER the week after her shot, she said that doctors did not find anything strange in her blood work, and nothing at that point to suggest clotting or low platelets — so she went home. But the symptoms continued to worsen over the following week.
"Then, when I went back to the ER, it was because my leg was so swollen that I couldn't really walk, so at that time they found the DVTs or the clots in my legs,” she said, saying they were in both legs and she had a low platelet count.
She added that she has been in the hospital for about a week, and she was in the intensive care unit until Sunday. When asked if she had any regrets, she said, "No, absolutely not. I just want to be clear, vaccines are good and they save lives."
The call for a pause mirrors a situation that occurred in Europe last month.
Some European governments paused the rollout of the Oxford University-AstraZeneca vaccine after a similarly tiny number of people suffered CVST blood clots. The AstraZeneca vaccine uses the same type of technology as the Johnson & Johnson shot.
Most European countries have since resumed administering the shot, but with a messy patchwork of different age restrictions and other guidelines. There is some evidence that as a result of this widely publicized fear, public trust in the AstraZenica vaccine is lower than others that are available.
The situation in Europe has divided experts, with some saying that governments could not ignore the possible link, and others saying that the pause may have increased infections and hesitancy, damage that far outweighs the dangers from these extremely rare side effects.
Meanwhile, Johnson & Johnson also announced it will "proactively delay the rollout of our vaccine in Europe," and that it will pause all vaccinations in clinical trials while it updates guidance for the trials.
This article was originally published on NBCNews.com.