Federal health officials on Tuesday ordered a temporary stop on administering COVID-19 vaccine produced by Johnson & Johnson after reports of six people experiencing a blood clotting disorder.
The Centers for Disease Control and Prevention and the Food and Drug Administration said in a joint statement the agencies were "recommending a pause" while more data pertaining to the single-shot vaccine is collected and reviewed.
Nearly 7 million people in the U.S. have received J&J shots, and roughly 9 million more doses have been distributed for future use, according to CDC data.
There have been no blood clotting cases reported so far tied to the Pfizer or Moderna vaccines, but if you've received the J&J single-dose vaccine -- and are concerned over potential side effects -- here's what you need to know.
MORE: Pausing Johnson & Johnson vaccines shows monitoring system is working: ExpertsIf you're one of those individuals -- and concerned over potential side effects -- here's what you need to know.
To date, only six cases have been reported -- all women, aged 18 to 48.
"I don't think people should be alarmed," Dr. William Schaffner, a professor of preventative medicine and infectious diseases at Vanderbilt University in Nashville, told ABC. "This is -- at the moment -- an association that is going to be further investigated."
Extrapolated, that very limited data set shows that a person's odds -- at least so far -- of similar complications are less than 1 in 1 million. Another study estimated that roughly 1 in 6 people hospitalized with COVID-19 experienced some kind of clotting.
Cerebral venous sinus thrombosis, or CVST, and low platelet counts were seen in individuals who've received the J&J vaccine. CVST is rare and seen in the draining venous sinuses of the brain. It's often associated with pregnancy, rare clotting disorders and hormonal contraception.
Experiencing a headache within a few hours, or even days, after getting the shot is often a common side effect -- not a serious issue.
Headaches are "much more likely to be from a more common entity ... like a vaccine side effect," said Dr. Todd Ellerin, an instructor at Harvard Medical School and an infectious disease doctor at Boston's South Shore Medical Center..
In Johnson & Johnson's February 2021 briefing with the FDA, one of the most commonly reported adverse effects to the vaccine was headache. But, as Ellerin noted, you should alert a health care professional if you "develop persistent headaches or chest pain, or shortness of breath or abdominal pain."
Medical professionals said symptoms associated with this type of blood clotting will be severe, and, as Ellerin noted, you should alert a health care professional if you "develop persistent headaches or chest pain, or shortness of breath or abdominal pain."
Vaccine side effects typically are felt within the first 24 to 48 hours after receiving a shot. If you're feeling what may be a severe or persistent new side effect outside of those first few days, contact a doctor, experts said.
Unlike the typical flu-like symptoms people experience after the vaccine, this "should be distinguishable," CDC Director Dr. Rochelle Walensky said on a call Tuesday with state governors. "Every one of these cases have presented at or after six days. ... What we're talking about is severe headaches that occur in the six-to-13-day period."
Experts time and again have emphasized that vaccines are overwhelmingly safe, but did advise keeping tabs on how you're feeling -- "anything that feels different in movements, strength, that's asymmetric on one side of the body vs. another," Dr. Deepak Srivastava, president of the Gladstone Institutes and a cardiologist, told ABC News. "But it's such a rare event that I wouldn't want millions of people to be alarmed."
There's no reason to take painkillers or do anything else to reduce your risk because it's already so small, experts agreed.
"If you're not having any symptoms, there isn't a prophylactic measure to take," said Dr. Laura Finn, director of hematology and bone marrow transplant at Ochsner Health. In fact, Finn said it's best to avoid painkillers like aspirin until the FDA and CDC determine what caused the rare clotting events.
Experts interviewed by ABC News applauded the FDA and CDC for being transparent about a potential safety concern.
Next, the CDC will convene its outside advisory committee to investigate whether there's a true link and offer recommendations.
FDA acting Commissioner Dr. Janet Woodcock said Americans can expect an update in "a matter of days."
"While it's concerning," said Dr. Jennifer Haythe, an associate professor of medicine, co-director of Columbia Women's Heart Center and a critical care cardiologist, "it's still overwhelmingly safe."
Should someone who's received the J&J jab feel "any symptoms of stroke -- blurry vision, difficulty talking, loss of strength in your arms or legs, fainting or loss of consciousness," they should call a doctor or 911 immediately.
While this specific instance of CVST has been rare -- again, fewer than 1 in 1 million vaccine recipients -- CVST is a risk of medication many Americans ingest daily: oral contraceptives.
"Oral contraceptive therapy is a known risk factor for CVST," Finn said. "This type of blood clot by location is actually rare, but it is more likely to occur in women than in men."
Olivia Davies, a fourth-year student at the Medical College of Wisconsin and who will be starting her residency at Massachusetts General Hospital this summer, is a contributor to the ABC News Medical Unit. ABC News' Sasha Pezenik contributed to this report.