US regulators on Tuesday recommended an immediate pause in Johnson & Johnson's vaccine rollout.
The CDC is investigating a possible association between the shot and six cases of rare blood clots.
Medical experts said the same concerns did not apply to Pfizer-BioNTech's and Moderna's shots.
For the first time in the US's vaccine rollout, regulators have pumped the breaks on an authorized coronavirus shot.
On Tuesday, the Centers for Disease Control and Prevention and the Food and Drug Administration together recommended a temporary pause in the distribution of Johnson & Johnson's vaccine while regulators investigate six reports of unusual blood clots.
All six of those cases involved women between 18 and 48 who developed central venous sinus thrombosis, an exceedingly rare type of clot in the brain. One of the women died, and another is in critical condition.
Experts aren't sure why these blood-clot cases developed - or even whether there's a definite cause-and-effect relationship with the vaccine. But there's no reason to suspect that rare clots would be associated with the US's other authorized vaccines, they said.
"Instead of taking away confidence from other vaccines, this should actually inspire further confidence in the oversight of these vaccines and definitely should not deter individuals scheduled for either Pfizer and Moderna vaccines to go through with their appointments," Dr. Vivek Cherian, an internal-medicine physician in Baltimore, told Insider.
Roughly 68 million Americans have been fully vaccinated with either the Pfizer-BioNTech or Moderna shot since December, and nearly 185 million doses have been administered in total. In that time, US regulators haven't discovered any significant safety concerns among people who've gotten those vaccines. (The CDC tracks vaccine side effects through a tool called V-safe, and healthcare providers are required to report adverse reactions via an online system called VAERS.)
"There's a lot of close scrutiny of these vaccines," Peter Gulick, an associate professor of medicine at Michigan State University, told Insider.
He added that Pfizer's and Moderna's shots were "months ahead of J&J" in their production and rollout and regulators had yet to see any red flags. Even his patients with HIV, Gulick said, haven't reported severe reactions.
J&J's vaccine, by contrast, was introduced more recently: It was authorized in late February and has been administered to just under 7 million Americans so far.
"When you start vaccinating people, you have a much, much, much larger sample size - in the millions," Cherian said. J&J's clinical trial, by contrast, had just under 44,000 people. "So when you have these exceedingly rare complications, sometimes you don't see that until you actually start."
Pfizer's and Moderna's shots carry 'virtually no risk' of clotting
In an average year, just two to five out of every 1 million Americans develop central venous sinus thrombosis.
J&J's vaccine hasn't come close to exceeding these normal background levels yet, with fewer than one CVST case for every million doses reported so far. But researchers have also seen some CVST cases among people who got AstraZeneca's coronavirus vaccine, which bears many similarities to J&J's.
By early April, European medical officials had identified 169 CVST cases out of more than 34 million people in the European Union who'd received AstraZeneca's shot. That's still just five cases per million shots, but it was enough for European regulators to conduct an investigation. The UK now recommends that people under 30 years old seek out other shots, if available.
Both AstraZeneca's and J&J's shot are viral-vector vaccines, which introduce a coronavirus gene into the body using a genetically engineered common-cold virus. Some scientists are wondering whether the platform itself could be linked to clotting.
Pfizer's and Moderna's vaccines, on the other hand, rely on mRNA technology, which uses a snippet of the coronavirus' genome to trigger an immune response.
At an American Medical Association webinar on Tuesday, Peter Marks, the director of the FDA's Center for Biologics Evaluation and Research, said the US had identified just three CVST cases among the 68 million people who got the double-dose mRNA vaccines.
In a Tuesday statement, Moderna also said it had carried out its own comprehensive analysis and found no association between its shot and CVST or other types of clotting.
"It's safe to say there's virtually no risk in individuals who received mRNA-based [vaccines], but we don't really have that information yet as far as the viral-vector," Cherian said.
But he added that even with the viral-vector vaccines, "far, far less than even 1% of individuals are actually having these side effects."
By comparison, a November study found that 20% of hospitalized COVID-19 patients developed blood clots in their veins.
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