Blood clots are a "very rare" side effect of AstraZeneca's shot, European regulators said Wednesday.
But five medical experts told Insider they aren't convinced the shot actually causes clotting.
Experts worry that public skepticism may unnecessarily derail AstraZeneca's global vaccine rollout.
The world's most-administered coronavirus shot is facing what may be its biggest challenge yet.
On Wednesday, European regulatory authorities said potentially fatal blood clots are indeed a rare side effect of AstraZeneca's vaccine. Following the announcement, the UK's regulator said people under 30 years old should seek out other shots, if available.
AstraZeneca's vaccine has already been rolled out to tens of millions of people around the world. In interviews with Insider, five medical experts emphasized that the shot is safe and effective.
"The risk of dying in an air crash is just astronomically higher than the risk of clotting after the vaccine dose, and yet we all get on a plane without a second thought," Johan Bester, director of bioethics at the University of Nevada, Las Vegas School of Medicine, told Insider.
Wednesday's announcement came after European medical officials reviewed fewer than 100 blood-clotting cases reported among more than 25 million people in the EU who've gotten AstraZeneca's shot. That's a rate of roughly 4.6 clot cases per 1 million shots - higher than expected, the review found, but still extraordinarily rare.
Given that, experts are more concerned about the consequences of public suspicion of the shot.
"All that people heard was: AstraZeneca's vaccine can kill you because of blood clots," Art Caplan, a bioethicist at New York University, told Insider, referring to the Wednesday announcement. "They're not watching the fine tuning and the careful recommendations."
Countries that have authorized the vaccine, he added, are now going to have to deal "with a very huge explosion of vaccine skepticism."
A non-issue? A blip?
The World Health Organization also issued a note of caution on Wednesday, acknowledging that a relationship between blood clots and the vaccine is "considered plausible but is not confirmed."
AstraZeneca, meanwhile, said these are "extremely rare events" that have occurred on a minuscule scale.
"There is no proof that the AstraZeneca vaccine leads to blood clots," Todd Ellerin, director of infectious diseases at South Shore Health in Massachusetts, told Insider. "Association is not the same as causation."
Alex Spyropoulos, a director at New York's Northwell Health who specializes in blood clots, told Insider that he, too, is not yet convinced there's a cause-and-effect relationship.
"This is almost a non-issue, a blip on the safety screen," Spyropoulos said. "The numbers do not elicit the type of hysteria we are seeing."
Although even a minuscule chance of a fatal blood clot sounds scary, no medicine carries zero risk. After a year of taking birth-control pills, about one in 1,000 women will develop blood clots. (The risk is about 1 in 10,000 for all young women, so it's elevated nearly 10-fold in birth-control takers.)
Even other authorized COVID-19 vaccines have faced safety scares. Johnson & Johnson's trial was paused in October because of an unexplained illness in a study participant. Moderna and Pfizer's shots also saw scattered reports of severe allergic reactions, but these cases have been extraordinarily rare and manageable.
Taison Bell, an infectious-disease physician at the University of Virginia, noted that the rate of blood clots among AstraZeneca recipients was close to the rate you'd see in a normal population.
"When we look at the risk of death from COVID-19, long haul from COVID-19, versus these very rare side effects, I think the overwhelming pendulum swings towards getting vaccinated," he said. "I'm not particularly concerned about this announcement."
Scientists are still studying the association between AstraZeneca's shot and blood clots, particularly to find out whether certain groups may face higher risk for biological reasons. In the meantime, the public should take comfort in the fact that surveillance systems are working to detect and investigate unusual cases, experts said.
AstraZeneca has struggled with transparency
Wednesday's concerns about AstraZeneca's two-dose vaccine are far from its first controversy.
The company's global trial results puzzled many experts in November, because AstraZeneca combined data from its Brazil and UK groups, even though they had different sample sizes and demographics. It also averaged the results of two different dosing regimens, one of which was administered by mistake.
Subsequent analyses, meanwhile, revealed that the vaccine's effectiveness could vary dramatically depending on the amount of time separating the two doses. Confusion about this disparate data led to a patchwork of mismatched vaccine authorizations and guidelines around the world.
The new blood-clot reports could make people even less likely to judge the shot on its merits, experts said.
"The perception that this is not as good as the Pfizer or the Moderna shot has taken hold among the public," Bester said.
Blood-clot concerns are their own 'worldwide crisis'
If countries continue to hold off on administering AstraZeneca's vaccine to some or all of their populations - or if their citizens refuse to take it - it could slow the global rate of vaccinations. That, in turn, would give the virus more time to develop dangerous mutations.
"When we try to play the game of which one is better, we lose the overall point that they're pretty much all good and have the potential, when widely deployed, to break the back of the pandemic," Bell said. "If we have distrust or a preference when there's not the supply to support preferences, that means on the public-health scale, we're not globally vaccinating, which means we're not getting protected."
AstraZeneca's vaccine remains the world's best hope at inoculating lower- and middle-income countries in particular, since the drugmaker has priced its shot far lower than other vaccines and has vowed to produce up to 3 billion doses this year.
That makes the blood-clot concerns "a worldwide crisis" of their own, Caplan said.
"The perception of danger is huge, and I think many people are going to say, 'I don't want that,' or 'I prefer something else,'" he said. "It's a big, big setback for those countries that invested heavily in AstraZeneca."
The US could still authorize the shot
An AstraZeneca spokesperson told Insider that the drugmaker still plans to apply for emergency authorization with the US Food and Drug Administration in the first half of April.
Experts think the vaccine is likely to get a green light, given positive trial results last month, which showed the shot was 76% effective at protecting against symptomatic COVID-19. But the vaccine's chances of being widely used in the US seem increasingly slim, given the large supply of other authorized vaccines.
Caplan said the FDA would likely take "a much longer and harder look" at the data now, given the blood-clot reports. The agency may even require a year and a half's worth of data before moving toward authorization, he said.
"At the rate we're vaccinating right now, we might be all vaccinated by the time this even becomes an issue" in the US, Bester said.
Because of that, many experts have been suggesting the US donate its supply agreement of 300 million AstraZeneca doses to other countries. However, the optics of such a move would be more difficult now.
"I'm sure in retrospect, it will be like, the US is shipping out the inferior vaccine, which is not the case," Ellerin said. "AstraZeneca is not the inferior vaccine, but it has done just about everything in its power to cause unforced errors."
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