AstraZeneca's coronavirus vaccine took yet another public relations hit yesterday, when the European Medicines Agency announced that the shot is linked to blood clots, and they should be listed as a "very rare" side effect of the vaccine.

What we're watching: Even before the link was announced, the U.S. didn't need the AstraZeneca vaccine, based on its existing supply of other shots. But what the Food and Drug Administration decides to do about the vaccine — if the company seeks U.S. authorization — will likely have global ramifications.

Driving the news: The EMA said the benefits of the vaccine still outweigh its risk, STAT reports. “This vaccine has proven to be highly effective to prevent severe disease and hospitalization,” said Emer Cooke, the EMA’s executive director. “And it is saving lives.”

• The U.K., however, recommended that people under 30 receive other vaccines, and other European countries have already restricted use of the vaccine to older populations.

Why it matters: The AstraZeneca vaccine has already been plagued by a series of self-imposed mishaps throughout its rollout. Even if blood clots are an exceedingly rare side effect — which experts stress they are — there's risk of further damage to the shot's reputation.

• "Very serious and very rare side-effects do occur, with essentially every medical intervention ever developed (not just vaccines)," said John Moore, a professor of microbiology and immunology at Cornell.
• "But with the spotlight on Covid-19 vaccines, the public is going to hear 'very serious' and tune out 'very rare.' So, there are going to be major perceptional and confidence problems moving forward."

The big picture: The shot is relatively cheap and easy to make, and is the main shot being used by the COVAX initiative. It's particularly important for developing countries.

• The U.S., on the other hand, doesn't need the vaccine; it has already purchased more than enough doses of other vaccines to cover its entire population.

Yes, but: What U.S. regulators say about the shot could still have global consequences, especially given its chaotic rollout.

• "Confidence in the AZ vaccine, which will be informed by the U.S. FDA position, will be critically important for global vaccine efforts," said Krishna Udayakumar, director of the Duke Global Health Innovation Center.
• "The U.S. FDA position will likely also impact what happens to the current and future U.S. supply of the AZ vaccine – even if we don’t use them, it would be helpful for the vaccine to have FDA EUA for the U.S. to confidently provide doses, via donations or loans, to other countries," he added.

What we're watching: The company hasn't yet submitted an application for emergency use authorization to the FDA, but it could do so in the coming weeks.

• If the company does seek FDA authorization, "I think it's unlikely they won't approve it, because despite this rare side effect, it’s a good vaccine," said Nahid Bhadelia, medical director of Special Pathogens Unit at Boston Medical Center.
• But even if it still authorizes the shot, it could put age parameters on eligibility.
• "The [problem] is even if authorized, the U.S. is probably going to be giving away the doses to other countries anyway, which makes the confidence impact more limited," said former FDA Commissioner Mark McClellan.

The bottom line: Finding alternatives to the AstraZeneca shot is "a real problem," Moore said.

• "The one advantage of the AstraZeneca vaccine was its low cost/high production characteristics. Perhaps [Johnson & Johnson] and Novavax can fill some of the gaps that may arise," he wrote in an email, adding that the Pfizer and Moderna vaccines are "problematic for global roll-out."
• "Otherwise, as of now, it’s the Russian and Chinese vaccines, and there remain real doubts about their safety, efficacy and production capacity for use on a global scale."