Doctors say fears of blood clots from J&J's shot are overdone and could delay the push to end the pandemic

• Coronavirus vaccines are facing their biggest safety scare to date.
• A small number of people immunized with Johnson & Johnson's or AstraZeneca's shot developed serious blood clots.
• Medical experts say a blow to public confidence in vaccination is a graver threat than the actual adverse events.

The race to immunize the world against the coronavirus is now facing its biggest safety-related challenge yet.

Two leading vaccines, developed by Johnson & Johnson and by AstraZeneca, have now seen their rollouts disrupted by concerns that they cause very rare but serious cases of blood clotting.

The US Food and Drug Administration and the Centers for Disease Control and Prevention jointly recommended on Tuesday that the US pause the rollout of the J&J vaccine as the agencies investigate. AstraZeneca's shot isn't available in the US.

The concern is a genuine one. Both vaccines have been tied to a small number of unusual cases of blood clots in the brain, though regulators haven't concluded that J&J's shot is responsible for the clots. In the US, six people have developed the clots and one person died.

People who recently received J&J's vaccine should watch for the signs and symptoms of blood clots, medical experts said, such as severe headache, shortness of breath, abdominal pain, or leg pain. If you experience such symptoms, contact a medical professional, they advised.

Still, experts emphasized that because COVID-19 is so devastating, the benefits of the vaccine probably outweigh the risk of these rare side effects.

"We're talking about an event that occurs in 1 in 1 million shots, versus if you catch COVID, the chances of you being hospitalized are 1 in 100 for most people," said Dr. Alex Spyropoulos, a blood-clot specialist and professor at the Feinstein Institutes for Medical Research, the research arm of Northwell Health said. "These are orders of magnitude different."

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The recent debate in Europe over AstraZeneca's vaccine suggests one path forward. After weeks of investigation, European regulators concluded that AstraZeneca's shot was causing rare blood clots. But they also said that the benefits of the vaccine outweighed the risks, and urged people to continue using it.

Some countries have said that younger people should get other vaccines, rather than AstraZeneca's shot, if those are available. That's because COVID-19 is less dangerous for younger people.

On Wednesday, the CDC will host an emergency meeting of its independent advisory group to discuss J&J's vaccine.

The US could take a similar approach to Europe. SVB Leerink industry analyst Danielle Antalffy said in a Tuesday investor note that she expects the CDC's committee will "recommend a demographic-based restriction to the recommendation for the product's use (e.g., not recommended for women under 50 years of age)."

Experts fear safety scares will boost hesitancy

Scientists have studied the J&J and AstraZeneca vaccines in trials involving about 70,000 people.

The J&J trial was, in fact, temporarily paused last October, after a 25-year-old man suffered from rare, serious blood clotting shortly after vaccination. Ultimately, researchers determined there was no clear cause and resumed the study, according to FDA documents on the vaccine.

But in general, ultra-rare safety problems are usually only identified once vaccines are given to millions of people. Nearly 800 million doses of coronavirus vaccines have now been given around the world.

The other two US-authorized vaccines, developed by Pfizer-BioNTech and Moderna, have not raised any major safety concerns.

Some people who got the shots initially had severe allergic reactions, known as anaphylaxis, but those events have been rare and manageable.

The J&J and AstraZeneca safety snafus are the first real speed bumps on the road to global immunizations. Unlike the Pfizer and Moderna shots, these programs were seen as instrumental to inoculating low-and-middle-income countries.

Both shots have advantages: AstraZeneca is selling its two-dose shot for a few dollars a dose, while J&J's vaccine requires just a single dose.

US regulators have said they hope to quickly resume giving the J&J vaccine after evaluating its safety.

What alarms medical experts more is that safety fears will lead to a surge in vaccine hesitancy.

"I am profoundly concerned about the ramifications of keeping the people's confidence in the relative safety of these vaccines and the relative danger of a raging pandemic that kills people," Dr. Monica Gandhi, an infectious-disease doctor and professor of medicine at the University of California, San Francisco, told Insider.

Gandhi's ultimate message to the public: "Keep calm and vaccinate."

The US and European data suggest J&J's and AstraZeneca's vaccine could cause these blood-clot cases in an exceedingly small proportion of people. The US has seen six such cases out of nearly 7 million people immunized with J&J's shot. European regulators have identified 220 blood-clotting cases out of 34 million AstraZeneca shots given.

In addition to the rarity, some experts, including himself, aren't convinced the cases are even caused by the vaccines, said Spyropoulos.

"What's much more worrisome than the extremely rare side effect, if that's what this is, is the public confidence in widespread vaccination. That's the most worrisome issue," Spyropoulos told Insider in a Tuesday interview.

With unclear path ahead, challenge of communicating science rises again

The long-term challenge will be a communication issue.

The US is far better at accomplishing remarkable scientific feats - developing multiple vaccines using new technologies in record time - than it is at explaining them to the public.

In this case, the safety worries require a nuanced discussion of balancing known benefits of the vaccine against potential, very rare, risks. Finding that proper tone is a perennial challenge in public health.

"It is difficult to communicate to the public that something scary has happened but it's rare," Gandhi said.

US regulators could have learned from the chaotic European review of similar safety events with AstraZeneca's shot, Gandhi said. Instead of urging a pause for the entire immunization program, Gandhi said regulators could have recommended restricting J&J's vaccine to older adults or just men as a less harsh measure. The vast majority of reported cases of blood clots linked to both J&J's and AstraZeneca's shots have been reported in younger women.

"There are things that could've been done that are a little less extreme than pausing it completely," Gandhi said "I wish we learned from the [European Medicines Agency] last week, because the problem was it led to so much fear. It is understandable that it's going to lead to fear, but we also desperately want to get through this pandemic and these are incredibly effective vaccines."