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As Covid-19 vaccines go into broad use, some rare side effects of vaccination will almost certainly emerge, like the reports of small numbers of people developing anaphylaxis. But so will medical events whose timing just comes down to random chance — and the potential ripple effects of those reports already have experts concerned.

Every single day, people die unexpectedly. They have strokes and heart attacks and seizures. On an average day, 110 people in this country may develop Bell’s palsy, a temporary facial paralysis, and another 274 will develop Guillain-Barré syndrome, a form of paralysis that usually resolves over time. The trigger for these medical events often isn’t known. But when they happen shortly after someone gets a vaccine — especially a new one — well, conclusions will be drawn.

“It is logical for people to say: That person had something done to them and something bad happened in the hours or days after that,” said Art Reingold, chair of the division of epidemiology and biostatistics at UC Berkeley’s School of Public Health. “And if it was you or your family member, you would be inconvincible that that wasn’t true.”


Except, of course, it often isn’t. Heart attacks occur most commonly in the morning, yet we don’t blame breakfast for causing them. A heart attack on the morning after a Covid-19 vaccine, though? That might be another matter.

But the public doesn’t have a great grasp of the concept that many problems that occur after vaccination probably aren’t tied to immunization itself. In part, that’s because that context has been missing from public health messaging about Covid-19 vaccinations.


“I think the lay public is fully, fully unprepared for understanding this,” said Kate O’Brien, director of the WHO’s immunization, vaccines, and biologics program.

Quickly distinguishing a true side effect signal from an abundance of noise will be critical to ease the alarm of a public already skittish about vaccines developed at “warp speed,” experts warned.

The risk of the public misinterpreting such anecdotal reports may be especially acute early on in the rollout, when elderly adults and people with health conditions have been prioritized to get the vaccine. Nursing home residents are in Phase 1a — currently ongoing — and seniors 75 and older are in Phase 1b. People aged 65 to 74 and those with medical conditions that increase the risk of severe Covid disease are in Phase 1c. These are people to whom medical events occur most commonly.

“Things are going to happen to them,” said O’Brien, though she noted that more medical misfortune would befall people in these groups if they were not vaccinated.

Helen Keipp Talbot, who is on the expert panel that devised the vaccine distribution priority lists for the Centers for Disease Control and Prevention, actually voted against putting nursing home residents at the front of the line, in part because vaccinating people who are in frail health first could inadvertently undermine confidence in the vaccine, given how common heart attacks, strokes and even deaths are in this population.

“All of the events are going to be temporally associated. But how do you explain that to the nurse’s aide who’s been taking care of that patient and loves her like her own grandmother? Who then decides that she’s not going to get vaccinated and tells everyone else not to get vaccinated?” Talbot told STAT.

“I fear a loss of confidence in the vaccine. That the vaccine will actually truly be safe, but there will be temporally associated events and people will be scared to use the vaccine,” said Talbot, an associate professor of infectious diseases at Vanderbilt University.

In some cases, there’s reason to believe reports of adverse events are likely due to the vaccine. Anaphylaxis — a potentially life-threatening allergic reaction — has been linked to multiple types of vaccines in the past.

Britain has reported several cases of anaphylaxis among people who have received the Pfizer vaccine. In the U.S., about 11 cases have been reported since the vaccine rollout began earlier this month, according to the CDC. Most followed receipt of the Pfizer vaccine, but a Boston doctor with a shellfish allergy developed a severe allergic reaction after receiving the Moderna vaccine. If and when other Covid vaccines are authorized for use, health authorities will be watching closely to see whether anaphylaxis is linked to all Covid vaccines, or merely those like the Pfizer and Moderna vaccines which are made using messenger RNA.

A three-year review of adverse reports logged into a U.S. national vaccine database found that anaphylaxis after vaccination is rare, occurring at a rate of about 1.31 per million doses of vaccine administered. Of those cases, 85% were in people with a history of allergies. None of the 33 cases — out of 25 million vaccinations — died. Some needed epinephrine, the drug in EpiPens, but others recovered after treatment with antihistamines.

Bell’s palsy, too, has been linked in at least one circumstance to a vaccine, an intranasally administered flu vaccine that was briefly brought to market in Switzerland.

Eight people in the Pfizer and Moderna trials, which enrolled nearly 74,000 participants in total, were diagnosed with the condition — seven in the vaccine arms and one in the placebo arm of the Moderna trial. The jury is still out on whether Bell’s palsy, which afflicts about 40,000 people a year in the U.S., is an occasional side effect of taking a Covid-19 vaccine.

As vaccinations begin on a larger scale, reports of other potential side effects will likely pop up. A few cases of something are anecdotes, not data, and definitely not proof of a causal relationship. While they must and will be studied, investigations of this sort take time.

The CDC and the Food and Drug Administration have a number of surveillance systems set up to monitor for potential side effects, as do some other countries. But it’s going to be important to look for, and be ready to react to, vaccine safety rumors anywhere they start to swirl, said Steven Black, a professor emeritus at the University of Cincinnati Children’s Hospital whose career has focused on vaccine safety.

“The reason I think you need to think globally is that vaccine scares are global,” said Black, who is also co-director of the Global Vaccine Data Network, a 17-country collaboration that studies vaccine safety and effectiveness. “We know very well that misinformation spreads much more quickly than information, so that a couple deaths in Brazil or a death in Indonesia or whatever, the public outcry could cause lack of confidence and undermine the whole vaccine program.’’

Reingold agreed.

“Once there’s a news report people have read, it doesn’t matter what we prove categorically two years later in a study, that perception will remain, and it’s hard to combat,” he said.

That’s why it’s so crucial to set expectations before and during a vaccination campaign. But public literacy about vaccination — the benefits, the risks, and how to balance the two — is poor, said O’Brien, who points to how often people insist they contracted flu from a flu shot, even though that is biologically impossible.

One of the ways vaccine experts try to combat the rise of rumors and unfounded claims is by knowing background rates of medical events, so that when things arise, one can get a sense of whether the number of cases is abnormal, or what might be expected to occur, whether or not a vaccination program was underway.

“If we know that Guillain Barré occurs in 1 in 100,000 people and a million people have been vaccinated, you would expect 10 cases, maybe,” Black explained. “But if you have 30, you begin to wonder: Maybe there’s an issue here? So, knowing background rates as a frame of reference is something that’s being promoted.”

Differentiating between what’s relevant from what isn’t is going to be tough, especially with multiple new vaccines (hopefully) starting to be used within a short period of time. When suspect medical events occur, it will be important to know which vaccine the individual in question received — though that may be a bigger challenge than you would expect. Record keeping for immunizations — especially adult immunizations — isn’t close to where it ought to be, O’Brien said.

Should it become clear that one, some, or all of the vaccines pose rare risks of some side effect, how will the public take the news?

“At some point, if events are occurring one in 100 million [vaccinations], people may be willing to accept that risk if it allows the world to get rid of Covid and go back to normal life,” Black said. “Because you have to remember, hundreds of thousands of people are dying of the disease.”

How that risk is communicated, though, will be critical, because many people can’t easily understand how to assess the significance of a rare risk. “They’re wary about very rare events and then they text while they’re crossing the street where they’re much more likely to get killed. Yeah, people do that very poorly,” Black said.

Giving people a comparison that makes sense to them can help, said Alison Buttenheim, an associate professor of nursing and health policy at the University of Pennsylvania, who works on vaccine acceptance.

The idea that 1 in 500,000 people who are inoculated with a Covid vaccine might have a serious side effect — this is a hypothetical example — might seem too risky to some people. Explaining to them that they run the same risk of being hit by lightning in a given year can put a different spin on that piece of information.

Likewise, pointing out that 1 in 500 New Jersey residents have already died from Covid-19 can remind people that the risk of not being vaccinated far exceeds the rare risks the vaccines may possibly pose, Buttenheim said.

At the end of the day, helping the public understand these issues requires communication — and that isn’t happening on a national level, said Bruce Gellin, president of global immunization at the Sabin Vaccine Institute, which promotes access to vaccinations.

The Department of Health and Human Services, not the CDC, has taken the lead on Covid vaccine communications efforts. But its output to date has been limited.

“It would have been nice if they could come up with a communications strategy at the same warp speed as they came up with vaccines,” said Gellin, a former director of HHS’s National Vaccine Program Office.

“My mantra is there’s a vaccine world and there’s a vaccination world, and they’re not necessarily connected by an arrow. [Operation] Warp Speed was largely about the vaccine world and about logistics,” Gellin said. “Warp Speed, unfortunately, was not about a vaccination program. And now what we’re seeing is that we’re now facing the vaccination program and are under-prepared.”

  • This article already starts with a bias. Tying heart attacks to breakfast is very dumb. So it’s possible to get a heart attack from covid or the vaccine but not any other side effect. Sadly we don’t need the experiences of the elderly to tell us to run from this one. This virus is no different from the flu, it’s the response that’s more lethal.

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