Vaccines against COVID-19 may begin arriving within days in the United States, as the fastest-ever vaccine development reaches a conclusion.
Experts say no corners have been cut. But the U.S. Food and Drug Administration (FDA) is considering authorizing the vaccines after a shorter-than-normal safety testing period.
Nothing sinister has turned up in clinical trials so far, but there are still unknowns.
With the COVID-19 pandemic killing more than 2,000 Americans every day on average, health officials must walk a fine line between urging people to get vaccinated and acknowledging what they do not know about the vaccines' safety.
"Communication is really critical here. We don't want to scare people, but we also don't want to lie to people," said Rupali Limaye, a vaccine behavior expert at the Johns Hopkins Bloomberg School of Public Health.
The vaccine from drugmaker partners Pfizer and BioNTech will get a hearing at the FDA on Thursday. Emergency use authorization could follow within days if all goes well. The agency will consider biotech company Moderna's vaccine next week.
The companies are ready to begin shipping the first doses as soon as they get the green light. The U.S. Centers for Disease Control and Prevention (CDC) has recommended that front-line health care workers and nursing home residents and staff get top priority.
While confidence in the vaccines has improved since September, just 37% of respondents say they would be comfortable being in one of the first groups to get it, according to the latest Pew Research Center survey. About 4 in 10 say they probably or definitely will not get vaccinated.
Vaccine makers usually collect safety data for several years before seeking FDA approval. But for COVID-19 vaccines, the FDA told drug companies it would make do with two months’ worth of data for at least half of study participants.
"The vast majority of side effects occur within 30, 40 days, and that's why the FDA made those recommendations," said William Moss, executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health.
Side effects that Pfizer and Moderna have reported so far include fatigue, muscle pain, headaches and fevers lasting up to a day or two.
These reactions are not serious or unusual, but they are more severe than most vaccines cause and could generate pushback.
"We need to be able to communicate that this is normal and remind people that the vaccine has undergone rigorous safety testing," Limaye said.
Health authorities maintain systems to keep track of side effects after the vaccines roll out to the public.
The CDC also is planning to use a text-messaging system to keep track of adverse reactions. But critics say the system is vulnerable to abuse by vaccine opponents filing false claims.
Tens of thousands of people have received the Pfizer and Moderna vaccines in clinical trials without major safety problems. But as distribution reaches millions of people, scientists will be watching for rarer side effects that might crop up.
"The trade-off here is we are in the midst of a terrible pandemic in this country," Moss said. "Unfortunately, because of that tragedy, we just do not have the time to look for these very rare side effects."
For example, some influenza vaccines have been linked to small upticks in a neurological condition known as Guillain-Barré syndrome, which ranges from temporary muscle weakness to, in unusual severe cases, paralysis and death. These increases are on the order of one or two cases per million people vaccinated, according to the CDC.
In a pandemic that has already killed nearly 300,000 people, experts say the public may have to weigh risks and benefits.
Misinformation is a serious concern. Anti-vaccine groups have been spreading false conspiracy theories about COVID-19 and the vaccines, especially on social media. Facebook said it is removing debunked claims from its platforms.
Scientists worry that anything that happens after people get their shots will be blamed on the vaccine.
For example, in the two months after the first 10 million people get vaccinated, about 4,000 people will have heart attacks, said Robert Watcher, chair of the University of California, San Francisco Department of Medicine. Another 4,000 will have strokes. Sixty will be diagnosed with multiple sclerosis. And 14,000 people will die.
"And the vaccines will have zero to do with any of them," he wrote on Twitter.
These are normal rates for these illnesses. They would happen with or without the vaccine.
But Watcher worries that people looking to undermine confidence in the shots will seize on these everyday misfortunes.
"(I)f somebody wants to turn every post-vaccine illness into a, 'See, I warned you' canard, there’ll be ample fodder," he wrote.