Pfizer and Moderna vaccines should be safe for most Americans, but 5 groups may want to wait for more data before getting shots

margaret keenan coronavirus vaccine — Margaret Keenan was the first person to be given the Pfizer coronavirus vaccine outside clinical trials.
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The FDA authorized Moderna's coronavirus vaccine for emergency use in people ages 18 and older on Friday.
Pfizer's COVID-19 shot got US clearance for people ages 16 and older a week ago.
The FDA said it's up to pregnant people and their doctors to decide whether to get Pfizer's vaccine.
The agency warns that anyone with severe allergies to a vaccine's ingredients shouldn't be immunized.
Experts also worry about vaccinating people who previously got COVID-19, particularly if these people have lingering symptoms.
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US vaccine regulators on Friday authorized the nation's second coronavirus shot for emergency use.

The Food and Drug Administration determined that the benefits of Moderna's coronavirus vaccine outweighed the known and potential risks, based on data from clinical trials. The agency authorized Pfizer's coronavirus vaccine a week prior.

Most participants in Pfizer's trial reported temporary side effects after being vaccinated, including fatigue, headaches, and pain at the injection site. In Moderna's trial, nine in 10 people who got the shots registered some side effects, but they were mostly mild or moderate.

Neither trial found any major safety concerns. Their results were consistent across genders, age groups, ethnic and racial groups, and people with preexisting medical problems.

Both studies were at or above the average size for a phase-three vaccine trial: Pfizer's late-stage trial involved more than 43,000 volunteers across six countries and 16 US states. Moderna's trial involved more than 30,000 participants throughout 30 US states.

But several populations were excluded from the trials - meaning it's still unclear whether the vaccines are safe for all Americans. Recent FDA reviews have said there's "insufficient data" to assess whether the vaccines are safe for immunocompromised people, young children, or pregnant women.

Some public-health experts have also questioned whether people who were already infected with the coronavirus should get vaccinated, particularly if they have lingering symptoms.

Here are five groups that may want to wait for more data on how the vaccine affects people like them. (At least two groups, young children and those with severe allergies to the vaccine's ingredients, will have to wait no matter what.)

Young children are often last to be included in trials

face mask kid school covid 19 — A girl removing her mask before posing for a school picture.
John Moore/Getty Images

Research has indicated that many children clear the coronavirus before it leads to severe disease, resulting in mild or asymptomatic cases. Young children have represented a small percentage of confirmed coronavirus infections in the US, according to the Centers for Disease Control and Prevention.

That means kids aren't a priority group when it comes to vaccinations. They're also difficult to include in trials because of ethical and safety concerns. Some parents may not feel comfortable signing their kids up to participate, and researchers often prefer to test vaccines in a small group of adolescents before enrolling young kids.

Pfizer's clinical trials, for instance, did not include people younger than 12. And the sample size of kids from ages 12 to 15 was too small to weigh the risks. Moderna's trial didn't include children at all.

But there's little reason to suspect that kids will react poorly to the shots.

"Children generally respond well to vaccines," Donna Farber, a Columbia University immunologist, told Business Insider. "They should respond well or comparably to a young adult - and maybe even better."

Pregnant people were excluded from trials too

pregnant woman covid 19.JPG — Nancy Pedroza experiencing contractions while taking a walk with her doula.
REUTERS/Callaghan O'Hare

Both the Pfizer and Moderna late-stage trials excluded pregnant people. They asked female volunteers to take a pregnancy test before each vaccination, and any with positive results were not allowed to continue.

Pfizer and Moderna plan to follow up with pregnant women who choose to get vaccinated now that the shots are authorized. That's how public-health experts eventually determined it was safe for pregnant women to get the flu vaccine, even though they were never included in the trials.

Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said on December 12 that pregnant women won't be denied the Pfizer shot if they and their clinicians decide it's the best option for them.

"The provider and the individual can make a decision about whether the benefits outweigh the risks, because certainly COVID-19 in a pregnant woman is not a good thing," he said. "So someone might decide that they'd like to be vaccinated, but that's not something we're recommending at this time. That's something we're leaving up to the individual."

Pfizer plans to include pregnant people in its vaccine study moving forward.

It's important to get this data as soon as possible, since pregnant people are at a higher risk of COVID-19 complications. A November CDC report found that pregnant women with COVID-19 were more likely to be admitted to the intensive-care unit, put on ventilators, need life support, and die than patients who weren't pregnant.

Pregnant people of color are particularly vulnerable. The report found that of all symptomatic pregnant women with COVID-19, 30% were Hispanic. But only 24% of women who gave birth in 2019 were Hispanic.

White women, on the other hand, represented 51% of women who gave birth in 2019 but only 24% of symptomatic pregnant women with COVID-19.

People with weakened immune systems must be particularly careful

covid safe hug — Michelle Grant hugging her mother through a plastic drop cloth in Wantagh, New York.
Al Bello/Getty Images

Both the Pfizer and Moderna trials tested the vaccine among more vulnerable populations, including healthcare workers and people with diabetes or HIV, but the cohorts weren't large enough for regulators to recommend the shot for immunocompromised individuals (people with a weakened ability to fight off infection).

These individuals must be particularly careful about getting vaccinated. If a patient's immune system isn't working properly, the shot might not work as well for them.

In its instructions to healthcare providers, the FDA said "immunocompromised persons, including individuals receiving immunosuppressant therapy, may have a diminished immune response" to the Pfizer and Moderna vaccines.

Both vaccines rely on a snippet of the virus' genome to trigger an immune response. For the most part, doctors recommend that immunocompromised people steer clear of vaccines that use live viruses.

Dr. Edward Jones-Lopez, an infectious-disease specialist at the University of Southern California, recently told Healthline that we could be "one or two studies away" from finding out whether coronavirus vaccines are safe for immunocompromised people.

People with severe allergies to the vaccine's ingredients must hold off

flu shot allergic reaction — zoranm/Getty Images

The main ingredient in both the Pfizer and Moderna vaccines is messenger RNA, a harmless snippet of the virus' genome that tricks the body into producing antibodies. The vaccines also include lipids - fatty substances that help that coded message cross the cell membrane.

The remaining ingredients aren't particularly allergenic: they're mostly salt, sugar, and a few substances that help stabilize other active ingredients. Pfizer said its vaccine doesn't contain any common allergens like nuts, eggs, or heavy metals, either.

Some doctors suspect that, in rare cases, people may be allergic to an ingredient called polyethylene glycol, found in the lipid component of Pfizer's vaccine. But most people ingest this ingredient daily in various foods, cosmetics, and medications (it's in the over-the-counter product MiraLAX).

Still, the FDA has not authorized immunizations for anyone with a severe allergic reaction (anaphylaxis) to the ingredients in either vaccine. Naturally, these individuals have been excluded from clinical trials as well.

Four people in Pfizer's trial, and three in Moderna's, developed a temporary form of facial paralysis called Bell's palsy. The FDA has said that it can't rule out with absolute certainty that Pfizer and Moderna's vaccine did not have an impact on these seven cases, but at the moment there's no reason to think the vaccine causes facial paralysis, experts recently told Business Insider.

But experts are still puzzled about a handful of recent anaphylactic reactions to Pfizer's vaccine.

Last week, two UK National Health Service workers had non life-threatening allergic reactions shortly after receiving the shot. Both workers had known, severe allergies, which required them to carry EpiPens. This week, two more people, this time in Alaska, had allergic reactions shortly after receiving Pfizer's vaccine. One of those people had no history of allergic reactions.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, suggested last week that people who are generally prone to severe allergic reactions might want special medical supervision while getting vaccinated.

But most Americans shouldn't be concerned.

"People with just run of the mill seasonal allergies in general, I don't expect them to have problems with this," Dr. Erin Reigh, a staff physician in the Section of Allergy and Clinical Immunology at Dartmouth-Hitchcock Medical Center, told ABC News.

People who previously had COVID-19 are in a gray area

coronavirus testing — A patient at St. George Hospital in Sydney in May.
Lisa Maree Williams/Getty Images

Pfizer and Moderna haven't specified whether their vaccines are safe for people who previously had COVID-19. Studies have suggested these people might have immunity that lasts anywhere from several months to several years. But the research is far from conclusive. They're likely to need a vaccine at some point.

"We want to look at vaccinating patients who have not been infected with COVID who are susceptible," Todd Ellerin, the director of infectious diseases at South Shore Health in Massachusetts, told Business Insider. "The post-COVID patients are not going to be your first, second, third, or fourth tier of groups that you're going to look at to want to vaccinate."

For people with long-lasting symptoms - known as long-haulers - it's possible that a vaccine could intensify an already aggressive immune response that for many has led to fatigue, muscle aches, trouble breathing, heart palpitations, or difficulty concentrating or sleeping.

"It'd be hard to engage a long-hauler in [a vaccine] study if they're having ongoing health problems," Natalie Lambert, an associate professor of medicine at Indiana University, told Business Insider. "Ethically there'd be big problems with having them take a vaccine, if you're worried about those effects."

Moderna did not include people who already had COVID-19 in its clinical trials. Pfizer, however, didn't screen participants for evidence of a prior coronavirus infection during its late-stage clinical trial. So it turned out that 3% of participants had been infected before. The data indicated the vaccine was just as effective in this group, but the FDA review said there wasn't enough evidence to know whether the vaccine prevented reinfection.

Ellerin said it would be unwise to inoculate previously infected people without more safety data.

"We have to keep our eye on the prize right now," he said. "Of course patients can get COVID more than once, so I'm not trying to tell you that they'll be excluded from this down the road, but we have a lot more to learn."

This article was updated on December 18 with information on the Moderna vaccine's emergency use authorization.

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