Pfizer's coronavirus vaccine should be safe for most Americans, but 5 groups may want to wait for more data before getting shots

• The FDA authorized Pfizer's coronavirus vaccine for emergency use in people ages 16 and older on Friday.
• The FDA said Saturday it was up to pregnant women and their doctors to decide whether to get vaccinated.
• The agency has warned that anyone with severe allergies to the 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.
• Visit Business Insider's homepage for more stories.

US vaccine regulators on Friday authorized the nation's first coronavirus shot for emergency use.

The Food and Drug Administration has determined that the benefits of Pfizer's coronavirus vaccine outweigh the known and potential risks to the public, based on data from clinical trials.

Most trial participants reported temporary side effects after being vaccinated, including fatigue, headaches, and pain at the injection site. People under 55 reported more side effects overall, but few participants had severe fatigue or headaches that stopped them from performing normal activities. Serious adverse events were reported in only 0.6% of vaccine recipients.

These results were consistent across genders, age groups, ethnic and racial groups, and people with preexisting medical problems. But several populations were excluded from the trials - meaning it's still unclear whether the vaccine is safe for all Americans.

While the trial was large, including more than 40,000 people, the vaccine hasn't been tested in some groups of people. In particular, a recent FDA review concluded that "there are currently insufficient data to make conclusions about the safety of the vaccine in subpopulations such as children less than 16 years of age, pregnant and lactating individuals, and immunocompromised individuals."

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.)

Children under 16 are often last to be included in trials

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.

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

Pfizer's phase-one clinical trial included primarily healthy people. Its late-stage clinical trials then tested the vaccine among more vulnerable populations, including healthcare workers and people with autoimmune diseases.

But the company excluded pregnant people. It asked female volunteers to take a pregnancy test before each vaccination, and any with positive results were not allowed to continue.

Pfizer plans to follow up with people who reported pregnancies during the study and those who choose to get vaccinated once the shot is approved. That's how public-health experts eventually determined it was safe for pregnant people to get the flu vaccine, even though they were never included in flu vaccine trials.

Pregnant women won't be denied the coronavirus shot if they and their clinicians decide it's the best option for them, Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said during a Saturday press conference.  

"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 who are HIV-positive and others with weakened immune systems must be particularly careful 

Pfizer's late-stage trials included some people with preexisting health problems. For example, the trials included a small number of people with HIV, but researchers said they didn't have enough time to follow up with this group before applying for emergency authorization. 

Volunteers were also excluded if they were considered immunocompromised (had 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.

According to the FDA's emergency authorization, "immunocompromised persons, including individuals receiving immunosuppressant therapy, may have a diminished immune response to the Pfizer-BioNTech COVID-19 vaccine."

For the most part, doctors recommend that immunocompromised people steer clear of vaccines that use live viruses. But Pfizer's vaccine instead relies on a snippet of the virus' genome to trigger an immune response.

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

The main ingredient in Pfizer's vaccine is messenger RNA, a harmless snippet of the virus' genome that tricks the body into producing antibodies. The vaccine also includes 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 ingredients found in common medications. The vaccine doesn't contain any common allergens like nuts, eggs, or heavy metals, either.

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

On Tuesday, two UK National Health Service workers had non life-threatening anaphylactic reactions shortly after receiving the Pfizer vaccine. Both workers had known, severe allergies, which required them to carry EpiPens.

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

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, suggested Thursday 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

Pfizer hasn't specified whether its vaccine is 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."

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 12 with information on the Pfizer vaccine's emergency use authorization.