By Jen Christensen, CNN

(CNN) -- The Pfizer/BioNTech Covid-19 vaccine being rolled out across the US should be safe for just about anyone -- even the frailest elderly people.

Under the emergency use authorization the vaccine got from the US Food and Drug Administration Friday, it can be given to anyone 16 and older. The Advisory Committee on Immunization Practices recommendation is that it should be safe for almost everyone.

A health worker in Alaska with no history of vaccine allergies did have an allergic reaction after receiving the Covid-19 vaccine Tuesday, but health experts believe these kinds of adverse reactions will be extremely rare.

"These vaccines are very well tolerated from the data we've seen," said Dr. Purvi Parikh, an allergist with Allergy & Asthma Network and co-investigator on several Covid-19 vaccine trials. She is also a professor at the NYU Grossman School of Medicine.

"We're in a life threatening pandemic. We have no defense. So, for most people, they should absolutely get it, there's very, very few cases that you could argue someone shouldn't get it."

Here's what we know so far about who should, and, possibly shouldn't, get a Covid-19 vaccine.

People with vaccine allergies

The FDA put only one group in the category of those who should not get the vaccine: people who have a known history of a severe allergic reaction to any component of the Pfizer/BioNTech vaccine.

A warning on the vaccine label adds that medical facilities should keep treatments to manage allergic reactions immediately available.

Two healthcare workers in the UK who had a history of severe reactions to vaccinations did have adverse reactions within minutes of getting the Covid-19 shot. Both workers recovered and are doing well, according to National Health System England.

In the UK people with vaccine allergies are not being vaccinated for now. In the US, the Centers for Disease Control and Prevention said people with a history of vaccine allergies can get the Covid-19 vaccine. Those patients, though, should be told about the unknown risks. Parikh said they may also want to, but are not required to, check in with their doctors.

"A doctor can dig through your history and see what are the chances are of you having a reaction or not," Parikh said. "It's just a very, very small percentage that need that evaluation."

Patients who have had allergic reactions to vaccines in the past should be monitored for 30 minutes after getting the vaccine, according to the FDA guidance. For people without allergies, the recommendation is a 15 minute observation time.

People with other allergies

People with other allergies, like food or mold allergies, should be fine to get the Covid-19 vaccine.

"In the clinical trials, we actually did not exclude people with allergies, even people with severe food allergies. We only excluded people if they had allergies to vaccine for the Pfizer trial, so that being said, there may have been thousands of people who had allergies and received the vaccine with no issue," said Parikh.

Pregnant and lactating women

Since there is not enough data yet on the safety of Covid-19 vaccines in pregnant women, it's up to them if they want to get the vaccine, according to Dr. Peter Marks, the director of the FDA's Center for Biologics Evaluation and Research.

"Covid-19 in a pregnant woman is not a good thing, so someone might decide that they would like to be vaccinated, but that's not something that we're recommending at this time," Marks said at a news briefing Saturday. "That's something we're leaving up to the individual."

The Pfizer clinical trials did not actively enroll pregnant women, but 23 volunteers became pregnant during the course of the trial. There have been no adverse events. Pfizer/BioNTech said it will continue to monitor those women.

According to observational data, the absolute risk to pregnant women is considered low. Since mRNA vaccines do not contain any live viruses, they should degrade quickly and won't enter the nucleus of the cell. They cannot cause genetic changes.

The American College of Obstetricians and Gynecologists (ACOG) recommended that vaccines "should not be withheld from pregnant individuals who meet criteria for vaccination."

The clinical trial did not include women who were breastfeeding, but mRNA vaccines are not thought to be a risk to a breastfeeding infant; therefore, ACOG said the vaccine should be offered to those women as well.

People with underlying medical conditions and the elderly

People with underlying medical conditions and the elderly can get the Covid-19 vaccine.

Late stage clinical trials of the Pfizer vaccine showed similar results for people with underlying conditions, compared to people who were healthy.

Older adults also tended to report fewer and milder adverse events after they were vaccinated in the clinical trials, according to the company.

The immunocompromised

The vaccine may be appropriate for people who have a suppressed immune system due to a condition or disease or because they are undergoing a treatment for a disease like cancer. This should be an individual's decision, according to CDC guidance.

There were immunocompromised volunteers with stable HIV infections in the Pfizer trial, but there is no specific data about this population, so there is insufficient evidence to draw conclusions about the safety of the vaccine with these patients.

"For pregnant women and the immunocompromised -- just at this point -- it will be something that providers will need to consider on an individual basis for patients," said Marks.

People who have or had Covid-19

Late stage clinical trial data suggested the vaccine was safe and helped protect people with past Covid-19 infections from reinfection. This was regardless of their past case being mild or severe.

A person who is currently sick with Covid-19 should, however, wait to get the vaccine after their symptoms have cleared up and they can come out of isolation. There's no recommended minimum time between infection and vaccination.

People who've taken a Covid-19 antibody treatment

There is no safety data on people who have been given an antibody therapy or convalescent plasma to treat a Covid-19 infection.

Since reinfection seems to be uncommon in the 90 days after the initial infection, as a precaution, the CDC recommends the person wait at least 90 days.

There's no data that shows a vaccine would protect someone who has been recently exposed. A person isn't fully protected until one or two weeks after they get the second dose of the vaccine.

Teens

Teens who are 16 and 17 can be vaccinated when there is the appropriate consent from an adult.

More than 153 teens ages 16 and 17 were included in the Pfizer trial and the early analysis of that data found no safety issues.

Details about how this age group reacted to the vaccine are limited, but the CDC said there "are no biologically plausible reasons for safety and efficacy profiles to be different than those observed in persons 18 years of age and older."