The Food and Drug Administration (FDA) has officially given its full approval to Pfizer’s COVID-19 vaccine. This opens the door for companies, universities, and other organizations to mandate COVID immunization. People can request medical exemption from getting the vaccine, however, experts say these exclusions are few and far between.

According to NBC News, each company or institution may determine what they will accept as a legitimate medical exemption based on recommendations by the FDA, the Centers for Disease Control and Prevention, and leading medical authorities.

“People will seek an exemption for all sorts of reasons, but there are not many valid ones,” noted Dr. John Grabenstein, director of Scientific Communications for the Immunization Action Coalition. Medical exemptions are usually based on “contraindications” or reasons why an individual should not get the vaccine, says NBC News.

In the case of the Pfizer vaccine, now called Comirnaty since its full approval, the only contraindication is a “known history of a severe allergic reaction to any component” of the drug. The rate of anaphylaxis, a severe and potentially life-threatening allergic reaction, occurred in about 2 to 5 cases per million doses, says Dr. Niraj Patel, chair of the American College of Allergy, Asthma and Immunology’s COVID-19 Vaccine Task Force.

“Putting this into perspective, you’re as likely to get struck by lightning as you are to have an allergic reaction to a COVID-19 vaccine,” he said.

Amanda Cohn, of the CDC’s vaccine planning unit, said that the Pfizer vaccine consists of messenger RNA and four lipid nanoparticles. One of these particles, called polyethylene glycol or PEG, could be an allergic component of the vaccine, and has been previously linked to anaphylaxis, according to Forbes.

The same lipid is found in the Moderna jab, another mRNA vaccine, but not in the Johnson & Johnson formulation. So, it’s possible that someone who is allergic to an mRNA vaccine could safely get the Johnson & Johnson shot, says Patel, who suggests that individuals should consult their allergist.

The CDC says that people who experienced an allergic reaction to the vaccine within four hours of getting it may also be exempt from getting another dose. The symptoms include hives, swelling and wheezing, says NBC News.

Patel says that people who are immunocompromised can safely get the vaccine and the FDA recently authorized a booster shot for this population because their need for extra protection. Patients who suffer from cancer and are undergoing treatment should consult with their doctor.

Pregnant women are also cleared by the CDC to receive a COVID-19 vaccine as long as they are not allergic to any of the components. Experts say anyone who is worried about their medical condition should ask their doctor for advice on the timing and type of vaccine that’s best for them.

Some people have experienced myocarditis, or heart inflammation, after an mRNA vaccine, says NBC News. The CDC recommends waiting until the symptoms are resolved before getting a second dose. For individuals with Guillain-Barre syndrome, the mRNA vaccine may be preferable over the Johnson & Johnson shot, which has been linked to a potential increased risk for the condition.

Given the information we have on the vaccines and their safety potential, Grabenstein says that there are very few reasons for legitimate medical exemptions.

“We’ve gone past the 200 millionth person to have gotten COVID vaccination just in the United States alone,” told NBC News. “So now we have extensive safety experience with these vaccines.”