A few reports of allergic reactions to the COVID-19 vaccines have made some people—especially those with allergies to medications or vaccines—a little nervous about the new shots. These aren’t the most common side effects associated with the new COVID-19 vaccines, but they are possible. Here’s what you need to know.
There are now two COVID-19 vaccines that have received emergency use authorization from the Food and Drug Administration: one from Pfizer and BioNTech, and one from Moderna. Both of them use mRNA technology to cause an immune response in the body that helps protect people from COVID-19 if they come into contact with the virus.
The two vaccines have very similar side effects, which tend to be flu-like and last a day or two. In clinical trials the vaccines most commonly caused pain at the injection site, fatigue, and headache, SELF reported previously. Some people also experience chills, a fever, and redness or swelling at the injection site. The side effects also tend to be worse after the second shot than they were after the first.
As of December 19, 2020, there were six confirmed cases of anaphylaxis following Pfizer/BioNTech COVID-19 vaccinations in the U.S., out of approximately 270,000 doses given, according to a presentation from the Centers for Disease Control and Prevention (CDC). (There’s no official CDC data yet regarding allergic reactions to the Moderna vaccine, which was just authorized a few days ago.) There were no reports of severe allergic reactions (anaphylaxis) to COVID-19 vaccines in the clinical trials. But the companies specifically excluded people with known allergies to components of the vaccines, and the Pfizer/BioNTech trials also excluded people with a history of allergic reactions to any vaccine.
One component of the vaccines, polyethylene glycol (PEG), is also found in many medications and household products (such as shampoo) and is known to cause anaphylaxis in a small number of people, the American College of Allergy, Asthma, and Immunology (ACAAI) says.
Our understanding of how PEG allergies may work is still developing, but some experts believe that people who have been previously exposed to PEG can develop antibodies to it. In a high enough level, those antibodies can cause an allergic reaction if someone comes into contact with PEG again. A recent study published in Analytical Chemistry estimated that about 7% of the general population could have a large enough amount of antibodies to PEG that might predispose them to anaphylaxis. But, again, researchers are still figuring out what may be causing the COVID-19 vaccine reactions and whether or not PEG sensitivities could play a role.
Considering that we’re talking about millions of vaccine doses being distributed across the country over the next few weeks, it’s not entirely surprising that there have been a few allergic reactions. But any case of anaphylaxis, which can be life-threatening, is an emergency.
For now, the National Institute of Allergy and Infectious Diseases (NIAID) is planning a large-scale study to learn more about what exactly is causing the reactions and how common they are. The CDC and the ACAAI have a few guidelines about who should and shouldn’t get the vaccines:
As with everything else related to COVID-19, our understanding of how best to use COVID-19 vaccines is still evolving. The vaccines are clearly important tools and will help us get a handle on the pandemic, but they do come with a risk for side effects—including, rarely, severe ones. So it’s important to get the full picture and to discuss any concerns with your doctor.