Vaccine Reactions

Any time a person takes a medication, even a preventative one such as a vaccine, they can have an adverse reaction. Vaccines are made to activate the immune system and help the immune system become familiar and form protection against a potential threat. Because vaccines activate the immune system, mild reactions are common. 

Sometimes a person can have allergic reactions to ingredients of the vaccine. These allergic reactions can be to the active ingredients but are more often to the inactive ingredients (excipients) used to stabilize or produce the vaccine. These allergic reactions can range from mild to severe, including a life-threatening allergic reaction called anaphylaxis. 

Sometimes mild reactions resolve without treatment or may improve with antihistamines like diphenhydramine. 

Anaphylaxis has some of the same symptoms of mild allergic reactions, but the symptoms are more severe and occur in more than one system. Symptoms of anaphylaxis occur abruptly and require emergency medical treatment. They do not resolve on their own.

Vaccine-related Reactions

Reaction Type and Severity  Examples of Symptoms Timing
Non-allergic Rash, fever, chills, irritability, muscle and joint pain, stomach upset, headache, fatigue, pain, redness or swelling at injection site or same arm Within 1-7 days after vaccination
Allergic and mild Itchy rash such as hives, or flushing, and/or swelling of lips or eyes or extremities Within
0-72 hours after vaccination
Anaphylaxis (allergic and severe) Symptoms of one or more systems: skin symptoms (itching, hives, flushing or facial swelling), breathing problems (shortness of breath, wheezing, cough), symptoms due to low blood pressure (confusion, disorientation, dizziness, lightheadedness, weakness, or fast heart rate), or gastrointestinal symptoms (nausea, vomiting, stomach cramps, or diarrhea). Occur abruptly, from 5 minutes to within 2 hours of vaccination

How common is anaphylaxis to vaccines?
Anaphylaxis to vaccines is historically rare. The Center for Disease Control and Prevention (CDC) reporting system has identified that they occur with about one in every million doses for most vaccines. Emerging data suggest these reactions may occur 5 times more often in the mRNA vaccines against COVID-19 (see Do the COVID-19 vaccines cause anaphylaxis?).

What is the role of the allergist in vaccine reactions?
An allergist can determine whether it is safe for your child to get a vaccine if they have a history of concerning allergies to vaccines or excipients or evaluate your child after they have had an allergic reaction to a vaccine.

How are vaccines assessed for safety?
Vaccine safety in the US is the responsibility of the CDC and Food and Drug Administration (FDA). Detailed procedures are in place for reporting and following up of reports of adverse reactions to vaccines. Reactions to vaccines that are suspected to be allergic reactions are reported to the through a Vaccine Adverse Events Reporting System (VAERS). The CDC safety officers and allergy consultants review the report to determine the likelihood of the reaction qualifying as anaphylaxis. The director of our Drug Allergy Program, Kimberly A. Risma, MD, PhD, has served as an allergist consultant for the allergic reactions to COVID-19 mRNA vaccines. 

Working in the lab.

Cincinnati Children's participated in the COVID-19 vaccine trials.

Anaphylactic Reactions to COVID-19 mRNA Vaccines are Rare

Confirmed anaphylactic reactions to COVID-19 mRNA vaccines are rare, likely due to a lack of preexisting IgE against the vaccine components, including PEG. Read more

Do the COVID-19 vaccines cause anaphylaxis?

The reports of COVID-19 vaccines causing infrequent episodes of anaphylaxis are being actively assessed for safety. The Pfizer BioNTech and Moderna COVID-19 vaccines are a new type of vaccine that uses messenger RNA (mRNA) to help the immune system develop protection against COVID-19. These mRNA vaccines use a delivery system called lipid nanoparticles (LNPs) that mimics a virus. The vaccine is not a live virus so the LNPs are packaged with specialized materials like cholesterol, synthetic lipids, and a stabilizer called polyethylene glycol (PEG). PEG is usually ignored by the immune system, but recent reports suggest that some people may have a rare allergy to PEG. A person having an allergy to PEG or other components of the LNP may have allergic reactions to these mRNA COVID-19 vaccines. There also may be other mechanisms for anaphylaxis to COVID-19 vaccines that we are not aware of yet. Read more

COVID-19 mRNA vaccine image.

The COVID-19 mRNA vaccines have mRNA (blue) hidden inside a tiny lipid nanoparticle core (red, gray) with polyethylene glycol (black squiggles) surrounding the particle to give stability.