Vaccines can rarely cause allergic reactions, but the risk is extremely low. An allergic reaction is an immune system overreaction, often to a non-active ingredient in the vaccine. While vaccines stimulate a protective immune response, they are overwhelmingly safe. The risk of a severe allergic reaction is far lower than the risk posed by the diseases they prevent, and it is distinct from common mild side effects.
The Mechanism of Allergic Reactions to Vaccines
A true allergic reaction, or hypersensitivity, differs from common post-vaccination side effects like fever or arm soreness, which are signs of a normal inflammatory response. An allergy involves a specific antibody called Immunoglobulin E (IgE). For an IgE-mediated allergy to occur, a person must have been previously sensitized to an ingredient, causing IgE antibodies to bind to mast cells throughout the body. Upon re-exposure, the IgE antibodies signal mast cells to rapidly release inflammatory chemicals, such as histamine, which causes the rapid onset of allergic symptoms.
Vaccine Components That May Cause Hypersensitivity
Immediate allergic reactions are typically caused by non-active ingredients, known as excipients, rather than the active antigens. Excipients are used to stabilize, preserve, or facilitate the vaccine’s manufacturing process. Although present in very small amounts, they can trigger reactions in highly sensitized individuals.
Common Excipients Causing Reactions
- Gelatin: Used as a stabilizer in some vaccines, such as the Measles, Mumps, and Rubella (MMR) vaccine.
- Egg Protein: Present in trace amounts in some influenza and yellow fever vaccines due to the egg-based manufacturing process. Most people with an egg allergy can safely receive these vaccines.
- Polyethylene Glycol (PEG) and Polysorbate: Used as stabilizers in certain vaccine formulations, including some mRNA and adenovirus-based vaccines.
- Trace Antibiotics: Such as neomycin, sometimes remain in minute quantities after being used during production to prevent bacterial contamination.
People with a known allergy to a specific excipient, like PEG, are at risk of reacting to any product containing that substance.
Identifying Symptoms and Severity of Post-Vaccination Reactions
Allergic reactions following vaccination can present across a spectrum of severity, from localized and mild to severe and systemic. Mild reactions often involve the skin, appearing as generalized itching, flushing, or hives (urticaria). These symptoms typically appear within minutes to a few hours after the injection. Swelling of the lips, tongue, or face, known as angioedema, is another common sign of an allergic response.
The most severe form is anaphylaxis, which is extremely rare, occurring at a rate of approximately one to two cases per million vaccine doses. Anaphylaxis is rapid in onset, usually developing within minutes but almost always within two hours of vaccination. It involves multiple body systems and is characterized by rapidly progressing symptoms.
Key signs include difficulty breathing due to throat or tongue swelling, wheezing, or shortness of breath. Cardiovascular symptoms, such as a rapid heart rate, weakness, dizziness, or a sudden drop in blood pressure, are also defining features. Gastrointestinal symptoms like vomiting or abdominal cramps may occur alongside severe skin involvement.
Protocols for Screening and Managing Allergic Risk
Healthcare providers follow established protocols to minimize the risk of allergic reactions and ensure immediate treatment if one occurs. Before administering any vaccine, providers screen individuals by asking about any history of severe allergic reactions to a prior dose of a vaccine or to any known vaccine component. This step helps identify high-risk patients who may need referral to an allergy specialist for further evaluation.
Following vaccination, the required 15- to 30-minute observation period is a safety measure. This observation window is designed to monitor for the immediate onset of an allergic reaction, particularly anaphylaxis, which occurs rapidly after exposure. If any signs of a severe reaction appear during this time, staff are immediately available to intervene.
Vaccination sites are well-equipped and prepared to manage a severe allergic event. The immediate and primary treatment for anaphylaxis is the administration of epinephrine, which rapidly reverses the life-threatening symptoms. The availability of epinephrine and trained personnel ensures that while allergic reactions are a rare possibility, they are managed effectively and promptly to prevent serious harm.