Experiencing a skin reaction after a vaccination can be unsettling, especially when the reaction involves hives, known medically as urticaria. While COVID-19 vaccines are safe and effective tools for preventing severe illness, they can occasionally trigger dermatological side effects. Hives represent one type of immune response that people may notice in the days or weeks following their shot. Understanding the nature of this reaction and knowing the appropriate steps for management are important.
Understanding Hives and Their Prevalence
Hives are characterized by raised, itchy bumps or patches on the skin that can appear anywhere on the body. These welts often look pale or pink, typically blanch (turn white) when pressed, and can change shape or move across the body over a period of hours. The vast majority of these post-vaccine reactions are mild and resolve on their own, usually within a few days to a week. The prevalence of urticaria following mRNA COVID-19 vaccination is low, reported in some studies to be around 1% to 2% of vaccinated individuals. This common skin reaction should be distinguished from a severe, immediate allergic reaction, which is rare and involves systemic symptoms.
The Immune Mechanism Behind Post-Vaccine Urticaria
The development of hives after vaccination indicates an activated immune system responding to the vaccine components. An immediate reaction, occurring within minutes to four hours, is typically a Type I hypersensitivity mediated by immunoglobulin E (IgE) antibodies, which drives anaphylaxis. More commonly, hives appear after COVID-19 vaccination with a delayed onset, sometimes appearing days or even up to two weeks after the shot. These delayed reactions are thought to be caused by a non-IgE-mediated mechanism, often involving a broader inflammatory response or T-cell activation.
This type of reaction is an immune inflammatory response to the vaccine itself, rather than a true allergy to a specific component. While components like polyethylene glycol (PEG) or other excipients have been suggested as triggers, skin testing for these materials often comes back negative in people with delayed urticaria. The delayed timing suggests a systemic response where the immune system recognizes the vaccine’s active ingredients, leading to the release of mediators like histamine that cause the welts. Some data suggests a higher rate of delayed urticaria following the Moderna mRNA vaccine compared to the Pfizer-BioNTech vaccine, possibly due to the higher mRNA dose or greater immunogenicity.
Immediate Steps and When to Seek Medical Care
For isolated cases of mild urticaria without other symptoms, the reaction can often be managed at home. Over-the-counter H1 antihistamines, such as cetirizine or fexofenadine, are the first-line treatment and are typically effective in controlling the itching and rash. Applying a cool compress to the affected areas can also provide temporary relief from the itching associated with hives. The median time for symptoms to resolve is approximately four days.
It is crucial to distinguish simple hives from the signs of a severe, systemic allergic reaction like anaphylaxis, which is a medical emergency. Anaphylaxis involves symptoms beyond the skin and typically occurs rapidly, within four hours of vaccination. Warning signs requiring immediate emergency medical attention include difficulty breathing, wheezing, throat tightness, swelling of the tongue or throat (angioedema), or dizziness due to a drop in blood pressure. If hives are accompanied by any of these systemic symptoms, call emergency services immediately; isolated hives alone are not an emergency.
Guidance for Subsequent Vaccine Doses
Experiencing hives after one dose does not mean you cannot receive a subsequent dose, such as a second shot or a booster. Urticaria alone, even if it occurred immediately, is generally not considered a contraindication to completing the vaccination series. However, before getting the next dose, it is highly recommended to consult with a healthcare provider or an allergist-immunologist. They can assess the severity of your initial reaction and provide personalized guidance.
For those who had a delayed reaction, the doctor may suggest pre-medication with an antihistamine taken shortly before the next shot, but this must be done only under medical advice. Even if hives recur after a subsequent dose, the reaction is not considered life-threatening and can be safely managed with antihistamines. In some cases, a provider might recommend receiving a different vaccine platform for the next dose, such as switching from an mRNA vaccine to a protein subunit vaccine. The decision to proceed should be made with your physician, balancing the small risk of recurrence against the significant protection the vaccine offers.