What is the cause of an adverse reaction to a vaccine (vaccination)?

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Last updated: April 11, 2025View editorial policy

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From the Guidelines

If you're experiencing a vaccine reaction, it's typically normal to have mild side effects like soreness at the injection site, low-grade fever, fatigue, or mild body aches for 1-3 days after vaccination, as indicated by the most recent and highest quality study 1. These reactions indicate your immune system is responding appropriately. For discomfort, you can take acetaminophen (Tylenol) 650mg every 6 hours or ibuprofen (Advil, Motrin) 400mg every 6-8 hours as needed. Apply a clean, cool washcloth to the injection site for swelling or soreness. Stay hydrated and rest if feeling fatigued. However, seek immediate medical attention if you develop severe symptoms like high fever (over 103°F/39.4°C), difficulty breathing, hives, swelling of face/throat, severe weakness, or symptoms lasting more than 3 days. These could indicate a rare but serious allergic reaction requiring emergency care, as noted in 1. Most vaccine reactions are simply your body building protection against the disease and resolve quickly without intervention.

Some key points to consider when experiencing a vaccine reaction include:

  • Local reactions are usually the least severe and most frequent, as stated in 1.
  • Systemic reactions (e.g., fever) occur less frequently than local reactions.
  • Serious allergic reactions (e.g., anaphylaxis) are the most severe and least frequent, as mentioned in 1 and 1.
  • Severe adverse reactions are rare, but it's essential to report them to public health authorities, including serious events, as emphasized in 1.

It's crucial to prioritize your health and seek medical attention if you experience any severe symptoms or if you're unsure about your reaction. Always consult with a healthcare professional for personalized advice and guidance. Remember, vaccine reactions are generally mild and temporary, and the benefits of vaccination far outweigh the risks, as supported by the evidence from 1 and other studies.

From the Research

Vaccine Reaction Overview

  • Vaccine reactions can range from mild to severe, with anaphylaxis being a rare but potentially life-threatening hypersensitivity reaction 2, 3.
  • The incidence of anaphylaxis after vaccination is estimated to be around 1.3 per million vaccine doses administered, with some vaccines having a slightly higher risk 2, 3.

Types of Hypersensitivity Reactions

  • Hypersensitivity reactions to vaccines can be classified as immediate (occurring within 4 hours of vaccination) or non-immediate (occurring later) 4.
  • Immediate reactions are potentially IgE-mediated and require an allergy workup, while non-immediate reactions may not require an allergy workup 4.
  • Local reactions, such as redness and swelling at the injection site, are the most common adverse event following vaccination and generally do not require an allergy workup 4.

Risk Factors and Contraindications

  • A previously known food allergy, such as an egg or milk allergy, is not typically a contraindication to immunization 4.
  • However, patients with a known allergy to specific components of vaccines, such as gelatin or latex, may require an allergy workup before vaccination 4.
  • The risk of anaphylaxis may be higher in certain individuals, such as those with a history of severe allergic reactions 3.

Management and Prevention

  • All personnel and facilities providing vaccinations should have procedures in place for anaphylaxis management, including prompt recognition and treatment of symptoms 2.
  • Healthcare providers play a critical role in maintaining public confidence in vaccines and should be knowledgeable about vaccine safety and adverse event management 5, 6.
  • Effective communication about vaccine safety and adverse events is essential for building trust and promoting vaccine uptake 6.

References

Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

Research

Vaccine-Associated Anaphylaxis.

Current treatment options in allergy, 2019

Research

Diagnosis and management of hypersensitivity reactions to vaccines.

Expert review of clinical immunology, 2020

Research

Evaluation of Adverse Reactions to Vaccines.

The journal of allergy and clinical immunology. In practice, 2021

Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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