Is a high fever after a flu vaccine considered a severe reaction and should it be reported to Vaccine Adverse Event Reporting System (VAERS)?

High Fever After Flu Vaccine: Reporting to VAERS

A high fever after a flu vaccine is not considered a severe reaction by itself and does not require VAERS reporting, unless it leads to hospitalization, disability, or is accompanied by other concerning symptoms.

Understanding Post-Vaccination Fever

• Fever is a common systemic reaction following influenza vaccination, typically beginning 6-12 hours after vaccination and persisting for 1-2 days 1, 2
• Fever occurs more frequently in those with no prior exposure to influenza virus antigens (e.g., young children) 1
• In healthy children, postvaccination fever rates vary by age: 11.5% in children aged 1-5 years, 4.6% in children aged 6-10 years, and 5.1% in children aged 11-15 years 1
• Among high-risk children aged 6 months-4 years, fever rates can be higher (27%) 1
• Placebo-controlled trials in older persons and healthy young adults show that split-virus influenza vaccine is not associated with higher rates of systemic symptoms (including fever) compared with placebo injections 1

What Constitutes a Severe Reaction

• Severe reactions that should be reported to VAERS include:

  • Immediate allergic reactions such as hives, angioedema, allergic asthma, and systemic anaphylaxis 1
  • Fever ≥40.5°C (≥105°F) within 48 hours of vaccination 3
  • Fever accompanied by unusual irritability, persistent crying for ≥3 hours, collapse, or seizures 3
  • Any adverse event resulting in hospitalization, disability, or death 1

• Severe allergic reactions are rare and typically result from hypersensitivity to vaccine components, most commonly residual egg protein 1, 4

VAERS Reporting Guidelines

• The Advisory Committee on Immunization Practices (ACIP) recommends that healthcare professionals should report to VAERS all clinically significant adverse events after influenza vaccination, even if they are not certain the vaccine caused the event 1
• Fever alone is one of the most frequently reported events in VAERS for children, along with injection-site reactions and rash 1
• A high fever that is not accompanied by other concerning symptoms and resolves within the expected timeframe (1-2 days) is generally considered a normal immune response 2, 3

Special Considerations

• Fever that begins ≥24 hours after vaccination or persists for more than 24 hours should not be assumed to be vaccine-related and requires medical evaluation 3
• In rare cases, high fever may be part of a hypersensitivity reaction, particularly in patients with certain risk factors such as those on TNF-alpha inhibitors like etanercept 5
• Research suggests that subjective reports of local and systemic reactions following vaccination may correspond with biological indicators of inflammatory status 6
• The overall frequency of symptoms after influenza vaccination is generally low, with studies showing only a 5.5% higher risk of flu-like illness in the first week following vaccination compared to later periods 7

Management of Post-Vaccination Fever

• Acetaminophen can be safely administered at a dose of 10-15 mg/kg every 4-6 hours as needed for fever or discomfort, but is not recommended as a routine prophylactic measure for most healthy individuals 3
• Prophylactic acetaminophen is specifically recommended only for those with a history of seizures or those with a family history of convulsions 3

Remember that the benefits of influenza vaccination in preventing influenza and its severe complications far outweigh the potential risks of common side effects like fever 2. While fever is a common reaction, it is rarely severe enough to warrant VAERS reporting unless accompanied by other concerning symptoms or meeting specific severity criteria.