What is the typical duration of fever after receiving a flu vaccine?

Duration of Fever After Flu Vaccination

Fever after influenza vaccination typically begins 6-12 hours after vaccination and lasts for 1-2 days. 1

Fever Characteristics After Flu Vaccination

• Fever is an infrequent systemic reaction that most commonly affects persons who have had no prior exposure to influenza virus antigens, such as young children 1
• Fever occurs in approximately 10% to 35% of children younger than 2 years after receiving the trivalent inactivated influenza vaccine (TIV) 1
• The frequency of fever is much lower in older children and adults 1
• Fever typically begins 6-12 hours after vaccination and can persist for 1-2 days 1

Age-Related Differences in Fever Response

• Among children, fever rates vary by age group:
  • 11.5% of children aged 1-5 years
  • 4.6% of children aged 6-10 years
  • 5.1% of children aged 11-15 years 1
• Among high-risk children aged 6 months-4 years, fever has been reported in up to 27% of vaccine recipients 1
• Adults experience fever less frequently than children after influenza vaccination 1

Other Common Side Effects

• The most frequent side effect of vaccination is soreness at the vaccination site, affecting 10-64% of patients and typically lasting less than 2 days 1, 2
• Other systemic reactions that may accompany fever include:
  • Malaise
  • Myalgia (muscle aches)
  • Headache 1
• These systemic symptoms follow the same timeline as fever, typically resolving within 1-2 days 1

Important Considerations

• The influenza vaccine contains only noninfectious viruses and cannot cause influenza 1, 2
• Respiratory disease or other symptoms after vaccination often represent coincidental illness unrelated to influenza vaccination 1
• Placebo-controlled trials have demonstrated that among older persons and healthy young adults, administration of split-virus influenza vaccine is not associated with higher rates of systemic symptoms (including fever) when compared with placebo injections 1

When to Seek Medical Attention

• Fever after vaccination is generally a benign clinical sign 3
• However, if fever persists beyond 2 days or is accompanied by severe symptoms, medical attention should be sought as this may indicate an unrelated illness 1
• Immediate allergic reactions (such as hives, angioedema, allergic asthma, and systemic anaphylaxis) rarely occur after influenza vaccination and require immediate medical attention 1

Managing Post-Vaccination Fever

• Antipyretics may be used to manage post-vaccination fever if needed 4
• Minor illnesses with or without fever should not contraindicate the use of influenza vaccine, particularly among children with mild upper respiratory tract infection or allergic rhinitis 1