Can the influenza vaccine cause flu-like symptoms in infants?

Can Influenza Vaccine Cause Flu-Like Symptoms in Infants?

Yes, influenza vaccine can cause mild flu-like symptoms in infants, but these are typically limited to fever, irritability, and local reactions—the vaccine cannot cause actual influenza infection. 1

Common Post-Vaccination Symptoms in Infants

The trivalent inactivated influenza vaccine (TIV), which is the only formulation approved for infants 6 months and older, contains killed virus and therefore cannot produce active influenza infection. 1 However, the vaccine can trigger immune responses that produce mild symptoms:

Local Reactions

• Injection site pain, tenderness, redness, and swelling are the most common adverse events after TIV administration in infants 1

Systemic Symptoms

• Fever occurs in approximately 10-35% of children younger than 2 years within 24 hours after immunization, though it rarely occurs in older children 1
• Drowsiness, irritability, and loss of appetite are commonly reported systemic reactions in infants receiving IIV4 (quadrivalent inactivated influenza vaccine) 1
• Mild symptoms including lethargy, nausea, headache, and muscle aches may occur but are less common than local reactions 1

Important Clinical Distinction

The killed vaccine might theoretically produce mild influenza-like symptoms by inducing some of the same cytokines associated with influenza disease, but this is fundamentally different from actual influenza infection. 1 These symptoms are:

• Self-limited and typically resolve within 24-48 hours 1
• Much milder than actual influenza illness, which in infants can present with high fever, febrile seizures (up to 20% of hospitalized cases), and signs mimicking bacterial sepsis 1
• Not contagious, unlike actual influenza infection 1

Special Consideration: Febrile Seizures

• Febrile seizures can occur after influenza vaccination in young children, particularly when TIV is administered concomitantly with PCV13 or DTaP vaccines 1
• The maximum additional risk is approximately 30 febrile seizure cases per 100,000 children vaccinated when vaccines are given simultaneously 1
• The benefits of timely vaccination outweigh this small risk, as febrile seizures rarely have long-term sequelae, and the alternative—actual influenza infection—carries much higher morbidity and mortality 1
• Routine use of antipyretic agents to prevent febrile seizures is not recommended, as they do not reduce incidence 1

Clinical Management Guidance

• Minor illnesses with or without fever are NOT contraindications to influenza vaccination in infants 1
• Parents should be counseled that mild fever and irritability are expected immune responses, not actual influenza 1
• Symptoms persisting beyond 2-3 days or worsening severity should prompt evaluation for other causes, as these would be atypical for vaccine reactions 2

Critical Pitfall to Avoid

Do not confuse post-vaccination symptoms with actual influenza illness or delay vaccination due to concerns about mild reactions. 1 The morbidity and mortality from actual influenza infection in infants—including hospitalization rates comparable to the elderly—far exceeds the minor discomfort from vaccine-related symptoms. 3, 4