What are the differences in side effects between live and killed (inactivated) influenza vaccines?

Differences in Side Effects Between Live and Inactivated Influenza Vaccines

The primary difference in side effects is that inactivated influenza vaccines (IIV) cannot produce signs or symptoms of influenza virus infection, while live attenuated influenza vaccines (LAIV) have the potential to produce mild signs or symptoms related to influenza virus infection due to their live virus content. 1

Overview of Vaccine Types

Inactivated Influenza Vaccine (IIV)

• Administration route: Intramuscular injection
• Composition: Contains killed viruses
• Approved population: Persons ≥6 months of age, including both healthy individuals and those with chronic medical conditions 1
• Immune response: Primarily stimulates systemic immunity 2

Live Attenuated Influenza Vaccine (LAIV)

• Administration route: Intranasal spray
• Composition: Contains live, attenuated (weakened) viruses
• Approved population: Healthy individuals 2-49 years of age 1
• Immune response: Stimulates both local mucosal and systemic immunity 2

Side Effect Profiles

IIV (Inactivated) Side Effects

• Local reactions:

  • Sore arm and redness at the injection site (most common) 3
  • Pain at injection site

• Systemic reactions (less common):

  • Fever
  • Malaise
  • Myalgia 3

• Rare adverse events:

  • Guillain-Barré Syndrome (GBS) risk estimated at approximately 1-2 cases per 1 million vaccinees 3

LAIV (Live) Side Effects

• Local reactions:

  • Nasal congestion (most common) 1, 3
  • Runny nose

• Systemic reactions:

  • Headache
  • Myalgias
  • Fever 1, 3
  • Potential for mild influenza-like symptoms 1

• Special concerns:

  • Increased risk of wheezing in young children, particularly those with history of asthma or reactive airway disease 1, 3

Key Differences in Safety Profile

1. Risk of infection: LAIV contains live viruses that can replicate and therefore has the potential to produce mild signs or symptoms related to influenza virus infection, while IIV cannot cause infection 1

2. Respiratory symptoms: LAIV is associated with more nasal/respiratory symptoms due to its intranasal administration and viral replication 1, 3

3. Age-related safety concerns: LAIV shows increased risk of wheezing in children under 5 years, particularly those with history of asthma or wheezing 1, 3

4. Contraindications: LAIV has more contraindications due to its live virus nature, including:

   • Children <2 years
   • Adults ≥50 years
   • Pregnant women
   • Immunocompromised individuals
   • Children with asthma or history of wheezing
   • Persons taking influenza antiviral medications (within 48 hours) 1, 2

Clinical Implications

• For healthy individuals aged 2-49 years without contraindications, both vaccine types are options, with different side effect profiles to consider 1

• For individuals with history of asthma, reactive airways disease, or other chronic disorders, IIV is the safer choice due to the potential for respiratory symptoms with LAIV 1

• For children under 2 years, adults over 49 years, pregnant women, and immunocompromised individuals, only IIV is appropriate 1

• Both vaccines are contraindicated in individuals with severe egg allergy (anaphylaxis) 1

Practical Considerations

• When discussing vaccine options with patients, explain that LAIV may cause more nasal/respiratory symptoms but avoids injection site reactions

• For children with any history of wheezing or asthma, the IIV is the safer choice despite requiring an injection 1, 2

• For healthcare workers or caregivers of severely immunocompromised individuals, IIV is preferred to avoid potential transmission of vaccine virus 2