Differences Between IIV (Inactivated Influenza Vaccine) and LAIV (Live Attenuated Influenza Vaccine)
The main difference between IIV and LAIV is that IIV contains killed influenza viruses while LAIV contains weakened live viruses that can replicate; this fundamental distinction drives their different administration routes, safety profiles, and patient eligibility. 1
Key Differences
Composition and Mechanism
IIV (Inactivated Influenza Vaccine):
- Contains killed influenza viruses that cannot replicate
- Administered via intramuscular injection
- Stimulates primarily systemic immunity 1
LAIV (Live Attenuated Influenza Vaccine):
Patient Eligibility
IIV Eligibility:
- Approved for persons aged ≥6 months
- Can be used in both healthy individuals and those with chronic medical conditions
- Safe for immunocompromised patients and their close contacts 1
LAIV Eligibility:
- More restricted use: only for healthy persons within specific age ranges (typically 2-49 years)
- Contraindicated in:
- Children <2 years
- Adults ≥50 years
- Pregnant women
- Immunocompromised individuals
- Children with asthma or history of wheezing
- Persons with chronic medical conditions 1
Contraindications and Precautions
IIV Contraindications:
- History of severe allergic reaction to any component of the vaccine or previous dose 1
LAIV Contraindications:
- History of severe allergic reaction to vaccine components or previous dose
- Concomitant aspirin or salicylate therapy in children and adolescents
- Children aged 2-4 years with asthma or wheezing history
- Persons taking influenza antiviral medications (within 48 hours) 1
Clinical Considerations for Vaccine Selection
When to Choose IIV:
- For patients <2 years or ≥50 years
- Immunocompromised patients
- Patients with chronic medical conditions
- Pregnant women
- Healthcare workers caring for severely immunocompromised patients 1
When to Choose LAIV:
- Healthy individuals aged 2-49 years without contraindications
- Particularly beneficial for children who might resist injections
- May provide better mucosal immunity which mimics natural infection 1, 2
Efficacy Considerations
- Both vaccines contain antigenically equivalent strains of influenza viruses (typically two influenza A strains and one or two B strains)
- Effectiveness varies by season, strain match, and patient population
- Some studies suggest LAIV may provide better protection in children, while IIV may be more consistently effective in adults 1, 3
Common Pitfalls in Vaccine Selection
- Failure to screen for contraindications: Always screen for asthma/wheezing history before administering LAIV to children
- Inappropriate timing: Avoid LAIV within 48 hours of antiviral medication use
- Overlooking close contacts: IIV is preferred for close contacts of severely immunocompromised individuals 1
- Egg allergy misconceptions: Current guidelines allow most patients with egg allergy to receive either vaccine with appropriate precautions 1
Practical Implications
- Both vaccines require annual administration due to waning immunity and viral antigenic drift 1
- Cost considerations: LAIV is typically more expensive than IIV 1
- Storage requirements differ between vaccine types and should be considered in vaccine program planning
By understanding these key differences, clinicians can select the most appropriate influenza vaccine for each patient, optimizing protection against influenza while minimizing risks.