Are All Influenza Vaccines Inactivated?
No, not all influenza vaccines are inactivated—there are two main types available: inactivated influenza vaccines (IIV) and live attenuated influenza vaccines (LAIV). 1, 2
Two Distinct Vaccine Types
Inactivated Influenza Vaccines (IIV)
- Inactivated vaccines contain killed virus particles that have been rendered noninfectious through chemical inactivation processes (typically using formaldehyde), and therefore cannot cause influenza infection 1, 3, 4
- These vaccines are administered via intramuscular injection and are approved for persons aged ≥6 months, including those with chronic medical conditions and pregnant women 1, 2
- Multiple formulations exist including standard-dose, high-dose (HD-IIV3 for ≥65 years), adjuvanted (aIIV3 for ≥65 years), cell culture-based (ccIIV3), and recombinant (RIV3) vaccines 1
Live Attenuated Influenza Vaccines (LAIV)
- LAIV contains live but weakened influenza viruses that are temperature-sensitive, cold-adapted, and attenuated, meaning they can replicate in the cooler nasal passages but not in the warmer lower respiratory tract 1, 2
- This vaccine is administered intranasally via nasal spray and is approved only for healthy, non-pregnant persons aged 2-49 years 1, 2, 5
- LAIV has the potential to produce mild signs or symptoms related to influenza virus infection, unlike inactivated vaccines 1
Key Clinical Distinctions
Manufacturing and Composition
- Both vaccine types are grown initially in embryonated chicken eggs and contain the same three influenza virus strains (trivalent formulations for 2024-25 season) 1
- For inactivated vaccines, viruses are chemically inactivated and processed into split virus or purified surface antigen preparations 1, 3, 4
- For LAIV, viruses remain live but are attenuated through cold-adaptation 2, 5
Population-Specific Recommendations
- LAIV should NOT be used in: children <2 years, adults ≥50 years, pregnant women, immunocompromised individuals (including solid organ transplant recipients), persons with asthma or chronic medical conditions, children 2-4 years with recurrent wheezing, and healthcare workers caring for severely immunocompromised patients 2, 5
- Inactivated vaccines are appropriate for: all persons ≥6 months including high-risk groups, pregnant women, immunocompromised individuals, and healthcare workers 1, 2, 3
Important Clinical Caveats
- The Advisory Committee on Immunization Practices (ACIP) temporarily withdrew its recommendation for LAIV during 2016-17 and 2017-18 seasons due to poor effectiveness against influenza A (H1N1), but reinstated the recommendation after strain improvements 6
- For adults ≥65 years, higher-dose or adjuvanted inactivated vaccines are preferentially recommended over standard-dose options 1, 7
- Healthcare workers caring for immunocompromised patients should preferentially receive inactivated vaccine rather than LAIV to avoid potential viral shedding 2
- Both vaccine types provide similar overall protection against influenza illness (approximately 50% effectiveness), though effectiveness varies by age, immune status, and antigenic match between vaccine and circulating strains 5, 6, 8