Flu Vaccine Types: Both Inactivated and Live Attenuated Forms Are Available
The "flu shot" specifically refers to the inactivated (killed virus) influenza vaccine administered by intramuscular injection, but a live attenuated influenza vaccine (LAIV) is also available as an intranasal spray. 1, 2
Inactivated Influenza Vaccine (IIV) - The Traditional "Flu Shot"
The inactivated influenza vaccine contains killed virus particles that have been rendered completely noninfectious through chemical inactivation processes. 1, 3, 2
Key Characteristics:
- Administered via intramuscular injection (0.5 mL dose) 3, 2
- Contains killed viruses that cannot cause influenza infection or produce any signs or symptoms of influenza virus infection 3, 2
- Approved for persons aged 6 months and older, including those with chronic medical conditions 2, 4
- Available in trivalent (IIV3) and quadrivalent (IIV4) formulations 1
Manufacturing Process:
- Vaccine viruses are grown in embryonated hens' eggs 1, 3
- Chemical compounds are used to inactivate (kill) the influenza viruses 1, 3
- Purified surface antigen preparations are created 1, 3
- Antibiotics are added to prevent bacterial contamination 3
Live Attenuated Influenza Vaccine (LAIV) - The Nasal Spray
The live attenuated influenza vaccine contains live but weakened influenza viruses that are temperature-sensitive, cold-adapted, and attenuated. 2
Key Characteristics:
- Administered via intranasal spray 2, 4
- Contains live but weakened viruses that produce mild or no symptoms 2
- Only approved for healthy persons aged 2-49 years 2, 4
- Available as quadrivalent formulation (LAIV4) 1
Important Restrictions for LAIV:
- NOT for persons younger than 2 years or 50 years and older 2
- NOT for persons with asthma or other chronic disorders 2
- NOT for immunocompromised individuals 2
- NOT for pregnant women 2
- NOT for children or adolescents receiving aspirin or salicylates 1, 2
- NOT for solid organ transplant recipients 2
Clinical Distinction
When someone refers to "the flu shot," they are specifically referring to the inactivated vaccine given by injection, which is the most widely used form and appropriate for the broadest population. 1, 2 The live attenuated vaccine is a distinct alternative delivery method with more restricted indications. 2
Safety Consideration:
Recipients of LAIV may shed vaccine virus for up to 2-7 days after vaccination and should take precautions to minimize contact with severely immunocompromised individuals. 2 In contrast, the inactivated vaccine cannot cause viral shedding because it contains killed viruses. 3