Influenza Vaccine Components
Flu vaccines contain three main components: inactivated influenza virus strains (typically two type A and one type B), residual egg protein from the manufacturing process, and may contain preservatives like thimerosal or antibiotics to prevent bacterial contamination. 1
Active Viral Components
- Inactivated virus strains: The vaccine contains highly purified, egg-grown viruses that have been chemically inactivated (killed) and rendered completely noninfectious 1
- Hemagglutinin antigens: Standard-dose vaccines contain 15 μg of hemagglutinin (HA) per vaccine virus strain in a 0.5-mL dose, while high-dose formulations contain 60 μg per strain 1, 2
- Subvirion or purified surface-antigen preparations: These are available depending on the manufacturer's process 1
The specific strains change annually based on predicted circulating viruses, with trivalent vaccines containing two influenza A strains and one influenza B strain, while quadrivalent vaccines add a second influenza B strain 1, 3
Manufacturing Residuals
- Residual egg protein: Because vaccine viruses are initially grown in embryonated hens' eggs, small amounts of egg protein remain in the final product 1
- Antibiotics: Manufacturers may add antibiotics during production to prevent bacterial contamination 1, 2
- Inactivating compounds: Different manufacturers use various chemical compounds to inactivate the influenza viruses 1, 2
Preservatives and Adjuvants
- Thimerosal: Multi-dose vials typically contain approximately 25 mcg thimerosal (a mercury-containing compound) per 0.5-mL dose as a preservative, though single-dose preparations may contain <1 mcg mercury or be preservative-free 1
- Adjuvants (in specific formulations): Some vaccines like Fluad contain novel adjuvants such as MF59 or AS03 to enhance immune response, particularly in elderly populations 1, 4
Important Clinical Considerations
The killed virus in inactivated vaccines cannot cause influenza infection or produce signs and symptoms of influenza virus illness 2, 5. This is a critical distinction from live attenuated vaccines, which contain weakened but live virus and are administered nasally rather than by injection 5, 6.
Common Pitfall to Avoid
Do not confuse inactivated influenza vaccines with live attenuated vaccines—inactivated vaccines contain completely killed viruses and cannot replicate or cause infection, making them safe for immunocompromised patients, pregnant women, and elderly individuals 2, 5. The presence of egg protein means patients with severe egg allergies require special consideration, though most can safely receive the vaccine 1.