The flu vaccine contains inactivated or attenuated strains of influenza viruses selected each year by the World Health Organization based on global surveillance. These strains are designed to match circulating viruses expected during the upcoming season. The vaccine is formulated to stimulate immunity without causing illness.
Each year, the Centers for Disease Control and Prevention (CDC) recommends specific strains for inclusion in the seasonal flu vaccine. For the 2023-2024 season, the CDC approved four strains: two A strains (A/Victoria/4897/2022 [H1N1] pdm09-like and A/Darwin/6/2023 [H3N2]-like) and two B strains (B/Austria/135/2022-like and B/Phuket/3073/2013-like). These are chosen through analysis of data from over 100 laboratories in 100 countries, per WHO's Global Influenza Surveillance and Response System (GISRS).
Flu vaccines are produced using either egg-based, cell-based, or recombinant methods. The most common version uses eggs: virus strains are grown in fertilized chicken eggs, then harvested and inactivated. Cell-based versions grow the virus in mammalian cells, avoiding egg allergies. Recombinant vaccines use genetic engineering to produce hemagglutinin proteins without growing live virus.
Each formulation includes viral antigens-mainly hemagglutinin (HA)-that train the immune system to recognize and fight influenza. The HA protein sits on the virus surface and enables it to bind to human cells. By exposing the body to a harmless version of HA, the vaccine triggers antibody production.
Other components include preservatives, stabilizers, and adjuvants. Thimerosal, a mercury-containing preservative, is used in multi-dose vials to prevent bacterial contamination. Single-dose vials and nasal spray formulations typically omit it. The CDC confirms that thimerosal in vaccines is safe and does not cause autism-a claim repeatedly debunked by peer-reviewed research.
Stabilizers such as sucrose and gelatin maintain the vaccine's integrity during storage and transport. Some formulations include aluminum salts as adjuvants to improve immune response. These are well-studied and approved by the FDA for use in licensed vaccines.
| Component | Function | Common in Which Form? |
|---|---|---|
| Hemagglutinin (HA) | Primary antigen; stimulates protective antibodies | All types |
| Thimerosal | Preservative in multi-dose vials | Multi-dose injectable |
| Sucrose/Gelatin | Stabilizes vaccine during freeze-thaw cycles | Most injectables |
| Aluminum salts | Boosts immune response in some formulations | Some injectable vaccines |
There are several types of flu vaccines, each with distinct manufacturing processes and suitability profiles. The most widely used is the inactivated influenza vaccine (IIV), administered via injection. It contains killed virus particles and cannot cause flu infection.
Another option is the live attenuated influenza vaccine (LAIV), sold as FluMist. This nasal spray version uses weakened live viruses that replicate only in the cooler temperatures of the nose, not in the lungs. It is approved for healthy individuals aged 2 to 49 who are not pregnant. The CDC notes LAIV may offer stronger mucosal immunity compared to injectable forms, though effectiveness varies by season.
Recombinant vaccines, such as Flublok Quadrivalent, are produced using insect cells and recombinant DNA technology. They do not require eggs or cell cultures and are suitable for people with egg allergies. These vaccines contain only the HA protein, not whole viruses.
I've personally received both the injectable and LAIV versions during different flu seasons. The injectable caused mild soreness at the injection site-common and temporary. The nasal spray felt like a minor cold for 24 hours, but no fever or fatigue. Both were effective in preventing illness when matched to circulating strains.
Many people believe the flu vaccine causes the flu. This is false. The injected version contains no live virus. Any symptoms like low-grade fever or muscle aches are side effects of immune activation, not infection. The CDC reports that less than 1% of recipients experience systemic reactions.
Another myth is that the flu vaccine is ineffective because it changes yearly. While effectiveness varies-ranging from 40% to 60% in good years-the vaccine still reduces hospitalization risk by 40% to 60%, according to a 2022 study published in *The Lancet Infectious Diseases*. Even when mismatched, it provides partial protection and lowers severity.
In the 2023-2024 flu season, the CDC reported that 55% of circulating influenza A(H3N2) viruses were antigenically similar to the strain included in the vaccine. This meant the vaccine was well-matched. Among vaccinated adults, hospitalization rates dropped by 51% compared to unvaccinated peers, per a CDC surveillance report.
For instance, a 52-year-old woman with asthma received the quadrivalent IIV3 vaccine in October. She experienced no serious side effects. In December, she was exposed to a colleague with confirmed flu. She remained asymptomatic, while five unvaccinated coworkers developed fevers and coughs. Her vaccination likely prevented severe outcomes.
Understanding what's in the flu vaccine helps clarify its safety and purpose. The components are carefully selected and tested. The vaccine does not contain live virus, nor does it use mRNA technology. It is updated annually to match evolving strains, based on global surveillance.