Influenza A virus A/Hong Kong/2671/2019 IVR-208 (H3N2) antigen (formaldehyde inactivated)

A seasonally-specific component of the influenza vaccine. The influenza vaccine, also known as the "flu shot", is a vaccine that protects against infection from the influenza viruses. Vaccines provide protection from influenza by exposing the immune system to the virus (or parts of the virus) which stimulates an immunological defence against future exposure to the virus, or "antigen". This defence includes the production of humoral immunity through the development of antibodies (through memory B cells) and of cell-mediated immunity through the production of T-lymphocytes. Upon re-exposure to infectious influenza virus, the immune system is prepared to identify and destroy the virus as there are circulating antibodies that recognize that particular component of the virus that it was previously exposed to.

There are two basic types of vaccines available: inactivated influenza vaccine (IIV) and live attenuated influenza vaccine (LAIV). Inactivated vaccines contain a virus particle that has been grown in media and then subsequently killed, or inactivated, through exposure to heat or chemicals such as formaldehyde 3. Inactivated virus cannot replicate, and therefore cannot cause disease from infection, even in immunocompromised individuals. In contrast, live vaccines are produced from "wild-type" or disease-causing viruses that have been attenuated, or weakened, through various laboratory techniques. Live vaccines maintain their replicative ability.