Inactivated flu vaccines contain inactivated (killed) influenza viruses. These viruses are not live and cannot cause the flu. The inactivated virus particles in the vaccine stimulate the immune system to recognize and fight the virus if exposed to the live virus later. More specifically, there are different types of inactivated influenza vaccines:
- Whole virus inactivated vaccines (contain the entire killed virus)
- Split virus vaccines (contain virus particles that are broken up)
- Subunit vaccines (contain only purified influenza virus antigens, such as hemagglutinin).
Thus, the type of virus in inactivated flu vaccines is the influenza virus itself, but it is inactivated or killed so it cannot cause infection. The viruses contained usually represent strains of influenza A and B, chosen each year based on predictions of circulating strains.
