A quadrivalent vaccine is designed to protect against four different strains of a virus. This type of vaccine is commonly used for influenza, targeting two strains of influenza A and two strains of influenza B. By including multiple strains, the vaccine aims to provide broader immunity and reduce the risk of illness during flu season. These vaccines are typically administered annually, as the strains of the virus can change from year to year. The quadrivalent formulation helps ensure that individuals receive protection against the most prevalent and harmful strains, enhancing overall public health efforts in controlling outbreaks.