Quadrivalent vaccines are designed to protect against four different strains of a virus. They are commonly used for preventing diseases like influenza and human papillomavirus (HPV). By targeting multiple strains, these vaccines aim to provide broader immunity and reduce the risk of infection. These vaccines work by stimulating the immune system to recognize and fight off the targeted viruses. For example, the quadrivalent influenza vaccine includes two strains of influenza A and two strains of influenza B. This approach helps ensure better protection during flu season, as the circulating strains can vary each year.