A trivalent vaccine is designed to protect against three different strains of a virus or bacteria. This type of vaccine is commonly used for diseases like influenza, where it targets three specific strains to provide broader immunity. By including multiple strains, the vaccine increases the chances of effectively preventing illness during an outbreak. These vaccines work by stimulating the immune system to recognize and fight off the targeted pathogens. When a person receives a trivalent vaccine, their body produces antibodies that can help defend against the influenza virus and reduce the severity of symptoms if infection occurs.