MF59 is an adjuvant, a substance that enhances the body's immune response to vaccines. It is an oil-in-water emulsion that helps improve the effectiveness of certain vaccines by promoting a stronger and longer-lasting immune reaction. Developed by Novartis, MF59 is primarily used in influenza vaccines. By stimulating the immune system, it allows for a reduced dose of the vaccine while still providing adequate protection against the flu. This makes it particularly useful in situations where vaccine supply is limited or when targeting vulnerable populations.