AS03 is an adjuvant system used in vaccines to enhance the body's immune response. It is designed to improve the effectiveness of vaccines by stimulating the immune system, allowing for a stronger and longer-lasting protection against diseases. Developed by GlaxoSmithKline, AS03 contains squalene, a natural oil, and other components that help activate immune cells. This adjuvant has been used in various vaccines, including those for influenza, to provide better protection and reduce the amount of antigen needed in each dose.