How is the active ingredient of a vaccine?

Different with each. The active ingredient of any vaccine is the part of the infecting agent, either weakened and live and whole, or a part of the organism after it has been heat- killed, that stimulates your body to make antibodies that protect against a recurring infection with that organism. This preparation is different for each vaccine, based on the type...
The main ingredient. The active ingredient, or main ingredient, of a vaccine is the part that causes a person to develop antibodies to protect against a specific germ (virus or bacteria). The other ingredients include chemicals to help the active ingredient work, chemicals to balance the pH (acidity), preservatives, stabilizers to keep the vaccine potent, etc...