The immune system needs those two weeks to gear up and make the immune cells and substances that can attack the flu virus when it first shows up in your nose or throat. By preparing ahead of time, your immune system can be ready to get rid of the viruses as soon as they enter your body and before they have time to make you sick. Because the flu changes from year to year, each year you need a new flu vaccine. But the immune system is still able to protect you against last year's flu type. Cancer vaccines , however, are different.
Cancers can vary widely among types of cancer and among individuals.
Because of this variation, the number of different cancer types, and the unpredictability of who might get cancer, preventive cancer vaccines are not yet possible. Current cancer vaccines are therapeutic , used to treat rather than prevent cancer, and given after a person already has cancer. Therefore, the goal of a cancer vaccine is not to prevent disease, but rather to get the immune system to attack existing cancerous cells. Fighting an established cancer is a difficult task, so most vaccines are not used alone, but in combination with additional substances that help stimulate the immune response in general, called cytokines or adjuvants .