A cancer vaccine, or oncovaccine, is a vaccine that either treats existing cancer or prevents development of cancer. Vaccines that treat existing cancer are known as therapeutic cancer vaccines or tumor antigen vaccines. Some of the vaccines are "autologous", being prepared from samples taken from the patient, and are specific to that patient.

Some researchers claim that cancerous cells routinely arise and are destroyed by the immune system (immunosurveillance); and that tumors form when the immune system fails to destroy them.

Some types of cancer, such as cervical cancer and liver cancer, are caused by viruses (oncoviruses). Traditional vaccines against those viruses, such as the HPV vaccine and the hepatitis B vaccine, prevent those types of cancer. Other cancers are to some extent caused by bacterial infections (e.g. stomach cancer and Helicobacter pylori). Traditional vaccines against cancer-causing bacteria (oncobacteria) are not further discussed in this article.

One approach to cancer vaccination is to separate proteins from cancer cells and immunize patients against those proteins as antigens, in the hope of stimulating the immune system to kill the cancer cells. Research on cancer vaccines is underway for treatment of breast, lung, colon, skin, kidney, prostate and other cancers.

Another approach is to generate an immune response in situ in the patient using oncolytic viruses. This approach was used in the drug talimogene laherparepvec, a variant of herpes simplex virus engineered to selectively replicate in tumor tissue and to express the immune stimulatory protein GM-CSF. This enhances the anti-tumor immune response to tumor antigens released following viral lysis, creating a patient-specific vaccine.

Tumor antigen vaccines work the same way that viral vaccines work, by training the immune system to attack cells that contain the antigens in the vaccine. The difference is that the antigens for viral vaccines are derived from viruses or cells infected with virus, while the antigens for tumor antigen vaccines are derived from cancer cells. Since tumor antigens are antigens found in cancer cells but not normal cells, vaccinations containing tumor antigens should train the immune system to target cancer cells not healthy cells. Cancer-specific tumor antigens include peptides from proteins that are not typically found in normal cells but are activated in cancer cells or peptides containing cancer-specific mutations. Antigen-presenting cells (APCs) such as dendritic cells take up antigens from the vaccine, process them into epitopes, and present the epitopes to T-cells via Major Histocompatibility Complex proteins. If T-cells recognize the epitope as foreign, the adaptive immune system is activated and target cells that express the antigens.

Viral vaccines typically work by preventing the spread of the virus. Similarly, cancer vaccines can be designed to target common antigens before cancer evolves if an individual has appropriate risk factors. Additional preventive applications include preventing the cancer from evolving further or undergoing metastasis and preventing relapse after remission. Therapeutic vaccines focus on killing existing tumors. While cancer vaccines have generally been demonstrated to be safe, their efficacy still needs improvement. One way to potentially improve vaccine therapy is by combining the vaccine with other types of immunotherapy aimed at stimulating the immune system. Since tumors often evolve mechanisms to suppress the immune system, immune checkpoint blockade has recently received a lot of attention as a potential treatment to be combined with vaccines. For therapeutic vaccines, combined therapies can be more aggressive, but greater care to ensure the safety of relatively healthy patients is needed for combinations involving preventive vaccines.

Cancer vaccines can be cell-based, protein- or peptide-based, gene-based (DNA/RNA). or live attenuated bacterial- or viral organisms.