What are cancer vaccines?

Vaccines for infectious diseases have changed the trajectory of humankind. In the 20th century alone, smallpox killed more than 300 million people worldwide, and polio killed or paralyzed half a million people each year. Today, thanks to vaccines, smallpox has been eradicated worldwide, meaning it's essentially extinct; and polio has been eliminated in many countries, so the disease is no longer endemic to those places.

With the success of vaccines for infectious diseases in mind, scientists have wondered if it might be possible to similarly harness the power of the immune system against other conditions. Now, researchers are working to develop vaccines for cancer.

But what, exactly, are cancer vaccines, and how do they work?

Related: The 10 deadliest cancers, and why there's no cure

How do regular vaccines work?

Vaccines, broadly, are substances that train the immune system to defend the body against a dangerous invader. They help the immune system recognize a pathogen by exposing the body to key features of that germ, such as proteins from a virus's surface. These features are called antigens, and when they're introduced through a vaccine, the immune system learns to recognize them as a threat.

Once the immune system becomes familiar with antigens from a virus or bacterium, it will then be able to quickly mount an attack against that pathogen if it ever comes into contact with the bona fide germ. That's how vaccines stop people from contracting infectious diseases, such as smallpox or measles. If a vaccine doesn't completely block an infection from happening, it can still make the resulting illness much less dangerous — think of the annual flu shot.

"They [vaccines] harness the inbuilt ability of our immune systems to recognize something that is foreign to our bodies," Dr. Vinod Balachandran, director of the Olayan Center for Cancer Vaccines at Memorial Sloan Kettering Cancer Center, told Live Science.

Some common vaccines against infectious diseases already help prevent cancer. The human papillomavirus (HPV) vaccine, for example, protects against the strains of HPV that are most likely to cause cervical cancer, and the hepatitis B vaccine protects against liver cancer caused by chronic hepatitis B infections. But these vaccines don't actually target the cancer itself; they instead take aim at viruses that increase the risk of cancer.

How do cancer vaccines work?

In contrast, when scientists talk about a "cancer vaccine," they're referring to a vaccine that targets cancer directly, rather than using the indirect methods mentioned above, Balachandran said.

Harnessing the immune system to fight cancer with a vaccine is tricky, though, because vaccines rely on the immune system's recognition of a pathogen as a foreign invader. But cancer grows from our own cells — and that means the genetic and molecular compositions of a cancer cell are relatively similar to those of a healthy one.

However, certain molecules are found only in cancer cells, and researchers are attempting to use those molecules to train the immune system to fight cancer. They call these molecules neoantigens, and they're introduced to healthy cells through processes such as genetic mutation.

"If you can identify the neoantigens in a cancer that the immune system can recognize, you can teach the immune system to recognize a cancer as foreign," Balachandran explained.

There are some neoantigens that everyone with a certain type of cancer may have, but neoantigens can also be specific to an individual. Researchers are still investigating the most effective neoantigens to target for different types of cancer. Unlike vaccines for infectious diseases, Balachandran said, cancer vaccines will likely need to be designed for individual patients, or made in small batches, to ensure they're targeting these different neoantigens efficiently.

In his own lab, Balachandran is conducting small trials with human patients to develop a vaccine for a deadly form of pancreatic cancer. After surgically removing patients' tumors, Balachandran gave the participants a regimen of immune-boosting and chemotherapy drugs alongside a personalized vaccine that targets specific neoantigens seen in their tumors. The vaccines contain mRNA, a genetic molecule that, in this case, carries blueprints for the neoantigens. Once inside the patient, the vaccine enables cells to build those neoantigens and show them to the immune system.

Related: New mRNA vaccine for deadly brain cancer triggers a strong immune response

Half of the vaccinated patients in the 16-person trial showed a boost in cancer-fighting immune molecules, and their cancers didn't return for the duration of the 18-month study. These results suggest that, at least in some patients, personalized cancer vaccines could decrease the likelihood that deadly cancers will return.

Do cancer vaccines treat cancer, or do they prevent it?

Balachandran explained that currently, many cancer vaccines are targeted at what physicians call "secondary prevention." This means that they're designed to stop cancer from returning in a person that's currently in remission, rather than preventing cancer from emerging in the first place.

That said, there are also therapeutic cancer vaccines that can treat existing cancers. These work like immunotherapies for cancer, by revving up the immune system to fight tumors.

As of 2025, one therapeutic cancer vaccine has been approved by the U.S. Food and Drug Administration. This vaccine, called sipuleucel-T (brand name Provenge), helps direct the immune system to attack a typically incurable form of prostate cancer. The vaccine contains a patient's own cells, which have been "activated" in the lab through exposure to a prostate-cancer protein. Trials suggest the treatment extends patients' survival by a few months.

Scientists are investigating many more cancer vaccines, including some for brain cancer and skin cancer, in clinical trials. Different vaccines are at different points in the clinical trial process; some are still in early research, whereas others, such as Merck and Moderna's vaccine for melanoma, are in the final stage of clinical trials..

If current efforts to design cancer vaccines for secondary prevention are successful, Balachandran hopes that researchers could one day design cancer vaccines for primary prevention — stopping people from ever developing cancer in the first place. A vaccine for primary prevention would work more like a traditional shot for infectious diseases by blocking the condition from ever emerging.

"If we now know that the immune system can also recognize cancer, it should, in theory, be possible to develop a vaccine against cancer, like we have been able to do against pathogens," Balachandran said. "It's an exciting time for the field right now."