Oct. 19, 2025, 8:45 AM EDTBy Kaitlin SullivanCovid vaccines may come with a tantalizing benefit that has nothing to do with the virus they’re designed to protect against: boosting the immune system to better fight tumors during cancer treatment.That’s according to new results presented Sunday in Berlin at the European Society for Medical Oncology conference. The research is still in the earlier stages — it has yet to be tested in a Phase 3 clinical trial — but experts say it shows promise.“I am cautiously optimistic,” said Stephanie Dougan, an associate professor of cancer immunology and virology at the Dana-Farber Cancer Institute, who was not involved with the work. “There is a scientific logic to why this could work.”Researchers found that among cancer patients getting immunotherapy, those who got an mRNA Covid vaccine within 100 days before starting their treatment lived longer.Only about 20% of cancer patients who get immunotherapy — which harnesses a person’s immune system to fight cancer cells — respond to the treatment. Finding a way to boost the effectiveness of immunotherapy drugs has been a feat researchers have been exploring for years, with little success.Typically, the immune-stimulating tactics employed in the past have either done too little to activate the immune system or done too much, triggering an overactive response that can damage the body. There’s a chance that mRNA Covid vaccines could exist in a Goldilocks zone.“Maybe we just needed something that was medium-strong, and this could potentially be it,” said Dougan, who emphasized the need for more research.That research will soon be underway: Dr. Adam Grippin, a senior resident in radiation oncology at the University of Texas MD Anderson Cancer Center who co-led the study, said his team is launching a Phase 3 clinical trial to confirm the initial results.In the research presented Sunday, Grippin and his co-authors looked back at survival rates among more than 1,000 people who had advanced non-small cell lung cancer and got immunotherapy as part of their treatment from 2019 through 2023. Of those, 180 people received an mRNA Covid vaccine within 100 days of starting treatment.The median survival for the group — when exactly half of those who underwent treatment are still alive — was nearly twice as long for those who were vaccinated compared to those who were not: about three years compared to just over 1.5 years.The researchers also compared the survival rates in a smaller group of patients getting immunotherapy for metastatic melanoma. Forty-three got an mRNA Covid vaccine; 167 did not. For those who didn’t get vaccinated, median survival was just over two years. Those who were vaccinated prior to treatment hadn’t yet reached their median survival point more than three years into follow-up.In further experiments in mice, the researchers got an answer they believe matches the way the vaccines work in humans.“It superdrives the immune system against tumors,” Grippin said.Creating a beaconVaccines that use mRNA are already a promising area of cancer research. Scientists have developed personalized mRNA cancer vaccines that are tailored to fight a person’s unique tumor, as well as ones that target genes that are commonly found in certain types of cancer, including pancreatic. (These developments come as the Trump administration has canceled half a billion dollars in funding for mRNA vaccine research for infectious diseases.)If Grippin’s later trial confirms the results of the early research, it could represent the next frontier for research on mRNA vaccines and cancer.Immunotherapy drugs work by boosting the immune system’s ability to fight cancer, often by enhancing the power of immune cells called T cells that attack invaders, or by making tumors easier for T cells to find.The mouse portion of the new research found that Covid mRNA vaccination appeared to make the immune system more attuned to recognizing tumors as a threat by stimulating dendritic cells, a type of white blood cell. When dendritic cells detect a threat, they turn on a sort of beacon that leads T cells to the perceived invader so they can attack. However, not everyone naturally has T cells that are capable of fighting tumors, which is why scientists believe immunotherapies only work in some of the cancer patients who take them. In these people, the immune system recognizes cancer cells as a threat, but their specific T cells are unable to stop the tumors from growing.“It’s just random chance whether you have those cells or you don’t,” said Jeff Coller, a professor of RNA biology and therapeutics at Johns Hopkins Medicine, who was not involved with the research.Getting an mRNA Covid vaccine doesn’t change whether a person has the specific T cells needed to fight their tumors, but it does appear to make it more likely that dendritic cells will detect a tumor as a problem and direct the T cells a person does have to the tumor. If those cells happen to be programmed to be able to kill tumor cells, having an mRNA vaccine that lights up the target before a person starts an immunotherapy can give their immune system a boost that helps the cancer therapy work better.Coller said one reason mRNA technology may be the best tool to elicit this response is because every cell in the body already contains mRNA.“We are really tapping into that natural process that your body already knows how to respond to,” he said. “You are using your body’s natural system to fight tumors.”Dougan said it’s possible that other factors could have accounted for better survival among people who were vaccinated prior to immunotherapy treatment. For example, a Covid infection may have weakened an unvaccinated person’s body and hindered their ability to fight off cancer cells. In the past, early studies like this one have shown promising results that didn’t pan out in later trials. “We have been misled by retrospective studies before,” she said.Grippin agreed the findings warrant a closer look. “This data is exciting, but all of these findings need to be validated in Phase 3 clinical trials to determine whether these vaccines should be used in our patients,” he said.Kaitlin SullivanKaitlin Sullivan is a contributor for NBCNews.com who has worked with NBC News Investigations. She reports on health, science and the environment and is a graduate of the Craig Newmark Graduate School of Journalism at City University of New York.