The COVID-19 mRNA-based vaccines that saved 2.5 million lives globally during the pandemic could help spark the immune system to fight cancer. This is the surprising takeaway of a new study that we and our colleagues published in the journal Nature.

While developing mRNA vaccines for patients with brain tumors in 2016, our team, led by pediatric oncologist Elias Sayour, discovered that mRNA can train immune systems to kill tumors — even if the mRNA is not related to cancer.

Based on this finding, we hypothesized that mRNA vaccines designed to target the SARS-CoV-2 virus that causes COVID-19 might also have antitumor effects.

So we looked at clinical outcomes for more than 1,000 late-stage melanoma and lung cancer patients treated with a type of immunotherapy called immune checkpoint inhibitors. This treatment is a common approach doctors use to train the immune system to kill cancer. It does this by blocking a protein that tumor cells make to turn off immune cells, enabling the immune system to continue killing cancer.

Remarkably, patients who received either the Pfizer or Moderna mRNA-based COVID-19 vaccine within 100 days of starting immunotherapy were more than twice as likely to be alive after three years compared with those who didn't receive either vaccine. Surprisingly, patients with tumors that don't typically respond well to immunotherapy also saw very strong benefits, with nearly fivefold improvement in three-year overall survival. This link between improved survival and receiving a COVID-19 mRNA vaccine remained strong even after we controlled for factors like disease severity and co-occurring conditions.

To understand the underlying mechanism, we turned to animal models. We found that COVID-19 mRNA vaccines act like an alarm, triggering the body's immune system to recognize and kill tumor cells and overcome the cancer's ability to turn off immune cells. When combined, vaccines and immune checkpoint inhibitors coordinate to unleash the full power of the immune system to kill cancer cells.