Respiratory viruses are common but can cause symptoms ranging from mildly uncomfortable to severe. New research indicates they could also lead dormant breast cancer cells to reactivate in the lungs.

A multi-institution study recently published in the journal Nature investigated how inflammation from the flu or COVID-19 may impact disseminated cancer cells – or dormant cells that have split off from a tumor and settled elsewhere in the body. In this case, the researchers looked at such cells in the lungs post-respiratory virus.

To conduct their study, researchers from the University of Colorado Anschutz Medical Campus, Montefiore Einstein Comprehensive Cancer Center, and Utrecht University used mouse models and cancer patient information from two different databases.

In mice, the team found that flu and COVID-19 infections can reawaken dormant cancer cells in the lungs, leading to the spread of metastatic cancer cells within days of infection and the formation of metastatic lesions within two weeks. The researchers say this was triggered by a protein called interleukin-6 (IL-6), which is released by immune cells during infections. The team says this suggests that inhibiting IL-6 could be a way to prevent this sort of activation from happening, or at least to lessen its impact.

While the lab portion of the study was in mice, the researchers also looked at human databases on COVID-19 infections prior to the availability of vaccines, which appeared to support the findings. First, the team analyzed UK Biobank data on cancer survivors who had been diagnosed at least five years before the COVID-19 pandemic began, meaning they were likely in remission. When comparing nearly 500 patients who tested positive for COVID-19 to more than 4,000 controls who did not, the researchers found that the first group was nearly twice as likely to die from cancer, particularly within the first year.

Meanwhile, the study also included breast cancer patient information from the U.S. Flatiron Health database. For this group, the team looked at lung metastasis incidence in more than 500 COVID-positive patients and more than 36,000 COVID-negative patients. Over a follow-up period of nearly four-and-a-half years, the COVID-positive patients were almost 50% more likely to develop lung metastasis than the controls.

Senior author and deputy director of CU Cancer Center, Dr. James DeGregori, says this suggests, “Dormant cancer cells are like the embers left in an abandoned campfire, and respiratory viruses are like a strong wind that reignites the flames.”

As a result of the findings, the team believes it may be a good idea for cancer survivors to take steps to avoid contracting respiratory viruses, including by getting vaccines, and to consult with their doctors on further precautions.

Going forward, Dr. DeGregori says, “By understanding underlying mechanisms, we will work hard to develop interventions that can limit the risk of metastatic progression in cancer survivors who experience respiratory viral infections. We also plan to extend our analyses, both in animal models and through mining of clinical data, to other cancer types and other sites of metastatic disease. Respiratory viral infections are forever a part of our lives, so we need to understand the longer-term consequences of these infections.”

If you'd like to contribute to other cancer research, click here!