London, UK – August 15, 2025

A groundbreaking new study has unveiled a concerning and previously unknown risk factor for cancer survivors: common respiratory viruses. Research, published in the journal Nature, provides the first direct evidence that infections from viruses like the flu or COVID-19 can awaken dormant breast cancer cells and significantly increase the risk of a relapse. The study reveals that it is not the viruses themselves, but the body’s powerful inflammatory immune response to them that acts as a “wake-up call” for these long-slumbering cancer cells. This discovery could fundamentally change how doctors advise cancer survivors about preventative health and may lead to new therapies to counteract this perilous biological process.

The link between a common viral illness and the re-emergence of a life-threatening disease has been a subject of speculation, but this new research provides a clear biological mechanism. The study, conducted by an international team of researchers from the University of Colorado Cancer Center and other institutions, utilized sophisticated mouse models engineered with dormant breast cancer cells. The results were startling. Within days of infecting the mice with either an influenza virus or a SARS-CoV-2 strain, the once-quiet cancer cells in the lungs roared back to life, proliferating more than 100-fold and forming new metastatic lesions. This dramatic reawakening, the researchers found, was driven by a cytokine, a signaling molecule called interleukin-6 (IL-6), which is secreted by the body to fight the viral infection.

Key Headlines

 * Dormant Cells Reactivated: A new study shows that common respiratory viruses, such as the flu and COVID-19, can reawaken dormant breast cancer cells.

 * Immune Response is the Trigger: The research reveals that the body’s inflammatory response to the virus, specifically the cytokine IL-6, acts as the primary trigger for this process.

 * Heightened Relapse Risk: The findings are supported by human observational data showing that cancer patients who had a viral infection faced a significantly higher risk of cancer-related death in the months following their illness.

 * Potential for New Therapies: The study opens the door to developing new drugs that could block the IL-6 pathway, potentially preventing the reawakening of dormant cells.

 * Focus on Prevention: The findings underscore the importance of cancer survivors taking extra precautions against respiratory infections, including vaccination, to reduce their risk of relapse.

The Science of the “Wake-Up Call”

The phenomenon of cancer dormancy, where a few stray cancer cells survive after treatment and lie inactive for years or even decades, has long been a major challenge in oncology. These “sleeper cells” can suddenly reactivate, leading to a metastatic relapse that is often fatal. The new research offers the first clear explanation for one of the potential triggers of this reawakening. The study found that when the body is fighting a respiratory virus, the immune system floods the body with inflammatory cytokines, including IL-6. This temporary spike in inflammation creates a pro-growth environment for the dormant cells, essentially flipping a switch that takes them from a quiescent state to an aggressive, proliferating one.

In a key experiment to prove this, the researchers used a drug to neutralize the IL-6 cytokine in the infected mice. The results were dramatic: without the IL-6 signal, the dormant cancer cells did not multiply nearly as much when the viral infection was introduced. This strongly suggests that IL-6 acts as a crucial switch for cancer cells, and that blocking its action could be a viable therapeutic strategy. Furthermore, the study also found that the reawakened cancer cells, once active, can suppress other immune responses, helping them to evade detection and elimination.

The Human Impact and What Comes Next

The findings from the animal models were corroborated by an analysis of human health data. By examining data from large medical databases like the UK Biobank and Flatiron Health, the researchers found a powerful association between a respiratory viral infection and an increased risk of cancer recurrence. Cancer survivors who had contracted COVID-19, for instance, were found to have a significantly higher chance of metastatic progression to the lungs. This association was strongest in the months immediately following the infection, mirroring the rapid expansion of dormant cells seen in the mouse studies.

The implications for cancer patients and survivors are significant. While the study’s authors stress that the results do not mean every respiratory infection will lead to a relapse, they highlight a clear, measurable risk. Dr. Julio Aguirre-Ghiso, one of the study’s co-leaders, advises that “individuals with a history of cancer may benefit from taking precautions against respiratory viruses, such as vaccination when available, and discussing any concerns with their healthcare providers.” The research also points to a new direction for pharmaceutical development.  Scientists can now explore the potential of anti-inflammatory drugs that target the IL-6 pathway to prevent cancer relapse. The new understanding of how common viruses can reignite the “embers” of a past cancer provides a sobering but actionable insight into a long-standing medical mystery, offering new hope for prevention and a better long-term outcome for cancer survivors worldwide.