Even if one is diagnosed with breast cancer early, they can live without any problems if they receive proper treatment. This was the case until the emergence of the novel coronavirus infection (COVID-19). Scientists have uncovered why the risk of dying from cancer nearly doubled during the COVID-19 pandemic. Animal experiments confirmed that the coronavirus can awaken dormant breast cancer cells in the lungs.
A team of researchers led by Professor James DeGregori at the University of Colorado School of Medicine noted, "We have found direct evidence for the first time that viruses like COVID-19 and influenza (flu) can activate dormant breast cancer cells that have metastasized to the lungs when infected in the respiratory tract," and published their findings in the journal Nature on the 31st.
Professor DeGregori said, "Respiratory viral infections not only awaken cancer cells but also cause them to proliferate and increase to an 'enormous number.'" Co-author Professor Julio A. Aguirre-Ghiso from Albert Einstein College of Medicine stated, "The results of this study suggest that for those who have battled cancer, taking preventive measures against respiratory viruses, like vaccination, could be beneficial."
◇Reigniting dormant infections
The research team discovered dormant cancer cells isolated from primary tumors in patients who had been diagnosed with breast cancer, prostate cancer, and skin cancer and had been cured. These cells are in a pre-metastatic stage that could pose problems for cancer survivors. About 25% of patients cured of breast cancer may have these cells that can lead to a recurrence and metastasis of cancer.
Scientists have long sought the cause of the reactivation of dormant cancer cells at the pre-metastatic stage. In 2018, the Cold Spring Harbor Laboratory presented evidence in Science that inflammation caused by smoking could reawaken dormant cancer cells. Researchers from Johns Hopkins University published in Nature in 2022 that inflammation caused by aging could also proliferate dormant cancer cells.
The research team considered that acute inflammation caused by respiratory infections like COVID-19 or flu could also reactivate dormant cancer cells. They genetically modified mice to induce breast cancer similar to that in humans and implanted dormant cancer cells into other tissues, including the lungs. They then infected the mice with the coronavirus or influenza virus.
A few days after the infection, the cancer cells that had been dormant in the mice's lungs began to activate, proliferate, and metastasized to other locations within two weeks. Professor DeGregori explained, "Dormant cancer cells are like ashes left in a campsite fire, and respiratory viruses are like a strong wind that reignites the fire."
◇Follow-up study on breast cancer patients confirmed results
The researchers discovered that the infected virus did not directly awaken the cancer cells. The cause was found to be interleukin-6 (IL-6), a signaling substance that induces immune responses. Mice engineered to not produce IL-6 showed little proliferation of dormant cancer cells even when infected with the virus. This aligns with previous treatments aimed at targeting IL-6 to suppress inflammation in COVID-19 patients.
The research team also found that T cells, a type of immune cell, protect cancer cells from other immune responses. Professor DeGregori remarked, "It was truly shocking that cancer cells draw on the immune system to distort it in order to protect themselves."
The researchers found similar occurrences might also happen in humans. A study by researchers at Imperial College London analyzed genomic data from 500,000 individuals in the UK Biobank and found that cancer patients who tested positive for COVID-19 had nearly double the risk of dying from cancer compared to those who tested negative.
The cancer-related mortality risk for those who contracted COVID-19 was highest during the months immediately following infection. This corresponds with the rapid proliferation of reactivated cancer cells observed in mouse experiments. In this investigation, cancer patients who died from COVID-19 were excluded.
Researchers from Flatiron Health, a U.S. healthcare provider, reached the same conclusion. Analyzing medical records of about 40,000 female breast cancer patients treated at 280 cancer specialty hospitals in the U.S., they found that patients infected with COVID-19 were about 50% more likely to metastasize to the lungs than other patients.
This paper in Nature provides clues on how to treat cancer patients during infectious disease pandemics. In a commentary published alongside in Nature, Professor John Alcorn from the University of Pittsburgh noted, "If we gain a clearer understanding of the process of reactivation of dormant cancer cells in the future, we could utilize existing treatments like IL-6 inhibitors," and emphasized the need to examine patients who have been treated for cancer but contracted the virus during a pandemic to verify whether dormant cancer cells have proliferated.
References
Nature(2025), DOI: https://doi.org/10.1038/s41586-025-09332-0
Nature(2025), DOI: https://doi.org/10.1038/d41586-025-02137-1
Nature(2022), DOI: https://doi.org/10.1038/s41586-022-04774-2
Science(2018), DOI: https://doi.org/10.1126/science.aao4227