Can a Respiratory Infection Trigger Breast Cancer Recurrence?

By: Dr. Chih-Wen Chen, Department of Medical Oncology

After completing cancer treatment, many patients ask: “Doctor, I feel fine. Why do I still need ongoing follow-up?” I often explain it this way: dormant cancer cells are like hibernating bears or a volcano that looks quiet on the surface. Everything may seem calm, but under the right conditions, they can “wake up” again.

What Is Cancer Cell Dormancy?

In 1954, Geoffrey Hadfield published a landmark paper in the British Medical Journal titled “The Dormant Cancer Cell,” introducing the concept of cancer dormancy. He proposed that after treatment, some cancer cells may stop dividing temporarily but do not fully die. Instead, they can travel from the original tumor site to distant organs such as the liver, lungs, and bones, and remain inactive until conditions become favorable again. His central argument was that recurrence is not always caused by brand-new cancer cells forming. It can happen because previously dormant cancer cells become active again. 

Clinically, this concept is especially relevant to hormone-related tumors such as hormone receptor-positive breast cancer and prostate cancer. These cancers rely on hormones as “fuel” for growth. When the environment is unfavorable, for example during endocrine (hormone-blocking) therapy, cancer cells may pause growth rather than disappear completely. That is why recurrence or metastasis can still occur years later, similar to how a bear hibernates when winter conditions make survival harder.

New Research: Viral Infections May “Awaken” Dormant Breast Cancer Cells

A breakthrough study published in Nature (published online July 30, 2025) reported that common respiratory viral infections, including COVID-19 and influenza, may awaken dormant breast cancer cells in the lungs, leading to new metastatic tumors. In mouse experiments, researchers created a situation where breast cancer cells were present in the lungs in a long-term dormant state, similar to what can occur in humans. When the mice were infected with influenza or SARS-CoV-2, the previously dormant cancer cells rapidly re-entered the growth cycle, and visible metastatic lesions formed within about two weeks. 

The study found that a key driver was interleukin-6 (IL-6), an inflammatory signaling molecule released by immune cells during infection. IL-6 often rises when people develop fever and inflammation during respiratory infections, helping the body recognize and fight pathogens. However, the experiments showed that this protective immune response can also unintentionally stimulate dormant cancer cells to “wake up.” 

Another striking finding involved CD4+ T cells, which are typically part of the body’s anti-cancer immune defense. After viral infection, the study suggests CD4+ T cells can form local immune “niches” in the lung that end up shielding the reactivated cancer cells. These niches suppress CD8+ T cells, which are among the immune system’s key cancer-killing cells, giving awakened cancer cells a safer environment to survive and expand. 

This is an important reminder that cancer cells do not only “come back” on their own. Under certain conditions, they may also take advantage of the immune system’s infection response to protect themselves.

Human Data: Signals from UK Biobank and Flatiron Health

The researchers also analyzed large datasets, including the UK Biobank and the U.S. Flatiron Health database, and reported patterns consistent with the animal findings. In the Flatiron analysis, female breast cancer patients who developed COVID-19 after their initial breast cancer diagnosis had a higher risk of later being diagnosed with metastatic breast cancer in the lungs, with an age, race, and ethnicity-adjusted hazard ratio of 1.44 (95% CI: 1.01–2.05). 

In plain terms, this aligns with the article’s message: among breast cancer survivors, COVID-19 was associated with a meaningful increase in subsequent lung metastasis risk in the months that followed. 

What This Means for Survivors: Protection and Follow-Up Matter

In medicine, there are two major schools of thought on how to address dormant cancer cells. One approach aims to intentionally wake up these hard-to-detect dormant cells and then eliminate them with precise therapy. The other approach focuses on keeping them dormant by maintaining immune function, controlling inflammation, and using treatments that suppress cancer cell growth (for example, endocrine therapy). Which approach is better remains unsettled, and the field has not reached a final answer.

One point is clear: dormant cancer cells may be reactivated by viral infection, so survivors should take infection prevention seriously. Vaccination against influenza and COVID-19, and in appropriate groups RSV vaccination, can be part of that protection strategy. Just as important is following medical advice, keeping regular follow-up appointments, and taking medications as prescribed. I frequently see patients stop medication or skip follow-ups because they feel well and have no symptoms. That can be risky.

Even when you feel healthy, it is wise to stay vigilant. Dormant cancer cells can be like underground magma that can flare up again, or a hibernating bear that wakes with disturbance. Regular monitoring and consistent treatment are among the most reliable defenses we have to reduce recurrence risk.

Do All Cancers Have Dormant Cells?

Not every cancer is known to enter a dormant state. Dormant cancer cells have been observed in breast cancer, prostate cancer, lung cancer, kidney cancer, melanoma, and certain blood cancers such as chronic leukemia and multiple myeloma. 

For other cancers, such as stomach cancer, pancreatic cancer, and some head and neck cancers, whether dormancy plays a major role is still unclear. More research is needed to understand which cancers involve dormancy and how best to prevent reactivation.

References

Hadfield, G. (1954). The Dormant Cancer Cell. British Medical Journal, 2(4888), 607–610. DOI: 10.1136/bmj.2.4888.607. 

Chia, S. B., et al. (2025). Respiratory viral infections awaken metastatic breast cancer cells in lungs. Nature. DOI: 10.1038/s41586-025-09332-0.