Dormant cancers can be reawakened by flu, COVID: Study

LOS ANGELES - Infections like influenza and COVID-19 may do more than cause temporary illness. A new study suggests they can also "wake up" dormant cancer cells, potentially increasing the risk of recurrence and metastasis years after treatment.

The research, led by Dr. James DeGregori at the University of Colorado Anschutz Medical Campus, was published July 30 in Nature. Scientists tested how respiratory viruses affect dormant cancer cells using mouse models of breast cancer, and then compared those findings with large patient health datasets.

How respiratory viruses reawaken dormant cancer cells

Cancer cells can sometimes break away from the original tumor and linger quietly in other parts of the body for years, a state researchers call dormancy. What causes them to reawaken has long been unclear.

The Colorado team found that in mice, influenza and SARS-CoV-2 infections triggered inflammation that reactivated these cells in the lungs. Within three days of infection, cancer cells began multiplying rapidly, and the effect persisted for months.

The awakening process depended on an inflammatory molecule called IL-6. When mice lacked IL-6, the cancer cells were far less likely to restart growth.

"Dormant cancer cells are like the embers left in an abandoned campfire, and respiratory viruses are like a strong wind that reignites the flames," DeGregori said.

The role of the immune system in cancer cell growth

The study also highlighted how immune cells influence cancer cell reactivation.

Researchers found that CD4+ T cells — normally part of the immune defense system — helped sustain awakened cancer cells by suppressing the activity of CD8+ T cells, which are more effective at killing cancer. When CD4+ T cells were depleted, more CD8+ T cells appeared in the lungs and were better at destroying cancer cells.

This suggests that during respiratory infections, shifts in immune response may give dormant cancer cells an opportunity to grow again.

What the study found in human cancer patients

What we know:

To test whether the mouse findings reflected real-world outcomes, researchers analyzed health records of thousands of cancer survivors.

• In the UK Biobank, people with a past cancer diagnosis who later tested positive for COVID-19 had nearly double the risk of cancer-related death compared to those who never tested positive.
• In a dataset of more than 36,000 women with breast cancer, having COVID-19 increased the likelihood of metastasis to the lungs by about 40 percent.

These findings suggest respiratory viruses may play a role in cancer progression long after remission.

What we don't know:

Scientists caution more research is needed before drawing firm conclusions for treatment. Key questions remain:

• Whether vaccines for flu and COVID-19 can reduce the risk of recurrence linked to infection.
• How different cancer types respond to viral infections.
• If targeting IL-6 or adjusting immune cell activity could help prevent dormant cancer cells from reawakening.

Why this matters for cancer survivors

For millions of cancer survivors in the U.S. and worldwide, these findings may reshape how doctors think about long-term remission care. Preventing or quickly treating respiratory infections may be an important part of reducing recurrence risk.

It also raises the possibility that anti-inflammatory therapies or immune-modulating drugs could one day be used to prevent dormant cancer cells from reigniting.

Big picture view:

The study highlights how viruses can have lasting effects on the body that go beyond the immediate illness. For cancer survivors, infections may be more than an inconvenience — they could represent a hidden risk factor for cancer’s return.

Researchers are now exploring therapies that target IL-6 signaling and immune pathways. Longer-term human studies are needed to see whether vaccines, antivirals or immune-based treatments could help reduce the danger of dormant cells reawakening.