A cold or flu awakens hidden cancer cells in the lungs.

Common respiratory infections, such as COVID-19 and the flu, can awaken dormant breast cancer cells that have spread to the lungs, paving the way for new metastatic tumors. The findings, published in the journal Nature and obtained in mice, have been supported by research showing increased mortality and metastatic lung disease among cancer survivors infected with SARS-CoV-2, the virus that causes COVID-19 .

According to research, respiratory viruses cause metastatic breast cancer cells to proliferate in the lungs.

Breast cancer is the most commonly diagnosed type of cancer in women. After an initial remission, cancer cells can remain dormant for years before metastasis (to the lungs or other organs) causes a relapse. Respiratory viral infections, such as SARS-CoV-2, are associated with inflammation, which can trigger processes that influence metastasis.

A surge in cancer mortality rates during the first two years of the COVID-19 pandemic prompted researchers at the University of Colorado , Montefiore Einstein Comprehensive Cancer Center (MECCC), and Utrecht University (Netherlands) to study the effects of influenza virus and SARS-CoV-2 infections on breast cancer outcomes in mouse models.

"Our findings indicate that people 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," said Julio Aguirre-Ghiso , co-director of the study.

Some evidence has suggested that inflammatory processes can reactivate disseminated cancer cells (DCCs), cells that have broken away from a primary tumor and spread to distant organs, often remaining dormant for long periods. “ During the COVID-19 pandemic, some studies suggested a possible increase in cancer mortality rates , reinforcing the idea that severe inflammation could contribute to the reactivation of dormant DCCs,” says Aguirre-Ghiso.

The researchers used mouse models of breast cancer with dormant cancer cells in the lungs, similar to the human disease. When mice were exposed to SARS-CoV-2 or influenza virus, these respiratory infections reactivated dormant cells, causing rapid metastatic proliferation and the formation of lung lesions in just two weeks.

The COVID-19 pandemic provided a unique opportunity to investigate the impact of respiratory virus infections—in this case, SARS-CoV-2—on cancer progression.

Researchers analyzed two large databases to assess the impact of COVID-19 on patients with a history of cancer.

In the UK Biobank , among cancer survivors in remission, those who tested positive for COVID‑19 had nearly twice the risk of dying from cancer compared to those who did not become infected, especially during the first year after infection.

In the Flatiron Health database, breast cancer patients who contracted COVID-19 were 50% more likely to develop lung metastases than those without a COVID-19 diagnosis. These results reflect a possible reactivation of dormant cancer cells, similar to what has been observed in animal models.

“Respiratory viral infections are a lifelong part of our lives, so we need to understand their long-term consequences,” said lead author James DeGregori .

The findings, he writes, "suggest that cancer survivors may be at increased risk of metastatic relapse following common respiratory viral infections."

It's important to note that the study focused on the period before COVID-19 vaccines were available.