A major study published in JAMA in August 2025 has raised new concerns about how viral infections affect cancer survivors. The findings suggest that viruses like COVID-19 and influenza may reactivate dormant cancer cells, potentially triggering metastasis.

During the first year of the pandemic, doctors noticed unusually high numbers of patients suddenly developing advanced cancers, particularly breast and colorectal. While many assumed this was due to delays in screenings and treatments, researchers argued that cancers do not typically progress so rapidly—leading them to investigate whether viral infections could be responsible.

Animal and human studies

Experiments in mice engineered with dormant breast cancer cells showed that even mild flu infections caused the cells to reactivate and spread within days. When infected with a mouse-adapted version of SARS-CoV-2, the cancer cells expanded dramatically within weeks.

Data from the UK Biobank, involving over 4,800 cancer survivors, revealed that patients who tested positive for COVID-19 in 2020 had nearly double the risk of dying from cancer compared to those who tested negative, especially in the months following infection. Another database of 37,000 breast cancer survivors showed a higher likelihood of lung metastases in those who had contracted COVID-19.

Why does this happen?

The study highlights inflammation as a key driver. Infections raise levels of interleukin-6 (IL-6), a protein critical to immune defense but one that can also fuel cancer growth. In mice, IL-6 reawakened dormant cells. Even after IL-6 levels normalized, tumors continued to grow, suggesting additional mechanisms. Researchers pointed to immune imbalance: CD4 T-cells suppress CD8 T-cells, which normally destroy cancer cells, creating conditions for uncontrolled spread.

Prevention and next steps

Scientists now want to explore whether other viruses can cause similar effects and how different cancers or organs might be impacted.

Until then, experts emphasize prevention. While flu and COVID-19 vaccines do not eliminate risk, they reduce the chance of severe infection. “Attention should not turn into fear that limits survivors’ lives,” said Professor Hugh Montgomery of University College London, stressing that caution remains a personal choice.

This research reveals a troubling possibility: respiratory infections may have lasting consequences, particularly for those with a history of cancer.