Cancer survivors infected with COVID-19 in the early months of the pandemic had a higher risk of dying from dormant cells reawakening, Colorado researchers found, though they don’t know whether people who get the virus now face the same risk.

Experiments in mice found that genetically modified animals were more likely to have signs of metastatic cancer in their lungs if infected with flu or COVID-19 than engineered mice that researchers didn’t give a virus, said James DeGregori, deputy director of the University of Colorado Cancer Center in Aurora.

That finding launched an international partnership to determine whether the same thing happened in people, he said.

DeGregori was one of the lead investigators, alongside scientists from Utrecht University in the Netherlands, Imperial College London, University College London, University of Connecticut, Albert Einstein College of Medicine in New York City, Children’s Hospital of Philadelphia, and the COVID-19 International Research Team, based in Massachusetts.

Two datasets, from the United States and the United Kingdom, both showed a higher risk that cancer survivors with confirmed COVID-19 infections would die from metastatic cancer, compared to survivors who didn’t test positive for the virus.

The British data showed people who tested positive had about twice the risk of those who tested negative, and the American data showed about a 44% increased risk, DeGregori said. A significant number of the U.S. patients never got tested for COVID-19, however, so the risk was likely higher because of cancer deaths in people with missed infections, he said.

The American data only included breast cancer survivors, while the British data included people who were in remission from any type of cancer. The risk of death was highest in the months immediately after an infection.

COVID-19 didn’t directly cause the cancer to spread, but created an environment where dormant cells elsewhere in patients’ bodies can wake up, DeGregori said. The body responds to an infection with inflammation to kill the virus, which helps the cancer cells, he said.

“It’s kind of like collateral damage,” he said.

Drugs exist that could block one specific molecule that ramps up inflammation, but they also suppress the immune system, which is a problem when the patient has a serious infection, DeGregori said.

“You have to balance the good it does with the bad it does,” he said.

The researchers didn’t have enough data to know whether people with more-severe cases of COVID-19 were more likely to die from their cancer, DeGregori said. They also couldn’t tell if flu infections had a similar effect, because most people who have the flu don’t seek medical care, he said.

Other questions for future studies include whether the risk is the same for infections with more recent COVID-19 variants and in vaccinated people, and whether other types of infections also can help cancer spread, DeGregori said.

Not all cancer survivors have dormant cells in their bodies, and of those who do, not everyone sees those cells wake up after an infection, DeGregori said. Still, survivors who are worried about their cancer spreading might want to get vaccinated against respiratory diseases and take steps like avoiding sick people, he said.

“We don’t want to scare people, but knowledge is power,” he said. “Anything that could limit the odds of infection should limit the odds of (cancer) awakening.”