Cancer charity KWF is going to support Dutch follow-up research into the link between Covid-19 and flu infections, and a recurrence of cancer in people.

Earlier international research on mice, carried out during the pandemic, has established that dormant cancer cells, which can’t be detected on scans but can remain present for years, were “awakened” by a respiratory infection.

Inactive breast cancer cells in mice injected with a virus increased a hundredfold over a short period.

The next phase of the research aims to find out how strong the link between an infection and the reactivation of cancer cells is in humans, oncologist and immunologist Hans Nijman, who is involved in the research, said.

“The fact that there may be a link is not to say that a flu virus is the cause of the reactivation of these cancer cells,” Nijman told NU.nl.

People who have had cancer should not be worried if they get flu, he said. “There is a big difference between association and causality. The mice study suggests a strong link,” he said.

Roel Vermeulen, a professor at Utrecht’s UMC teaching hospital and who was involved in the American research, found signals in British figures that pointed to a possible link in humans.

An analysis of the number of deaths from cancer in former cancer patients who had been diagnosed with the disease 10 years before, and who had Covid in the first year of the pandemic, showed they were twice as likely to die from recurrent cancer in the two years following the infection.

The scientists will also investigate whether vaccinations, which had not been developed at the time, can reduce the risk of the recurrence of cancer. It is thought they might because it protects against the effects of infections on the immune system.

The research will also throw new light on the reasons why dormant cells are reactivated and why this happens in some people and not in others.