How a bug could bring your breast cancer back

For most people, becoming infected with Covid-19, flu or any other respiratory virus usually leads to unpleasant but temporary symptoms. But for some cancer survivors – specifically women in remission from breast cancer – these viruses may be more horrifying.

New research suggests even fairly ­innocuous viral infections may increase the risk of breast cancer coming back.

The study, published in Nature, found that respiratory bugs can reactivate breast cancer cells that have settled in the lungs of mice and become dormant.

The results were borne out by a second study, by the same researchers, which looked at thousands of breast cancer ­survivors whose details were recorded in a US medical database. These women had been diagnosed with breast ­cancer at least five years earlier and so were almost ­certainly in remission.

Those who had caught Covid-19 in 2020 (before vaccinations were available) were nearly 50 per cent more likely to develop metastatic breast cancer (cancer that has spread) in their lungs, than women who tested negative for the virus.

The researchers also looked at data from cancer survivors generally and found that those who’d had Covid had almost double the risk of dying from cancer – the risk was ‘even greater in the months following infection’, James DeGregori, a professor of biochemistry and molecular genetics at the University of Colorado School of Medicine, who led the research, told Good Health.

Experts suggest the results highlight the role of ­regular Covid and flu vaccinations for those who have had breast cancer.

‘Dormant cancer cells are like embers in a campfire – and ­respiratory viruses are the wind that can reignite them,’ says ­Professor DeGregori.

Animal tests, he said, showed that after Covid or flu infection, there was a 100-fold surge in ­levels of previously dormant breast cancer cells that had ­settled in the lungs.

‘Breast cancer cells can stay in the lungs in very small ­numbers and in essentially a quiescent state,’ he told Good Health. ‘Our data suggest that if you are a ­cancer patient who has these dormant cells, you may end up living a perfectly normal life, dying without them ever awakening.

‘But if you get a respiratory virus such as flu or Covid-19, the chance of those cells awakening is much greater – and with it your chance of dying from them.’

One in five women with breast cancer that is treated successfully develops secondary tumours in other parts of the body – sometimes decades later. These tend to be more aggressive and respond less well to therapies.

Why this happens to some women and not others is not completely understood.

But it is linked to whether tiny sleeper cancer cells migrate from the original tumour and settle in areas such as the bone marrow and lungs, often very early on in the disease’s initial progression. For many women, these cells stay quiet and never reactivate.

But for some, a trigger causes them to reawaken, leading to new tumours.

Professor Kefah Mokbel, a consultant breast surgeon and chair of the multidisciplinary breast cancer programme at the London Breast Institute, told Good Health the study backed up previous ­findings that widespread inflammation – such as that triggered by a viral infection – in the body can reawaken cancer cells.

At the same time, viral infections can upset the immune ­system, making cells which are supposed to protect the body against cancer less effective and suppressing the activity of other immune cells that would normally attack and destroy cancer.

‘This creates fertile ground for dormant cancer cells to grow into metastases,’ says Professor ­Mokbel. ‘We must be cautious, but the data strongly suggest that viral infections can influence cancer recurrence and spread.’

But Dani Edmunds, research information manager at Cancer Research UK, says that people should not be overly worried that a bout of flu or Covid is going to lead to their cancer returning.

‘Current data from this study is not sufficient to show that Covid-19 was the cause for metastatic cancer in humans,’ she says.

‘For example, key details about the cancer care given to patients during the pandemic are missing from the human datasets.’

This is important because the pandemic caused widespread disruption of cancer services – from screening to treatment. This was not taken into account by the researchers.

Dani Edmunds adds: ‘Findings from mouse and human samples should not be directly compared.

‘And while the researchers ­identified a mechanism by which respiratory infection could increase breast cancer metastasis in the lungs of mice, they ­provide no evidence this is applicable in humans.’

Researchers who took part in the Nature study are now looking at whether respiratory infections might also be linked to increased recurrence rates in other types of cancer and what impact ­vaccinations for flu or Covid might have.