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Covid could combine with influenza this winter to cause a super-infection doubling the length of hospital stays compared with coronavirus alone, scientists have warned.
A report from the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) said people who had contracted both viruses in UK hospitals needed an average of 16 days of treatment compared with seven for those just with Covid.
The report warned that social distancing had reduced the circulation of all respiratory viruses and, because of waning population immunity, the next flu season would be worse than usual. A dual infection may also increase the chance of transmitting Covid, scientists fear.
"Co-infections with Sars-CoV-2 and other respiratory viruses are expected to occur this winter, with the potential to place pressures on the NHS and care services," it said.
"Based on animal and clinical data, there is a realistic possibility that co-infection with Sars-CoV-2 and influenza may cause increased disease severity than would be expected if influenza and Sars-CoV-2 acted independently.
"We recommend that individuals with symptomatic respiratory infections self-isolate even if they receive a negative test result for Sars-CoV-2, as this will reduce respiratory virus transmission and potentially societal burden."
The Nervtag scientists said clinical symptoms of flu and Covid were similar and recommended that laboratories test for both. They said hospitals should also instigate control measures to avoid cross-infection, with flu testing introduced in emergency departments to prevent hospital transmission.
Minutes released on Friday by the Scientific Advisory Group for Emergencies (Sage) show scientists believe co-infection with both viruses "represents a significant challenge" this winter. They said it was vital that people stayed at home if they were unwell, even if testing negative for Covid.
Sage said there was evidence that a co-infection with flu and coronavirus increased the risk of death.
Vaccine manufacturers have already warned that the flu jab may fail to protect people this winter because global Covid surveillance prevented laboratories from gathering sufficient data on dominant variants.
The World Health Organisation made recommendations about what to put in northern hemisphere flu jabs in late February, but the global genetic sequencing of flu had dropped by up to 94 per cent in the months preceding that decision.
In 2015, a mismatched flu jab contributed to the largest spike in mortality in a generation when 28,189 excess deaths were recorded, leading to a fall in life expectancy in England and Wales for the first time in two decades.
The Academy of Medical Sciences said a potential surge in respiratory viruses this winter could push the NHS to breaking point, warning that between 15,000 and 60,000 people could die of flu.