There were more than 3,700 patients with flu in English hospitals last week - a sharp rise of 79%, new data shows.
NHS bosses said the sharp increase is "very worrying" and has been seen across both general hospital and critical care beds – those for the sickest patients.
An average of 3,746 people with flu were in hospital across the seven days to December 25, up week on week from 2,088, according to NHS England.
The number was just 772 at the start of the month.
The average number of critical care patients with flu rose from 149 in the seven days to December 18, to 267 in the week to Christmas Day.
At this point in 2021, just 34 patients were in hospital with flu, with only two in critical care.
NHS England’s national medical director Professor Sir Stephen Powis said the latest flu numbers "sadly" show health chiefs' "fears of a ‘twindemic’ have been realised".
Meanwhile, staff absences due to Covid-19 at hospital trusts in England have risen by 47% in a month, from an average of 5,448 a day in the seven days to November 27 to 8,029 last week.
The total number of staff off sick was up 20% from 52,556 at the end of November, to 63,296 a day last week.
NHS Providers’ interim chief executive Saffron Cordery, said: “It’s very worrying to see flu admissions rising so steeply, alongside so many other pressures, contributing to unacceptably high bed occupancy rates which make it much harder to ensure safe high quality care.
“Increased staff sickness is another concern, compounding severe workforce shortages.
“This is impacting on care right through the system, including ambulances, hospital A&Es, mental health and community services.
“Staff are working flat out in very difficult circumstances to minimise disruption for patients.
“The worry is that as these pressures intensify, with the potential further impact of industrial action, we could see more delays in the new year.”
NHS England’s national medical director Professor Sir Stephen Powis urged eligible people to get vaccinated for flu and Covid.
He added: “Sadly, these latest flu numbers show our fears of a ‘twindemic’ have been realised, with cases up seven-fold in just a month and the continued impact of Covid hitting staff hard, with related absences up almost 50% on the end of November.
“As well as flu, the NHS continues to be under significant pressure, with high bed occupancy, more than 12,000 beds taken up by patients medically fit for discharge, and demand for the 111 service remaining high, so please do make the most of 111 online, and only call 999 or visit A&E in an emergency.
“It is clear this is no time to be complacent and the risk of serious illness is very real, so with nearly 350,000 available vaccination appointments next week it is important that everyone eligible comes forward and gets their Covid and flu jabs at the earliest opportunity.”
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