Health Secretary Jeremy Hunt updated the Cabinet on the spread of flu on Tuesday, telling them that hospital admissions in the first two weeks of the year were double the rate in the comparable period in 2010-11, the year after the swine flu outbreak.
There are suggestions the latest figures, for the week ending 21 January, will show the rate of increase has slowed in the last week, but flu is notoriously unpredictable and any optimism will be tempered by the severity of symptoms shown by many patients.
The UK last saw a significant serious outbreak in 2013-14, with the last two winters bringing unusually low levels of influenza.
The NHS has been concerned about the potential for a severe flu outbreak since the autumn when it became clear that one of the strains was responsible for the worst flu season in a decade in Australia.
The A-strain ‘Aussie flu’, H3N2, is one of three that has been evident this winter, and despite concerns it has not been dominant or the most harmful.
B strains made up six out of 10 cases in the most recent statistics, with the majority caused by ‘Japanese flu’ B-Yamagata, which is not covered by the cheapest vaccine that has been offered by doctors and pharmacies.
The trivalent vaccine covering three major strains cost NHS doctors and GPs £5 per-shot, while the quadrivalent vaccination that includes B-Yamagata was £8.
The more expensive vaccine has been given to children, who receive the vaccine by nasal spray.
Concerns that flu may reach epidemic levels are based on its rate of increase in the first fortnight of the year, based on the number of people at GP surgeries presenting with flu-like illnesses, measured by prevalence per 100,000 patients.
The baseline is 13.1 cases per 100,000, but since mid-December has risen from 11.4 to 53.1 cases per 100,000, with a 40% increase in the last week for which figures are available.
Some 120 people were confirmed to have died from flu, an increase of 85 on the previous week.
The Royal College of GPs said the figures represented a 150% increase since the turn of the year.
In England, a rate above 107 patients per 100,000 presenting with flu-like symptoms is considered an epidemic, though the Department of Health says an epidemic can only be formally declared by the Chief Medical Officer.
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The epidemic threshold differs for the other home nations.
In Scotland an epidemic is more than 418.9 cases per 100,000, in Wales 75.4 per 100,000, and in Northern Ireland 142.4.