The flu vaccine used to fight this winter's outbreak is only working in 30-40% of cases, the director of the Worldwide Influenza Centre laboratory in London has told Sky News.
Vaccination is the first line of defence against flu outbreaks, and laboratories around the world collaborate to try and develop the most effective defence against a virus that is constantly evolving.
The vaccine developed this year is intended to protect against several strains, including the H3N2 virus, dubbed "Aussie flu" after Australia suffered its worst ever outbreak in the Southern Hemisphere winter.
Dr John McCauley, director of the laboratory based at the Francis Crick Institute, said the threat posed by H3N2 may have been over-stated, and figures show it is responsible for only one in four cases.
But the vaccine intended to control it has not proved as effective as hoped.
"The vaccine for this particular influenza, H3N2, is not particularly effective, it is basically considered of moderate effectiveness, which means 30-40% of cases will be avoided among people who have the vaccine," Dr McCauley said.
"At a public health level that is a lot less flu and it may also be reducing the impact of the virus on those that do get it."
Cases of flu have spiked in the last week according to Public Health England, with hospital admissions two and a half times higher than in the same week last year, and a 78% increase in cases reported by GPs.
Britain has experienced only moderate flu levels in the last two winters, with the last really serious outbreak in 2014, and before that the 2009-10 swine flu outbreak.
Dr McCauley said if cases increase at the current rate it could become significant.
"What the prospect for this winter is I think too early to say. We know it is not going to be a mild winter, but whether it is moderate or severe we wait and see. The numbers are increasing, and if they continue increasing it's going to be quite an influenza season"
The impact is being felt particularly at GPs' surgeries, and the chair of the Royal College of GPs described the current rates as "staggering".
Dame Helen Stokes Lampard told Sky News: "The whole of the NHS is feeling the strain at the moment, and there is a lot of talk about the pressure accident and emergency is under, but actually the figures in GPs are staggering.
"There's been an 80% increase in the number of people coming to see their GP with flu or respiratory illness, and that's a huge increase in workload.
"It means every surgery in country will be feeling the pressure and there will be fewer appointments available for routine issues as we focus our energy on those who really need us, the acutely unwell."