The A(H3N2) subtype of the virus is thought to have changed so significantly from the strain in the vaccine that it will not protect patients in the vast majority of cases, Public Health England (PHE) has said.
But health leaders have hastened to add that vulnerable patients should still receive the vaccine to offer protection against other influenza strains – such as A(H1N1)pdm09 and flu B – which could also circulate this winter.
PHE researchers looked at 1,314 flu patients presenting in primary care across the UK. They found that the vaccine worked in a paltry 3% of cases, compared to the usual 50% effectiveness observed in the UK.
‘Genetic drift’, a natural process by which simple genetic changes accumulate over time, is thought to be responsible for the mismatch between the A(H3N2) strains in the vaccine and those circulating in the UK population.
Study author, Dr Richard Pebody, also PHE’s head of flu surveillance, said: ‘It’s not possible to fully predict the strains that will circulate in any given season, and there is always a risk of a drift occurring as we have seen this year.
‘The World Health Organisation monitors influenza globally and each year recommends the strains of flu virus that should be included in the flu vaccine for the forthcoming flu season. It takes from February through to August/September to produce sufficient quantities of the flu vaccine. If a change in the virus is detected once production has started it is not possible to change it.’
Doctors have been urged to prescribe antivirals to vulnerable groups to reduce the risk of serious illness and death.
Deputy CMO Professor John Watson said: ‘The latest data show that levels of flu are generally decreasing in the UK. We do see "drift" in the flu virus from time to time, but even so, I want to reassure people that it is still the best overall way to protect yourself and your family from flu, along with good hand hygiene.
‘Antiviral drugs are available and effective, and doctors should prescribe them for those at greatest risk of becoming seriously ill due to flu.’
Flu vaccines must be readministered every year because the strains are known to change over time. But a US study unveiled last month suggested that a universal flu vaccine offering long-term protection could be on the cards within five to seven years.
The mismatch comes as flu levels reached their highest ever for three years earlier in the season.