Flu vaccine 'barely effective' against main viral strain
This year's seasonal flu vaccine is barely able to protect people from the main strain of flu being spread in the UK, health officials say.
Doctors are being urged to use antiviral drugs quickly to protect vulnerable patients.
Evidence shows the vaccine is stopping only three out of every 100 vaccinated people from developing symptoms.
But Public Health England says people should still get vaccinated to protect against other strains of flu.
Flu is a constantly shifting target and that makes it difficult to develop a vaccine. It is why a new jab is needed each year.
Twelve months ago, the World Health Organization settled on the three most likely strains of flu that would be circulating this winter.
But one of them has since mutated so significantly that the vaccine seems to offer little protection.
It works in just three out of every 100 people. A flu vaccine normally works in 50 out of every 100.
The strain in question, H3N2, is also a particular worry as it primarily kills the elderly.
There have been outbreaks in care homes and overall there has been a higher-than-expected number of deaths in elderly people this year.
Prof Nick Phin, from Public Health England, told the BBC: "We have seen an increase in excess deaths, probably the biggest increase we've seen since 2008-09, so I'm sure that a significant contribution to this will have been the vaccine not being as effective as it usually is."
Public Health England reached the conclusion after a study on 1,314 patients hospitalised with flu in the UK.
Similar levels of viral mutation have been reported in the US and Canada.
The mutation was also detected in the Australian flu season - during the northern hemisphere's summer - but the vaccine was already in development.
Public Health England has already said this is the worst flu season out of the past three years, but is circulating at nowhere near epidemic levels.
Dr Richard Pebody, the head of flu surveillance at Public Health England, said: "Throughout the last decade, there has generally been a good match between the strains of flu in the vaccine and those that subsequently circulate, so it's crucial that these results do not discourage people in at-risk groups from having flu vaccination now, or in the future."
He said the vaccine would still protect against swine flu and influenza B, "both of which may yet circulate this season" so he urged at-risk people to get vaccinated.
He added: "Our findings also mean that the early use of antivirals to treat and help prevent serious cases of flu in vulnerable patients is even more important this season."
The deputy chief medical officer, 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."