The flu jab most frequently given to adults may not offer protection against the most common strain of the virus, Public Health England (PHE) has warned.
In a letter to GPs in the south west, the body warned that people could still get ill if they have had the “trivalent” flu jab, which protects against three strains of the illness.
The jab protects against two Influenza A strains, including the lethal “Aussie flu” strain, which has already claimed lives in the Republic of Ireland, and one Influenza B strain.
But it does not protect against one type of the virus, Influenza B/Yamagata, which has accounted for the vast majority of Influenza B cases reported by labs, they warned.
PHE confirmed to The Independent that 21 of the 25 cases tested in the penultimate week of 2017 were from this Yamagata strain.
It warned practices that most adults and staff have had the three-strain jab and this is likely to include hundreds of thousands of NHS staff vaccinated as part of a drive to limit the impact of staff sickness over winter.
“It is possible that cases of flu will be seen among individuals, both staff and patients, who have accepted this vaccination,” the letter says.
A four-strain “quadrivalent” vaccine, that is given to children as a nasal spray, does protect against this Influenza B strain.
Local NHS clinical commissioning groups (CCGs) determine whether to offer the three or four-strain jab to adults, and neither PHE or NHS England were able to give a figure for how many patients might have had either version.
However the three-strain, trivalent vaccine is cheaper so is thought to be the most widely used.
Flu outbreaks have already heaped pressure on hospitals and care homes in the region, and the letter says there is still time to get vaccinated.
“Quadrivalent vaccine will have greatest effect given the mixed picture of circulating strains,” the letter says.
“But trivalent vaccine will still offer better protection than having no vaccine in many cases, even if it may not protect against all of the circulating strains.”
This comes as the latest update from PHE shows that flu is heaping even more pressure on hospitals which, this week, were told to cancel all non-urgent care to focus resources on emergency departments.
In the last week of 2017, 114 patients were admitted to intensive care units with flu and 421 people were hospitalised, accounting for almost half of the 1,078 flu admissions in the last three months.
It also shows that uptake of the flu jab in vulnerable adults and children – aged six months to 65 years old – has fallen behind rates seen last year.
Only 46.3 per cent of this group received a vaccine, compared to 46.8 this time in 2016-17.
Professor Paul Cosford, Medical Director, Public Health England, said: “Our data shows that more people are visiting GPs with flu symptoms and we are seeing more people admitted to hospitals with the flu.”
He added: “PHE outlined the vaccines available and the additional benefits provided by the quadrivalent vaccine in its guidance on vaccination known as the Green Book. It is up to the local NHS to decide which vaccine to procure for the adult programme.”
Director of policy and strategy at NHS Providers, Saffron Cordery, said of the national figures: “The NHS has a long history of advance planning to deal with flu and has done more than ever this year to minimise the risk. The growing impact of flu comes as services are already at or beyond full stretch."
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