Parents have been urged to vaccinate their children against the flu following a surge in serious cases among under-fives.
Flu hospitalisations in young children are nearly 20 times as high as last year, figures suggest.
This week, 230 under-fives were hospitalised, compared to just 12 at the same time last year.
Estimates show hundreds more under-fives have been admitted to hospital with flu over the last six weeks.
Pregnant women have also been advised to vaccinate themselves.
The NHS is writing to over 800,000 parents to encourage them to take their children to get the vaccine at a GP practice ahead of Christmas. Children under five are given a nasal spray, unless it is not medically appropriate.
Uptake in children aged two to three is behind last year’s figures, with just under 35% of children having received their flu jab so far, down 9% on last year.
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Nearly 18 million jabs already administered
NHS National Director of Vaccinations and Screening Steve Russell said: “With almost 18 million jabs already administered, our flu vaccination programme continues to make great strides in protecting the public, but it is vital we make sure no group falls behind.
“Young children, whose health can of course be affected by illness, can also pass on flu to other vulnerable family members, so we encourage parents to think about getting their flu vaccination at thousands of available sites ahead of the Christmas period.”
Who can receive a free vaccine?
Children born between 1 September 2018 and 31 August 2020 can receive a free NHS flu vaccine, along with all pregnant women.
Pregnant women also have lower uptake of the vaccine, with 29.6% having had the vaccine this year and 34.4% last year.
Respiratory diseases more common in winter
Professor Sir Stephen Powis, NHS National Medical Director, said: “It is very encouraging to see good flu vaccination uptake already but as we go through winter and we spend more time indoors, respiratory diseases become more prevalent so it is vital we do whatever we can to protect ourselves and others.”
“Flu when you are pregnant can cause complications and evidence suggests it could cause your baby to be born prematurely. Young children can also be at risk, especially if they have long-term health conditions, and we have seen an increase in hospitalisations in recent weeks.
“So our message to pregnant women and parents of young children alike is it’s not too late to come forward for this vital protection.”
It comes as the number of people in hospital with the flu jumped by 40% in the past week.
Official figures showed an average of 482 patients a day were in hospital with the flu last week, compared to 344 the week before.