Flu levels are continuing to rise in England as health experts warned parents of two and three-year-olds to get them vaccinated.

Rates are estimated to be highest among people aged 15 to 44, with 13.4% of laboratory samples from this age group testing positive, according to the figures from the UK Health Security Agency (UKHSA).

Rates are second highest in those aged five to 14 year-olds, with 10.4% of samples from this age group testing positive.

It comes as the NHS revealed it administered 13.2 million flu vaccinations since September.

The UKHSA said flu vaccine uptake this year is comparable to last for those aged 65 and over, people at risk and pregnant women, but lower in two and three-year-olds, at 20.3% and 21.8%.

Dr Conall Watson, consultant epidemiologist for the UKHSA, said: “We’re now seeing flu spreading in all age groups over recent weeks.

“Children aged two to three may have little natural immunity to flu and can become severely ill.

“The recent flu season in Australia saw a high number of children hospitalised and there have already been intensive care admissions of young children in the UK.

“That is why we are urging parents to get their children vaccinated as soon as possible.

“It is also important that schoolchildren get vaccinated as it not only protects them but also reduces the spread of flu.

“Again, parents should ensure they fill in the school consent form so their child is able to get the nasal spray through their school programme.”

In total around 33 million people in England can get a free flu vaccine this year, including everyone aged 50 and over, all primary-age children and some secondary-age children.

The vaccine is also being offered to pregnant women, people in care homes, frontline health and social care staff, carers, those aged six to 49 with a specified health condition, and household contacts of people with weakened immune systems.

NHS director of vaccinations and screening Steve Russell said: “The NHS has delivered more than 13.2 million flu vaccinations over the last two months providing crucial protection to the most vulnerable in our society, including almost two thirds of over 65s, ahead of the first winter we will see flu in full circulation since before the pandemic.

“Today’s latest data from UKHSA shows that flu infection levels continue to rise, up again on last week, and so it remains vital that anyone who has been invited for an NHS flu jab comes forward and books as soon as possible – either through your GP, local pharmacy or at one of the hundreds of sites now on the new national booking service.”

All two and three-year-olds are eligible for a flu nasal spray vaccine, which will be offered by their local GP.

The new data shows 6.1% of samples from all age groups tested positive in the week to October 30, up from 5.2% in the previous week.

Hospital admissions for flu decreased slightly during the week and were highest in the 85 and over age group, the UKHSA said.

Covid-19 hospital admissions also fell but were highest in the 85 and over age group.

Overall levels of flu are still low, meaning it is too early to predict how big the wave of infections will be.