The UK's top health agency has issued a warning after uncovering a staggering trend showing a surge in flu cases among young children.

The UK Health and Security Agency (UKHSA) has found that cases of the flu have continued to increase among schoolchildren across the country - and it may suggest the flu season window is shifting. The agency found that, in week 42 of 2025, flu is now "above baseline" in some indicators,

Flu positivity has increased by around 6.1 percent, health experts found, more than a percentage point more than recorded the previous week, and the number of people hospitalised with the virus has risen alongside it.

The severity of those cases has also increased, as the UKHSA data shows the number of hospitalisations rose from 1.29 to 1.73 per 100,000 people in the same week. The agency has suggested that the jump in cases indicates that the country is "seeing signs of a possible earlier flu season".

While the number of cases is most notable in schoolchildren, epidemiologists have warned they could easily spread to other, more vulnerable sections of the population.

Dr Jamie Lopez Bernal, Consultant Epidemiologist at UKHSA, said it is critical that parents vaccinate their children to prevent other people from developing "serious illness". He said: "We continue to see a rise in flu activity this week, with positive tests among children still increasing.

"That is why it's important that parents return consent forms so that children can get the nasal spray vaccine when it is offered in schools. If your child has missed the vaccine at school, get in touch with your local school Imms team to find how you can get a vaccine."

"Those children in high risk groups can also go through their GP. And all those in other eligible groups who are more vulnerable should take up the free flu vaccine as soon as possible."

Dr Lopez Bernal added that it was equally important for other population groups to get their own vaccinations as flu continues to circulate in the UK this autumn. He said: "It is likely that flu circulation will spread into other groups of the population over the coming weeks, therefore it is important that all eligible groups get vaccinated.

"The vaccine remains our best defence against severe illness and hospitalisation from flu and is offered to everyone aged over 65, pregnant, or in a clinical risk group as well as school-aged children and two and three-year-olds."

The warning comes after the UKHSA warned that this year's influenza season could be worse than last year's with GP visits, hospital admissions and ICU stays for flu-like illnesses on the rise compared to 2024. According to the agency, the last wave was driven by the H1N1 strain of influenza A and later influenza B, which pushed the season well into March.

Flu warning as Brits urged to get vaccine after UKHSA uncovers staggering trend - The Mirror