Health experts have sent a warning to parents around the UK following a surge in flu cases in the past week.
The latest surveillance data on Covid-19 and flu, published by the UK Health Security Agency (UKHSA), suggests the flu season has arrived early this year.
The UKHSA says there has been a rise in hospitalisations and ICU admissions among children under five in England.
The new data also suggests an increasing number of calls being made to NHS 111 and a slight rise in people seeking help from their GP for flu-like symptoms.
The UKHSA has urged everyone eligible to get a flu vaccine, saying Covid restrictions over the past two winters means people have little natural immunity.
The latest data suggests hospital and intensive admissions for the respiratory disease are rising quickest in children under five.
However, overall flu levels are still at a relatively low level, meaning it is difficult to predict how big the wave will be.
This year's flu jab is a good match for the type of seasonal flu (H3N2) that is circulating.
In 2017/2018, the H3N2 flu strain led to a bad UK flu season, with around 20,000 deaths and 40,000 hospital admissions.
Dr Mary Ramsay, Director of Public Health Programmes at the UK Health Security Agency, said: "Our latest data shows early signs of the anticipated threat we expected to face from flu this season.
"We're urging parents in particular not to be caught out as rates of hospitalisations and ICU admissions are currently rising fastest in children under 5.
"This will be a concern for many parents and carers of young children, and we urge them to take up the offer of vaccination for eligible children as soon as possible."
She added: "It's possible that we're already seeing the benefits from so many people taking up their Covid-19 autumn booster in England. Encouragingly, the latest data shows a small decrease in Covid-19 cases and hospitalisation rates over the past week.
"There's no room for complacency though, as cases could rise again at any point and we need to be armed in readiness through vaccination of everyone who's eligible. Don't delay, please come forward for both Covid and flu vaccinations as soon as you're offered them.
"Vaccines remain our best protection against severe disease and hospitalisation this winter but it's also vital that we all remain cautious as we head into winter when people tend to mix more indoors and the risk increases.
"If you are feeling unwell, avoid contact with elderly people or those with underlying health conditions."
Around 33 million people in England are eligible for a free flu vaccine this year, including all primary-age and some secondary-age children, who will be offered the nasal spray.
Those eligible for the flu jab are:
- People aged 50 and over;
- Those aged six months to 49 with a specified health condition;
- Secondary school-aged children focusing on Years 7, 8 and 9 with any remaining vaccine offered to Years 10 and 11;
- Primary school-aged children; pregnant women; those in care homes, frontline health and social care staff;
- Carers and the household contacts of people with weakened immune systems.
These people can get a jab from their GP surgery or pharmacies offering an NHS vaccine service.
GPs are also inviting children aged two and three years old (as of August 31) for the nasal spray vaccine.
