The UK could face a ‘twindemic’ of Covid-19 and flu infections this winter, with both infections expected to circle widely, the UK Health Security Agency (UKHSA) has warned.

Reduced social mixing during the Covid-19 pandemic has led to lower levels of natural immunity to the flu this year, especially amongst children below the age of five, making the population especially susceptible to infection, it said.

UKHSA warned that the H3N2 influenza subtype is currently the most widely circulating worldwide and has already caused early waves of infection in southern hemisphere countries like Australia.

Coupled with expected growing levels of Covid-19 in circulation, NHS director for vaccinations and screening Steve Russell said this winter ‘could be the first time we see the effects of the so-called twindemic’ of flu and Covid-19.

With 33 million people offered a flu jab and 26 million offered an autumn Covid booster vaccination this year through pharmacies and other NHS services, Mr Russell urged those eligible to ‘come forward for vaccines in order to protect themselves and those around them’.

Dr Susan Hopkins, chief medical advisor at UKHSA, warned the flu H3N2 subtype ‘can cause particularly severe illness’, and in combination with widely circulating Covid variants this ‘poses a serious risk to our health’.

The last time H3N2 was dominant in the UK, in 2017-18, there were 41,730 flu-related hospital admissions in England and approximately 22,000 excess deaths.

However, early evidence from Australia suggests that current flu vaccines are well matched to the current subtypes in circulation and can provide effective protection against severe illness.

Dr Hopkins added: ‘Most eligible groups have been selected because they are at higher risk of severe illness. Younger children are unlikely to have built up any natural immunity to flu and therefore it is particularly important they take the nasal spray vaccine this year.’

All primary school children and some secondary school students will be eligible for the flu vaccine this year, which will largely be administered by nasal spray at school. GP surgeries are also offering the spray to children aged 2 and 3.

Despite record uptake of the flu jab in peopled aged 65 last year, the latest data suggests that less than 40% of pregnant women are currently vaccinated and only half of two- and three-year-olds have had flu vaccinations.

Those in high-risk groups may be at severe risk of serious illness or even death without the vaccination.

Earlier this week, UKHSA warned that vaccinators should leave a seven-day gap between the flu jab and the Novavax Covid-19 vaccination.

In September, Pfizer’s new bivalent vaccine targeting the Omicron Covid variant was added to the autumn booster campaign.

Over two million Covid booster jabs had already been given to patients two weeks into the autumn campaign, as Covid infection numbers have begun to rise once again.