Although Covid-19 cases are once again in decline, experts say a potentially harsh flu season has begun in the U.K.

Some southern hemisphere countries have already experienced serious flu outbreaks after two years of relatively low circulation. Australia, for example, has had its most severe season in five years.

The U.K.’s Health Security Agency on Thursday reported flu cases had jumped over the last seven days, with hospitalisations rising fastest in children under five.

Experts say the population is more vulnerable to flu than usual this year. Imperial College London professor of experimental medicine, Peter Openshaw, says: “Every winter there is this sort of expected crisis caused by influenza and we’re expecting this crisis to be even bigger than it would normally be, because no one has got that much background immunity this time.”

However, some scientists, including University of Georgia professor John Drake, say a worse flu season is not guaranteed.

Officials urged eligible members of the public to get their flu shots and Covid-19 boosters as soon as possible as cases rise.

So far, vaccination levels in pre-schoolers — as well as pregnant women and under-65s at increased clinical risk from flu — are lower than in previous years.

Openshaw says flu has a serious impact on the body and can bring complications such as bacterial pneumonia and ear and sinus infections.

“Flu is a nasty illness. It doesn’t just affect the nose and lungs. It can cause very severe disease, particularly in people with no immunity,” he says.

UKHSA director of public health programmes, Dr Mary Ramsay, said: “Don’t delay; please come forward for both Covid-19 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.”

At the moment, all adults over 50, as well as pregnant women and younger people with certain health conditions, are eligible for both a Covid-19 booster and flu shot in the U.K. Pre-schoolers, elementary school children and some older kids also qualify for a free flu vaccine.