People who catch flu and Covid at the same time this winter are twice as likely to die than those who only have coronavirus, according to the UK Health Security Agency chief executive, Dr Jenny Harries.

The former deputy chief medical officer for England warned that the UK faces an “uncertain” winter – with both flu and Covid-19 circulating for the first time – and urged people to take up both the coronavirus and flu jabs if eligible.

Asked how worried the public should be about flu this winter, she told Sky’s Trevor Phillips On Sunday: “We should be worried about flu each winter. I think people still don’t realise it can be a fatal disease.

“But I think the important thing about this winter is, we are likely to see flu, for the first time in any real numbers, co-circulating with Covid. So the risks of catching both together still remain. And if you do that, then early evidence suggests that you are twice as likely to die from having two together than just having Covid alone.

“So I think it’s an uncertain winter ahead – that’s not a prediction, it’s an uncertain feature – but we do know that flu cases have been lower in the previous year so immunity and the strain types are a little more uncertain,” she said.

Harries also warned that the UK could have a multi-strain flu this year, with lowered immunity, as last year’s Covid restrictions meant that levels of the virus were extremely low.

She said that on average, about 11,000 people will die from flu each year. “The difference here is because we have, if you like, skipped a year almost with flu, it’s possible we might see multi-strain flu – we usually get one strain predominating,” she added.

Harries said there are four strains of virus in this year’s flu vaccine, after taking advice from the World Health Organization (WHO), the Joint Committee on Vaccination and Immunisation (JCVI) and looking to countries in the southern hemisphere, where winter and therefore flu season arrives earlier.

“So we’ve got a pretty good array in our toolbox to try and hit whichever one becomes dominant but it could be more than one this year, and people’s immunity will be lower. So I think the real trick here is to get vaccinated in both Covid and flu, but obviously to continue to do those good hygiene behaviours that we’ve been practising all through Covid”, Harries added.

Harries also said that making children wear masks in school would not be at the top of her list of Covid-safe measures.

“I think the important thing is we should make sure children aren’t in school if they are actually infectious. We’ve got a very good testing programme and we know that at the start of the term we expected to see a surge in cases”, she told the BBC’s The Andrew Marr Show.

Harries also said the dominance of the Delta variant globally has caused other coronavirus variants to “become extinct”, but warned we still need to “stay alert”.

The NHS aims to immunise a record 35 million people this winter, the most ambitious programme of flu jabs in its history. Free flu shots are available for about 30 million frontline health and social care workers, people aged 50 and over, children up to school year 11, those who are pregnant and those at clinical risk.