Catching flu at the same time as Covid-19 nearly doubles the risk of death from the virus, Public Health England (PHE) has said.
A new study of hospital patients who contracted both diseases from January to April this year found a 43 per cent mortality rate compared to 23 per in people who caught coronavirus alone.
Although the high death tolls for both cohorts reflects the vulnerable status of those patients, officials are warning that anyone who gets both flu and coronavirus at once could be in "serious trouble".
PHE also highlighted the risk of being hospitalised by influenza and then catching Covid-19 from other patients or staff.
The agency is launching its biggest ever flu vaccination campaign, aiming to make 30 million doses available across the winter. For the first time, all primary school and Year Seven children will be offered the flu nasal spray vaccine in an effort to stop spread between families.
The flu vaccine will also be offered to household contacts of people on the NHS shielded patient list and all health and all social care workers who have direct contact with the people they care for.
Once uptake in the most at-risk groups has been maximised, the newly eligible 50 to 64-year-olds will be invited for vaccination later in the season. Experts have said these lower risk individuals may not be called for a jab for several months.
Professor Yvonne Doyle, the medical director at PHE, said: "People still think that the flu is just like a cold – it's not. And those who are going to get a bad dose of flu may well end up in hospital at a time when they may be exposed to other problems like Covid.
"If you get both you are in some serious trouble, and the people who are most likely to get both these infections may be the very people who can least afford to in terms of their own immune system or the risks of serious outcomes."
A typical flu season in the UK kills between four and 20,000 people. Last year, approximately 8,000 people died due to the seasonal virus, with around 5,000 hospitalised.
PHE aims to vaccinate 75 per cent of the eligible groups this year. Two and three-year-olds will be able to get a vaccine from their GP.
Professor Jonathan Van-Tam, England's deputy chief medical officer, said: "Flu can be deadly and it is easily spread in children and adults. The vaccine is the best way to protect yourself from becoming ill with the flu, especially if you are in a vulnerable group."