People are being advised to get a flu jab to help protect against the "double danger" of flu and coronavirus.
Research shows people can catch both diseases at the same time, with serious and sometimes deadly consequences.
More people will be offered a free flu vaccine this year - anyone over 50 in England is eligible.
A vaccine for coronavirus is not available yet and experts are worried the UK could see the virus rip through the population this winter.
Research from Public Health England looking at Covid-19 illness between January and April among nearly 20,000 hospital patients suggests risk of death is more than doubled for people who catch flu on top of coronavirus, compared to coronavirus alone.
Flu by itself can also be a serious condition - it kills around 11,000 people in England each year and hospitalises many more.
People at high risk from flu are also most at risk from Covid-19.
Who will be offered the flu vaccine in England?
- people who were required to shield from coronavirus and anyone they live with
- people with some medical conditions including diabetes, heart failure and asthma
- pregnant women
- pre-school children over the age of two
- all primary school children, as last year, and, for the first time, Year 7 pupils
- initially all people over 65, before the programme is extended to the over-50s
- healthcare and social care staff
- people living in a residential or nursing home
- people who are the main carer for an elderly or disabled person
The NHS will get in contact with those who are eligible.
Prof Jonathan Van-Tam, the deputy chief medical officer for England, 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.
"This winter with Covid still circulating, and the increased risk to life if you are ill with both viruses simultaneously, it is even more vital to get the free jab as soon as you can."
People who are not eligible for a free jab could pay for one at pharmacies and supermarkets, although availability is limited currently while stocks are prioritised to those who need it the most.
Boots has temporarily suspended its bookings for anyone under the age of 65 and is temporarily limiting existing stocks to those at highest risk - people 65 and over.
NHS England confirmed there is no nationwide shortage of flu jabs, but that those eligible for the free vaccine would be immunised in phases, with the highest risk groups receiving it first.
It is unclear how bad flu might be this winter - some years are worse than others - but experts say the vaccine is a good match for the strain of flu that will be circulating.
Flu, like coronavirus, is a viral infection that is passed on through coughs and sneezes. Social distancing, masks and handwashing should help reduce the spread of both.
Most people with flu recover at home in a week, but people with chronic conditions or who are over 65 should call NHS 111.
If you think you have either flu or Covid-19, stay at home and self-isolate.
Book a coronavirus test if you have:
- a high fever
- a new. continuous cough
- a loss of, or a change to your sense of smell or taste