People should "not be complacent" over flu, experts have said as new research has shown dire consequences of co-infection of flu and Covid -19.
Those infected with both flu and Covid-19 have a serious increased risk of death, Public Health England (PHE) said.
Officials warned that both influenza and Covid-19 could be circulating at the same time as they urged people who are eligible to get vaccinated.
The flu vaccination programme has been expanded this year so more people than ever will be eligible for a vaccine – up to 30 million people in England.
It is hoped that a new campaign will encourage those who are eligible to accept their invitation when it is sent out.
The campaign has been launched as a new study showed that during the first peak of the pandemic, people who were infected with both the new coronavirus and flu had a significantly higher risk of death.
While the numbers were small, the risk of death was nearly six times greater among those co-infected compared to the general population.
The risk of death was also higher compared to being infected with Covid-19 alone.
PHE researchers analysed data in almost 20,000 people who were tested for both Covid-19 and flu between January 20 and April 25.
Fifty eight were identified as having 'co-infection' of the two viruses.
Overall 43% of people with co-infection died compared to 27% of those who tested positive for Covid-19 alone.
Those who died "tended" to be older, PHE said.
Flu usually kills around 11,000 people each year in England and many more are hospitalised.
People more susceptible to the effects of flu are also more at risk from Covid-19, officials said.
Three of England's most senior health officials have implored eligible people to get vaccinated when they get their invitation.
In previous years, the rates of people getting their flu jab have been low among some groups.
PHE's medical director Professor Yvonne Doyle, said: "We're encouraging anybody who is eligible to accept their flu vaccination this year, particularly with the winter we're going to face.
"People still think that the flu is just like a cold. It's not. The flu is an extremely unpleasant condition.
"If you're in a risk group, it can be really dangerous, and it can kill you.
"The vaccine is good, it's safe, and it does protect people. So it's worth having it."
On theories on competing viruses, Prof Doyle said that the first peak of the pandemic coincided with the end of the flu season last year so the full interaction between the two viruses is not yet fully known.
But she added: "If you get both, you are in some serious trouble. And the people who are most likely to get both of these infections may be the very people who can least afford to in terms of their own immune system or their risk for serious outcomes.
"Please protect yourself against flu this year.
"This campaign will... reinforce the importance of taking flu seriously and not being complacent about that."
Flu can be horrible and sometimes serious for little ones. The free nasal spray vaccine is the best way to protect them and others from #flu. If your child is 2 or 3, ask your GP today.
— NHSLewishamCCG (@NHSLewishamCCG) September 21, 2020
Professor Jonathan Van-Tam, the deputy chief medical officer for England, added: "We want to deliver the most extensive flu programme possible. And we will have enough vaccine this year for 30 million people in total.
"And this obviously is very important in a year where we face the possibility that flu and Covid-19 and may at some point this winter, circulate together and at the same time."
He said that alongside the PHE data, there have been studies in mice which found "bad outcomes" among those infected with both flu and Covid-19.
"There are multiple, plausible reasons why it's a very bad idea to have Covid-19 and flu at the same time. And of course, that possibility is real for this winter," he said.
When asked about the rising number of Britons joining the so-called anti-vax movement, Professor Van-Tam added: Of course I recognise that the anti-vax movement is real. But if you look at the annual PHE survey, actually the British public have an exceptionally high confidence in the UK vaccine programme and pretty much all aspects of it.
"I'm in high risk group, as soon as I'm called for my flu vaccine, I'm off to get it because I recognise the data on the bad outcomes that may be associated with having flu and Covid at the same time and since one is vaccine preventable at the moment, I don't want to take that risk. It's as simple as that."
Dr Nikita Kanani, medical director for primary care for the NHS in England, added that about 32 million people would be contacted and encouraged to take up their flu jab.
Health Secretary Matt Hancock said: "This year more than ever, it's vital that those eligible for the flu jab get it this winter so you can protect yourself, your family and the NHS.
"We're pulling out all the stops to prepare for this uniquely challenging winter and we have enough vaccines for 30 million people this year, more than we've ever done before.
"With the simultaneous risk of flu and Covid-19, make sure you get your flu jab if you're eligible, don't gather in groups larger than six and remember 'Hands Face Space' so we can look after each other."
This year people eligible for the flu vaccine include:
– Primary school children and Year 7 pupils will be offered the flu nasal spray in schools and two and three-year-olds will be offered the vaccine through their GP.
– Those age 65 and over, people with long-term health conditions and pregnant women will be offered the vaccine through their GP or pharmacy.
– Household contacts of people who were instructed to 'shield' during the first wave of the pandemic.
– Health and social care workers who have direct contact with the people they care for.
– Once the first at risk groups have been contacted, the vaccine programme will also be rolled out to include people over the age of 50.