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'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."
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.
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."
This year people eligible for the flu vaccine include:
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.
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.