NHS leaders are urging nurses, doctors and other healthcare workers to have a flu vaccination to protect their patients this winter.
Vulnerable groups, such as children, pregnant women and older people are also reminded to have their free jab.
Sir Bruce Keogh, NHS England's medical director, told the BBC he was worried about how staff would cope with a major flu outbreak.
He said the NHS was under "severe and unrelenting" pressure.
His comments come following reports of a much higher incidence of flu in the Australian winter and the possibility that the same strain of the virus will be seen in Europe.
NHS bosses said many people with flu showed no symptoms so healthcare workers could be unintentionally infecting vulnerable patients.
Getting vaccinated was the best way to stop the spread of flu and prevent deaths, they said.
NHS staff are offered the vaccine free and NHS England will also offer it free to more than one million care-home workers this winter.
It wants employers to report how many people don't get the vaccines - at some hospitals only 30% of staff have the jab while at others it is nearer to 90%.
NHS England said 21 million people would be eligible for the vaccine in total this winter.
It has expanded the nasal flu vaccination programme to include children from the age of two up to and including those in school Year 4. Children in reception year can now get their vaccine in school instead of going to their GP.
Sir Bruce said: "Many people are very worried that this winter will be particularly difficult - the thing that I worry about most is that we have an outbreak of flu or an outbreak of norovirus which puts an added strain on the NHS service."