Latest figures from the Health Protection Agency inidcate that the number of flu cases is continuing to increase.
Several indicators include GP consultation rates, the proportion of calls to NHS Direct and new admissions to intensive care of high dependency units.
Children aged five to 14 in particular have been the group most affected by flu so far this season.
Dr Richard Pebody, head of seasonal flu surveillance at the HPA said: “Vaccination against flu is still the most effective way of preventing the virus in people who are in an ‘at risk’ group, as they are more vulnerable to developing complications from flu.
"This includes people with underlying conditions such as heart problems, diabetes, lung, liver or renal diseases and those with weakened immune systems, as well as older people and pregnant women."
Healthcare staff are being urged to have the flu jab to help protect themselves and their patients from the illness. As the temperature drops over the winter, the number of people with coughs, colds and flu increases, putting extra pressure on the service.
The NHS has started a campaign to get hospital workers to have the vaccination as well as vulnerable members of the public.