Staff, patients and visitors in some hospitals in the south have been asked to wear face masks to reduce the spread of the disease.
They include main hospitals in Andover, Basingstoke, Bournemouth, Poole, Portsmouth, Slough and Winchester.
Other hospitals, such as Dorset County and St Mary’s on the Isle of Wight reported very few flu cases in the first week of December.
The predominant strain of flu this winter, the drifted H3N2 strain, means there is “less natural immunity in the community”, the UKHSA said.
It said the strain typically affected older adults more severely than H1N1 strains.
However children and young people aged five to 14 have the highest positive test rates for flu in England.
Health authorities have urged patients with flu symptoms not to attend hospital, except in the most serious circumstances such as shortness of breath.
Dr Sutton said the disease could be treated with rest, paracetamol and good hydration, and should improve within three or four days.