Flu is circulating at its highest level for three years, amid concerns the vaccine may be less effective.
Official figures show 74 outbreaks have been reported in England in the past week, with most in care homes.
Meanwhile, doctors at Southampton General Hospital have warned the virus has mutated to avoid the immunity provided by the flu jab.
Public Health England said the jab would still offer "some protection" and was the best option available.
There has not been a major flu season in the UK since the winter of 2010-11.
Cases remain below the level seen then, but are already higher than the peak from the past three flu seasons.
Increases in weekly flu cases have been reported in England, Wales and Northern Ireland, but remain stable in Scotland.
Public Health England data show 107 people have been admitted to hospital in the past week.
Doctors will be closely watching the figures as children return to school for an indication of how bad the flu season will be.
The main circulating flu virus this winter is influenza A type H3N2, which disproportionately affects the elderly.
It has been the source of mounting concern that strains of the virus have mutated to make the seasonal flu jab less effective.
The US Centres of Disease Control has warned that half of H3N2 viruses tested there were "drifted strains" which were different to the one protected by the flu jab.
Of 24 samples tested by Public Health England, five are of drifted strains.
Dr Richard Pebody, head of seasonal flu surveillance at Public Health England, said: "This indicates a proportion of circulating flu viruses circulating in the UK are likely to be drifted.
"Further analysis will take place, but it is not clear yet if these drift variants will become the dominant strain nor if such a drift will result in reduced effectiveness of the vaccine.
"Existing vaccine is likely to still provide some protection and getting vaccinated remains the best way to protect yourself from flu."
Doctors at Southampton General Hospital have warned of a "significant rise" in admissions and put part of the blame on flu mutating.
Dr Ben Marshall, a specialist in respiratory medicine at the hospital, said: "We have seen the number of patients, mainly those who have respiratory conditions such as asthma or COPD, being admitted as medical emergencies increase from 25-to-30-a-day to more than 50."
He said half of cases were H3N2.