Respiratory experts in Southampton have warned a strain of flu not covered by the seasonal vaccine is contributing to a significant rise in emergency hospital admissions.
Dr Ben Marshall, a specialist in respiratory medicine at Southampton General Hospital, said the number of patients admitted with respiratory illnesses had doubled.
Around half were found to have the influenza A H3 virus, a severe strain of the virus which is included in the annual seasonal influenza vaccine but, this year, has changed significantly (mutated) since the vaccine was prepared.
"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 a day," he explained.
"'Using a novel swab test, we have been able to rapidly discover what type of virus they are suffering from and a large proportion of them have had the influenza A H3 strain, which is now evading the immune response generated in recipients by the current seasonal vaccine."
Dr Marshall said despite the large numbers, the majority of patients admitted with respiratory conditions complicated by illnesses such as flu and other viruses were being treated and discharged "within days" - but the volume was adding "significant strain" to already stretched hospitals.
Dr Tristan Clark, a specialist in infectious diseases and respiratory viruses at Southampton General Hospital, said national figures showed cases of influenza were rising steadily week-on-week and warned that high levels of influenza activity could continue for another eight weeks.
"The flu season began in the UK around four weeks ago, which is a bit earlier than last year, and admissions are up significantly," he said.
"Based on the current trend and increasing hospital admissions, there is a high possibility we could continue to see such levels of activity for the next eight weeks."
Dr Clark, who is also a researcher in infectious diseases at the University of Southampton, explained flu preparations were made every year based on the flu season already seen in other parts of the world.
"We can sometimes predict what a flu season is going to be like based on the activity in the southern hemisphere before it reaches north of the equator and we know countries like Australia had a bad year," he said.
"Unfortunately, since the vaccine was prepared, the influenza A H3 strain has changed significantly, making the vaccine less effective at protecting against the virus - something we occasionally see."
He added, although less effective, people should still get vaccinated to offer some degree of protection - particularly those with chronic illnesses who have still not had the jab.
In addition, the antiviral drug Tamiflu remains active against all strains of influenza and is effective at reducing severity of symptoms, especially when given early in the course of the illness.