Norovirus: new strain may be behind 63% increase in cases Symptoms of the norovirus, a strain of which is seen here under an electron miscroscope, usually last for around two days. Photograph: Rex Features/CDC/Phanie Most people contracting the norovirus winter vomiting bug are being hit by a new strain, which may explain the huge increase in cases this year, say health officials. The new strain of norovirus, called Sydney 2012, was first detected in the UK in October but has now become the dominant strain, the Health Protection Agency said on Tuesday. HPA genetic testing of patients in England and Wales affected by norovirus last October, when it began circulating again, identified Sydney 2012 as one of a number of strains that were affecting a growing number of patients, which also included another strain called New Orleans 2009. At the time none of what the HPA called a "cocktail of different strains" was dominant, the HPA said. But the latest testing of the most recent outbreaks, completed this week, showed that Sydney 2012 had overtaken all others to become the dominant strain. Sydney 2012 was first seen in Australia - and named after the place it was first identified. It has also been seen in France, New Zealand and Japan, the agency said in a statement confirming the new strain's presence here for the first time. The HPA stressed that Sydney 2012 did not affect patients any worse than previous strains of norovirus, which is highly contagious. This new strain does not cause more serious illness than others, and the methods of managing cases and outbreaks are the same for any strain of norovirus. These measures include washing the hands thoroughly and regularly, particularly after using the toilet and before eating," it said. However, an HPA spokeswoman said the mutation of the virus may be why 63% more people have been hit by norovirus so far this winter than during the same period last year. Until now experts had been baffled by the fact the virus peaked unusually early this winter. "Dr David Brown [the director of the agency's virology reference department] says that it may explain ... why the epidemic has come early this year," she said. But it does not definitively explain the surge. There are no definitive answers about this. David says that it can lead to an increase in cases but it doesn't necessarily do that," she said. HPA data released on Tuesday showed an estimated 1.19 million people in the UK - a big increase on last year - have been affected by norovirus so far this winter in a surge that has closed dozens of hospital wards and also caused outbreaks in some schools, workplaces and cruise ships. Some 150,000 people were struck by it over the festive season alone. It usually affects about 3 million people between October and April. The new strain was the subject of a scientific paper published in the journal Eurosurveillance on 3 January, which called it Sydney 2012. It is always difficult to predict the norovirus season and this year is no different. Noroviruses mutate rapidly and new strains are constantly emerging. At the start of the season it is normal for outbreaks to be caused by a range of different strains. However, as the season progresses particular strains are more successful and become dominant," said Dr Brown. The emergence of a new strain does not mean that it causes more serious illness and managing outbreaks and those with the illness remains the same. There is no specific treatment for norovirus infection other than to let the illness take its course, with symptoms usually lasting around two days. Keeping hydrated is very important and you can take over-the-counter medicines to relieve headaches and aches and pains," he added. The HPA does not know at what point after October the Sydney 2012 strain became the dominant type of norovirus circulating, the spokeswoman said. During the last two weeks of 2012, the HPA confirmed 529 cases of norovirus.