The norovirus variant, known as Sydney 2012, was identified in a scientific paper last week and Britain's Health Protection Agency HPA.L said genetic testing showed it was now causing more cases in England and Wales than other strains.

Sydney 2012 does not carry worse symptoms than others but, like other norovirus strains, it can cause violent and projectile vomiting, diarrhoea and sometimes fevers, headaches and stomach cramps.

Norovirus cases have risen earlier than expected this winter in Britain, across Europe, Japan and other parts of the world.

Although norovirus mostly causes just a few days of sickness, it is responsible for millions of infections every year and is notorious for its ability to evade control.

In the United States, the Centers for Disease Control and Prevention say norovirus causes 21 million illnesses a year, with 70,000 cases requiring hospitalisation and around 800 ending in death.

Ian Goodfellow, a scientist who has studied norovirus for 10 years, describes it as "the Ferrari of the virus world" and "one of the most infectious viruses of man".

Latest HPA data showed a dip in reported norovirus cases over the Christmas period - something scientists had predicted - but with 4,140 cases so far in England and Wales, infections are still 63 percent higher than at this time last year.

For every laboratory-confirmed case, scientists estimate there are 288 unreported cases, since the vast majority of people affected don't go to a doctor. This means the number affected so far in Britain is more than 1.2 million.

"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," said David Brown, director of the virology reference department at the HPA.

"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."

There is no specific treatment for norovirus infection other than to let the illness take its course and try to stay hydrated by drinking regularly. Symptoms usually last around two days. (Editing by Tom Pfeiffer)

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