The majority of recent cases of norovirus were caused by a new strain according to health experts.
The new variant of the winter vomiting bug called Sydney 2012, has become the "dominant strain" and will have caused many of the cases of the recent outbreak, officials said.
While other strains are still in circulation, Sydney 2012 is responsible for the majority of recent cases in England and Wales.
Health officials said that Sydney 2012, which has also been identified by health experts in France, New Zealand and Japan, is not more serious illness than other strains.
The HPA said yesterday there have been 4,140 laboratory-confirmed cases of norovirus so far this season - but for every reported case, an estimated 288 are not flagged up.
This means as many as 1.19 million people could have contracted the illness this season - a 63% rise on the previous year.