A new and deadly strain was behind most of the norovirus illnesses that affected dozens of people throughout New Zealand late last year.
There were 87 cases of norovirus in October and November last year, compared with 14 in 2011 and 23 in 2010, ESR senior scientist Joanne Hewitt said.
Of the 87 in October and November last year, 67 were blamed on a new strain known as the Sydney-2012 - making it the predominant strain.
The Sydney 2012 variant has mutated so it is not recognisable by the human immune system, according to Professor Peter White from the University of New South Wales team that identified the virus.
The virus had taken about four years to reach its current "pandemic label", he said.
Those infected suffer 48 hours of severe illness including vomiting, diarrhoea, nausea, headaches and possibly fever.
Ms Hewitt said the emergence of new strain prompted a warning for healthcare institutions to be prepared for a severe norovirus season with the rise of the Sydney-2012 variant.
The virus kills around 200,000 people around the world each year, with the very young and old and those with poor immune systems the worst affected.
There is no vaccine or treatment.
What can be done to limit its spread:
• don't go to work;
• don't visit hospitals;
• don't visit other people's homes; and
• don't visit childcare centres.
- APNZBy Rebecca Quilliam Email Rebecca