Health officials are concerned clusters of the harmful H1N1 virus, responsible for the 2009 swine flu pandemic, are appearing unseasonably early.

One woman is critically ill in Hawke's Bay Hospital with the virus and eight others have been admitted to the hospital's intensive care unit.

Dr Nicholas Jones, Hawke's Bay District Health Board medical officer, said the virus had struck before the usual winter flu season, which was cause for concern.

"We don't have an explanation for that. From what I can tell, there hasn't been a big change with it [the virus], which is one of the things you worry about," he said.


"The testing seems to show that it's similar to the one that was around in 2009."

Since then, H1N1 had been part of the standard influenza vaccine and people who were vaccinated need not worry, he said.

In South Canterbury, Dr Daniel Williams, Medical Officer of Health, said there had been a "significant" cluster of influenza-like illnesses in Geraldine. It was not unusual to have some cases out of season, but it did appear to be worse this year.

University of Otago Associate Professor of Health Michael Baker said calling the virus swine flu was now largely redundant since it had become one of the three strains circulating the planet every winter.

"It's best to say it's seasonal flu. It just happens to be the same strain that caused the pandemic."

A Ministry of Health spokesman said he was not aware of other clusters around the country, however, it was not a notifiable illness so it was possible it was occurring elsewhere.

So far this year 540,000 doses of vaccine have been distributed — almost half the target of 1.2 million people vaccinated by July 31.

Influenza immunisation is free from a GP or nurse for at-risk New Zealanders.