Ministry of Health spokeswoman Dr Alison Roberts said tests in Wellington, Auckland and Christchurch had confirmed influenza B viruses in circulation in the community, as well as small numbers of influenza A virus.
"Influenza B infections are generally less severe than influenza A and they're more commonly associated with outbreaks among school-age children rather than older age groups. As a result, influenza B is less commonly linked with hospital admissions and the more severe disease. But they can still be very unpleasant," Dr Roberts said.
"The influenza B viruses circulating this year, particularly in Auckland and Wellington schools, are predominantly the Influenza B Hong Kong strain and a few of the Influenza B Shanghai strain. The B Shanghai virus is in the influenza vaccine this year after the Shanghai virus circulated in 2004, whereas the 2004 vaccine contained the B Hong Kong strain.
"People who were vaccinated last year are likely to have some remaining protection against the B Hong Kong strain. It's possible children may not have been exposed to influenza B strains, which is why there has been such an impact in various schools around the country."
Dr Roberts said the annual influenza immunisation is the best form of defence against the disease, and she strongly urged people to get immunised, particularly those who are most vulnerable and at risk. For these people, influenza can be a serious disease.
The subsidised influenza vaccination campaign for high-risk New Zealanders -- people aged 65 and over, as well as adults and children with long-term health conditions such as heart disease, stroke and diabetes -- runs until the end of July.
"There's still time for people who haven't had their vaccination to go and get one, and I would strongly encourage this," Dr Roberts said.
Children with high-risk conditions are encouraged to have influenza vaccine and are funded under the Ministry of Health's subsidised influenza vaccination campaign.
The influenza vaccine is also available for children who do not have high-risk conditions, but it is not publicly funded.
Dr Roberts said it was also important to remind people that being fit, active and healthy does not protect against influenza “ it's highly contagious and anyone can catch it.
Influenza activity in New Zealand peaked last year at the end of September, a month later than the peak in 2003 and 2002.
It is well-documented that influenza B predominates or co-dominates every second year in the southern hemisphere. This happened in New Zealand in 1991, 1993, 1995, 1997 and 2001. Dr Roberts said the last big year for influenza B in New Zealand was 2001.
The three influenza strains in this year's vaccine are:
A/Wellington/2004 (H3N2) A/New Caledonia/1999 (H1N1), and B/Shanghai/2002.moht.nz