Health Minister Jonathan Coleman today announced the influenza immunisation programme will be extended until the end of next month.

The extension follows reports that an unusual pattern of sickness is developing in the North and South Islands, with health professionals recording peaks of different influenza strains - something not seen in more than 30 years.

"Ministry of Health surveillance suggests that influenza has not yet peaked this winter. To help ensure people have the protection they need we are extending the funded vaccination season until 31 August," Dr Coleman said.

"The A-H3N2 strain, which is covered by the vaccine, appears to be the predominant type - this strain affects the elderly and very young more severely than other strains."

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The influenza vaccine is also free for people with Down Syndrome and those with cochlear implants.

"Although the vaccine was delayed worldwide by a few weeks, the health sector has quickly caught up with previous years' distribution," says Dr Coleman.

As of today, almost 1.19 million flu vaccines have been distributed across the country, protecting over a quarter of our population.

Dr Lance Jennings, Canterbury District Health Board virologist and spokesman for the National Influenza Specialist Group, said the country do a lot better with the number of flu vaccinations handed out so far.

"I think we're about 17,000 doses shy of last year's record of 1.20 million doses distributed, so it would be nice to reach that target and protect those at greatest risk and who haven't received their vaccine as yet."

As the Southern Hemisphere influenza season was still getting underway, there was still no data on whether any new viruses had been discovered yet.

However, samples were sent to the World Health Organisation's southern hemisphere laboratory in Melbourne last week.

He also put a dampener "one of the myths" about the effectiveness of the flu jab.

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"There is always that 'oh I got the flu jab and I developed the flu afterwards' but there are other respiratory viruses that result in considerable burdens in terms of hospitalisation of young children with pneumonia-like conditions and that's prevalent at the moment as well. There are also other viruses that mimic influenza."

Dr Jennings said not much could be done about preventing the respiratory viruses currently circulating.

National Influenza Specialist Group [NISG] figures reveal CanterburyDHB has been issuing the most flu vaccinations as at June 30 with 34 per cent. Their neighbours in the South CanterburyDHB patch are second at 32 per cent, with North Island DHBs - Hutt ValleyDHB third on 31 per cent, followed by Auckland and Capital & CoastDHBs both sitting on 29 per cent - rounding out the top five.

Data obtained by the Herald from the Environmental Science and Research latest influenza report, dated July 5, reveals there has been 131 reports of the A strain in the North Island this season, compared with two in the South Island.

However, the South Island - which has a smaller population - has had 30 reports of the B strain, compared with 29 in the North Island.

Reports of influenza-like illnesses were peaking so fast they had almost reached the total number for last year, despite two months of the flu season to go.

Concern about the virus spreading is high as children head back to school next week.

Dr Jennings warned North Islanders battling Influenza A that they would get hit by Influenza B.

"I suspect what we're going to see in the North Island is the H3N2 [A] virus dominating and then as the season progresses they're going to be affected by the Influenza B virus because that quite often follows A later in the season. That's one possible scenario."

Dr Jennings described the latest patterns as "fascinating". The last time a pattern like this had emerged was in the 1980s.

The viruses depended on which viruses had circulated the year before, he said.

"The main issue is the difference in the viruses circulating and that's a fascinating observation. "In the South Island last year, we had a significant H3N2 year, in the latter part of our winter we had two peaks, H1N1 and then the H3N2 virus impacted and so that effectively will have primed the [South Island] population and that may be reflected in the dominance of the B virus this year down here," he said.

The H3N2 virus - or Hong Kong virus - first emerged in the pandemic of 1968. Since then it had continued to "quite rapidly evolve", he said.

However, Dr Jennings said there was some good news - all the viruses currently circulating are all covered by this years flu vaccine, available until July 31.

Last year the country experienced a relatively mild winter.

Influenza H3N2 - the Switzerland strain - was a new virus that emerged last year, he said.

It surfaced in Auckland four weeks ago, before spreading to the Bay of Plenty.

"But now Waikato has been severely affected and Bay of Plenty have a large number of virus identifications - they're increasing. Whereas Auckland and Counties Manukau regions, it's been ongoing for over four weeks or so, and there has been a lot of influenza activity. But it's been in the last three weeks in the North Island that it's really escalated, and the last week in the Waikato."

The A virus is usually more serious, but both are fatal.

In 2005, five children, including three with no reported medical conditions, died after contracting Influenza B.

"In general, Influenza A viruses, particularly the H3N2 virus, have a severe impact on the very young and the elderly and what we tend to see in institutions, particularly old people's homes, when the H3N2 virus enters those communities we see increased mortality."

Influenza B viruses tended to produce severe infections, but had lower death rates.

The Hawke's Bay District Health Board yesterday confirmed its reports of influenza-like illnesses had doubled in the past week.

Chief medical officer John Gommans said shortness of breath and chest infections were the lead causes of presentations and admissions to Hawke's Bay Hospital, which had been very busy.

Waikato District Health Board's medical officer of health Dr Anita Bell also confirmed a sudden increase in influenza-like illnesses and Influenza A cases in the district, as well as respiratory infections.

Dr Jennings said the flu spread throughout winter as people spent more time indoors and it revelled in the drier, colder conditions.

"People do die from influenza, people do get admitted to hospital with influenza and the important thing is if parents or individuals are concerned they should phone their GP and consult with them."

New Zealand has experienced relatively mild winters in the past three years, last experiencing significant peaks in influenza-like illnesses in 2012 and 2010.

What to expect

What can those in the North and South Island expect this winter?

In the South Island, people will continue to ride out the B strain virus, while in the North Island, the A strain will continue to spread, likely peaking after school holidays, before the B strain virus will kick in.

When should I see my GP?
Immediately if you're over 65 or pregnant or over 65/under 5 with a history of respiratory conditions. Or, seek help if you can't lift your head off the pillow and are having trouble breathing.

How long are you contagious for once you notice influenza symptoms?
Up to 10 days for children or between three and five days for adults. So if you're sick at work, you're likely spreading it to your colleagues.

How much of the population has taken a flu vaccination so far?
As at 30 June, 1.17 million doses had been handed out, which is about 26 per cent of the population. A total 1.2 million doses were handed out last year.

How do you catch influenza?
Most commonly by air particles, from people sneezing - flu can spread up to two metres away - or not washing their hands.

When will it all end?
Flu season typically lasts until September.