Medical experts in New Zealand are bracing themselves for the arrival of a deadly flu that is sweeping the US which has already killed 20 children.

Authorities here say they are prepared and this year's flu vaccine has been upgraded to cope with the vicious strain.

The H3N2 virus struck early in the northern hemisphere winter and has spread quickly.

In New York alone, where a state of emergency has been declared, more than 19,000 cases of influenza have already been reported - almost five times the number reported at the same time last year.


Last week, 7 per cent of deaths in the US were blamed on pneumonia and the flu, the Centers for Disease Control and Prevention said.

The country's health system is creaking under the strain and vaccine shortages have been reported in some areas.

The National Influenza Specialist Group (NISG) in New Zealand says the US flu resembles the same 'Victoria' strain which struck Christchurch last year.

A serious outbreak of the H3N2 strain of virus, which caused nausea and hallucinations, resulted in Christchurch Hospital struggling to cope last winter.

Six people ended up in intensive care, and at its peak around 50 patients were isolated as the health board battled to bring it under control.

The 2013 flu vaccine has been adapted to include the Victoria strain which top virologist Dr Lance Jennings says should provide ``adequate protection'' against the new strain terrorising the States.

But while last winter was ``a relatively mild to moderate'' flu season - outside of Canterbury - Dr Jennings believes that the H3N2 virus will have an impact.

``This virus will affect New Zealand and we are likely to see regions more severely affected,'' said the NISG spokesman, speaking today (Tuesday) from an influenza conference in Phoenix, Arizona.


The outbreak was the talk of the conference this week, he said.

While tens of thousands of Americans die every year from flu, even in non-epidemic years, it's estimated that around 400 people die each year in New Zealand, either directly or indirectly.

Influenza can make other conditions, including respiratory or heart problems, even worse.

Flu immunisation is free for New Zealanders at high risk of more severe disease and complications, including pregnant women, people aged 65 and over, and anyone with on-going health conditions.

A spokeswoman for NISG today said this winter's vaccine will be available early March, after it arrives in the country, is approved by medical regulatory body Medsafe, and then rolled out to all doctor's surgeries.

Dr Jennings said the vaccination programme was planned "well in advance'' to ensure there was enough stock to last a bad winter.

The flu season usually starts in June, he said, with peak activity late June, July, or August, depending on the season.

The best protection against it is still the flu jab, Dr Jennings said, adding that people should not become complacent after a mild season.

"Influenza is a serious disease, and unfortunately people die from it.''