Three more North Shore students have been tested for the influenza A virus after displaying flu-like symptoms, Health Minister Tony Ryall has said.

School groups from Rangitoto and Northcote Colleges on the North Shore are being monitored for swine flu after visiting Mexico, where the strain of influenza A is believed to have killed scores of people.

The World Health Organisation fears the new virus, a mix of human, pig and bird flu strains, has the potential to become a global pandemic. It jumped species, infecting more than 1000 people across Mexico and killing as many as 86; 20 deaths have been confirmed.

On Sunday, 10 students from Rangitoto tested positive for influenza A and three students from Northcote College have since been been tested after showing signs of illness.

They were part of a group of fourteen students and two parents from the school who returned from Mexico on Saturday on Air NZ flight NZ1.

Twenty-two Rangitoto Spanish language students and their three teachers returned from a three-week trip to the Central American country on a different fligh - NZ5 - the same day.

Some were sick with flu-like symptoms, and the public health service was notified on Saturday by a GP and a North Shore Hospital physician.

Initial tests showed 10 of the group had some form of influenza A, which health officials believe is highly likely to be swine flu.

Director of Public Health Mark Jacobs this morning told a media conference he was happy with the way the suspected swine flu cases had been handled.

"Until late yesterday afternoon we didn't even know for sure these people had influenza.

"What they had was mild upper respiratory infection, so it could just have easily been a cold," he said.

"It could turn out to be one of the many other forms of influenza A."

Dr Jacobs said New Zealand was in quite a different situation to Mexico where there were potentially many infections but only a small number of tests for the virus had been carried out.

In brief:
• Swine flu is a subset of the influenza A virus known as H1N1
• The virus has killed at least 20 people in Mexico and infected over 1000
• Cases have also been confirmed in Canada and the US
• There are suspected cases in France and New Zealand
• Northcote and Rangitoto College groups returned from Mexico on Saturday
• Ten Rangitoto students have tested positive for influenza A and the group is being kept in home isolation
• Passengers arriving in NZ on North American flights are being screened and given information on the flu virus
• The potential influenza epidemic status has been upgraded to yellow

A Rangitoto College teacher has been admitted to North Shore Hospital with flu-like symptoms and Mr Ryall said she was expected to be sent home this afternoon after being treated with Tamiflu.

Another member of the group was also hospitalised over the weekend but has since been discharged after being treated with antivirals.

"The remaining members of the party are being treated at home and remain in isolation," the Ministry of Health said in a statement.

Samples from the students who tested positive for influenza A are being sent to the World Health Organisation laboratory in Melbourne to ascertain whether it is the subset H1N1 swine influenza.

Health Minister Tony Ryall said his officials believed the students were "highly likely" to have swine flu.

"All precautions are being taken to allow for this. However, I am also informed none of the affected patients are considered seriously ill," he said.

At this morning's media conference he stressed that anyone who thought they might be at risk of the virus should contact a health professional.

"If you've travelled from North America or Mexico to New Zealand in the last fortnight and you develop flu-like symptoms, please be in touch with your general practice or with the public health authorities."

NZ 'well prepared' - Key

Prime Minister John Key earlier said New Zealand was well prepared to handle swine flu and had increased surveillance.

Mr Key said the Government was invoking its flu plan and health officials hiked the potential influenza epidemic status to yellow, just below the code red response phase.

"I think New Zealanders can be confident that the government has a plan, that plan was put in place quite a number of years ago when we were concerned about avian bird flu," Mr Key told Breakfast on TV One.

Mr Key said the plan set up since the 2003 bird flu scare meant New Zealand had good stocks of the Tamiflu vaccine - about 1.4 million treatments and the single gateway into New Zealand via Auckland meant control was easier.

Tamiflu has been released from Middlemore Hospital to the Auckland Regional Public Health Service to treat patients and those who have had contact with them.

Mr Key said other options included people not being allowed onto Air New Zealand flights if they were ill and those who arrive at Auckland being sent by a doctor stationed there since 5am for checks.

Warning signs of severe flu are rapid breathing or difficulty breathing, chest pain, severe vomiting or if a child is difficult to wake up.

Flight NZ1

The 364 passengers on Air NZ flight NZ1, on which the Rangitoto students returned, are being urged to see their GP or a health professional if they develop flu-like symptoms.

Air New Zealand said filters on its airconditioning killed 99.9 per cent of airborne viruses including influenza.

Dr Jacobs told this morning's media conference it was "unlikely" the virus could have spread throughout the plane.

He said those most at risk were people sitting beside an infected passenger, though influenza A could also be spread if someone coughed or sneezed on later touched by someone who put their hands in contact with their mouth.

"In theory its possible for it to spread through going to the airplane toilets and things like that. But the less direct contact someone's had with someone with influenza on a plane, the lower the risk."

Clinical director of the Auckland Regional Public Health Service, Julia Peters, said while the weekend focus was on managing the sick school students, her staff would today be following up with other passengers.

"Now that we know that this is an influenza A virus, possibly the swine flu virus, our attention will definitely be turning to all the contact tracing.'

Dr Peters said the sick had respiratory illness symptoms including blocked nose, cough, fever, aching muscles, headache, loss of appetite, sneezing and watery eyes.

The sick and the well had agreed to stay in "home isolation" until the preliminary test results were known.

"Where there are non-symptomatic people, we have said other household members can go about their normal business," she said. "If the preliminary results are all clear it is likely we would be allowing people who are well to go to school and work [today]."

David Boyens, of Mairangi Bay, told the Herald last night he was relieved that tests on his 16-year-old twins Thomas and Anja, who went on the trip, had put them in the clear.

But because swine flu could not yet be ruled out for the whole group, health officials had told them to act as if they had been exposed. They would be given Tamiflu and were in home isolation.

"I was down to go to Australia [today] but I was told I can't - just cancel that and stay at home."

Passengers arriving in NZ

All passengers arriving in New Zealand from North America are being screened and given information about the flu.

They are also completing passenger locator cards so they can easily be contacted if required.

Health Minister Tony Ryall has upgraded the alert level to code yellow - one below the highest code red. "[That means] increased surveillance and people arriving on North American flights will be told that if they have flu-like symptoms they should be in touch with their doctor or other health authorities."

Sick people should use tissues to cover sneezes and coughs; avoid contact with others and stay at home; wash and dry hands often; and avoid touching their eyes, nose and mouth.

Global spread

Canada today became the third country to confirm human cases of swine flu with public health officials reporting six people falling ill in Nova Scotia and British Columbia.

In the US, health officials said 20 swine flu cases had been reported so far in New York, Ohio, Kansas, Texas and California. There are suspected cases in France as well as in New Zealand.

On Saturday, the World Health Organisation activated stage three of its six-stage global influenza plan. Stage three puts nations on "pandemic alert", urging them to intensify surveillance for unusual outbreaks of influenza-like illness and severe pneumonia, but is less serious than the declaration a pandemic is under way.

Stage 3 is activated when there is "no or very limited human-to-human transmission".

The WHO declared the outbreak a public health emergency of international concern, but said there were gaps in knowledge on the virus.

Director-general Margaret Chan said symptoms of swine flu were similar to seasonal flu, but could include severe pneumonia causing death. Sometimes there were no symptoms.

The WHO said the current seasonal flu vaccine did not contain swine flu virus and it was not known whether the vaccine would protect against it.

The US Centres for Disease Control and Prevention said Tamiflu and Relenza seemed to work against the new virus.

- With NZPA, AP