Health officials will soon know if Northlanders were on flights into Auckland which may have carried a virus threatening to create a world pandemic.
The flu strain contains a mutation of bird, pig and human viruses, and appears to have begun in Mexico.
Northland District Health Board's acting medical officer of health, Loek Henneveld, has not heard whether any Northlanders or visitors to the region arrived on flights NZ1 and NZ5 on Saturday.
Those were the flights on which two school groups returned from Mexico, several of whom have become ill with a strain A influenza, possibly swine virus.
Authorities are now awaiting further test results from the World Health Organisation (WHO) laboratory in Melbourne to confirm whether it is swine flu.
At this stage, only people seated within a certain distance of those who were now ill were being given high priority as far as tracking and testing were concerned, Dr Henneveld said.
A public health "code white" alert was put in place when it was learned 10 members of two school groups had become ill after visiting Mexico.
But public health and border control sectors have gone into a nationwide code yellow response since the positive identification of the A strain.
Code yellow is one below the ultimate code red response which would see borders closed and other shut-downs put in place.
Northland travel companies say they are advising anyone wanting to book travel into North America, including Mexico, of the health risk. Stephen Gillingham, of the Whangarei branch of House of Travel, said travel consultants would be offering the same advice as the Government was to anyone recently returned from those destinations: If you feel ill then seek medical advice and stay away from other people while you wait for your illness to be identified.
All people coming into New Zealand are being warned that the flu strain may be here. Anyone admitting to feeling ill on arrival from North America will be required to have medical tests.
Senior health advisers said yesterday the chances of the disease having spread through the planes' air filtration systems was minimal.
Cross-contagion was more likely to occur through coughing, sneezing and touching.
In a media teleconference, the experts said the illness appeared to be a fairly mild form of flu.
Dr Henneveld said by yesterday afternoon, about 20 people had been positively identified as having died of swine flu in Mexico City, which has a population of about 20 million people and where the majority of cases have been reported.
Elsewhere in North America, 11 sick people had been confirmed as having the swine flu strain but none had died.
At this stage, it was not known how virulent or deadly the strain was compared with other flu viruses.
Swine flu cases contained so far, travellers advised of risk
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