Planning is under way for a blood-testing programme by the Ministry of Health to find out more about swine flu as Health Minister Tony Ryall warned that a second wave could be more severe.
In a speech to the National Health Emergency Management Forum, Mr Ryall said a serological prevalence survey - a series of targeted blood tests - was being considered to find out more about spread, levels of exposure, and susceptibility in the population to H1N1.
"We don't know how many New Zealanders caught the virus in the first wave. Because it was mostly mild, many people cared for themselves at home while others may have been infected but had no obvious symptoms."
Officials estimate between 10% and 25% of New Zealanders have been ill with swine flu.
At least 50% of a population needs to be infected to provide sufficient community immunity.
Initial figures from the United Kingdom said as many as 30% of a population could become ill, with the same amount experiencing only mild symptoms.
Information from the tests would aid future responses including vaccination strategies.
Deputy Director of Public Health Darren Hunt said the wide variance of the estimates meant that health authorities understood only the "tip of the iceberg".
"We don't really know the proportion of the population who have been exposed, therefore we don't really know who might be susceptible to the virus in the future."
From whom blood samples would be taken, and how many were needed, were issues still to be worked through - a timeline had not been finalised but it was work that needed to happen as soon as possible, Mr Hunt said.
Much of the testing would focus on antibodies in the blood to the virus - what was the length of time they were present in the body, and what antibody levels said about immunity levels, were questions which scientists would be looking to find answers for, Mr Hunt said.
But before that could happen, tests had to be validated to make sure scientists were looking at the right antibodies.
