Overwhelmed public health authorities would give up trying to count all cases of influenza during a severe pandemic.
In the early stages of a pandemic, when there might be clusters of flu cases in defined areas, medical officers of health would try to trace everyone with symptoms and those who had been in contact with them.
In the Government's pandemic plan, this is the "stamp-it-out" phase. It would follow the "keep-it-out" period of likely border closure and precede the worst phase of "manage it".
But at the height of a pandemic as severe as the 1918 Spanish flu, during which a third to a half of the New Zealand population was infected, identifying every case would be impossible. On that scenario and based on today's population, 1.3 million to two million people could be infected.
"That would make it very difficult to follow every individual case," Ministry of Health senior public health adviser Dr Andrea Forde said yesterday. Alternative systems, such as contacting households, schools and employers, would be needed and were under development.
Health authorities globally are on the alert for the possibility of a severe flu pandemic that could arise from the H5N1 bird flu virus affecting parts of Asia and eastern Europe. It has killed or led to the culling of millions of birds, but relatively few people have been infected; 148 people have been confirmed as having caught the virus, of whom 79 have died.
The virus does not spread easily between humans, but virologists fear it could change to do so.
In New Zealand, human cases of any type of bird flu are notifiable, meaning they must be reported to a medical officer of health. These are public health doctors at district health boards who can be given wide powers under the Health Act by the Minister of Health to control travel and isolate patients during outbreaks of infectious disease.
But human influenza - and that would include pandemic flu, because, by definition, it would then be spreading easily between humans - is not notifiable, although it is classified in the act as an infectious disease.
Case numbers of seasonal flu are monitored by a system in which GPs nationally report all cases each week from May to September of flu-like illness. The national rate of flu is calculated from this.
"Early on [in a pandemic], we want to identify every case and contact-trace every case," Dr Forde said. "But in the middle of it, when lots of people are sick, you are probably looking at monitoring workplace and school absenteeism and maybe computer-assisted telephone interviews."
The idea of the latter was that a computer would phone people at home to ask about flu cases and record details. A website could be set up for employers to record absenteeism.