One of the scientists who found that 29.5 percent of the nation's population has some immunity to H1N1 swine flu says the new research helps explain why relatively few older people were infected.
"The elderly population of New Zealand was largely protected from the serious effects of last year's pandemic because of good fortune - previous influenza viruses, particularly from before 1957, gave them some protection," said Otago University Associate Professor Michael Baker, of Wellington.
About 12 percent of the population - 480,000 people - had some immunity before the swine flu epidemic even began in March last year: a lot of older people had apparently been exposed to a similar virus earlier in their lives and their immune systems "remembered" that.
But Prof Baker said the study also showed that younger people with certain health conditions, including pregnancy, were particularly vulnerable to serious illness and death from the virus and should consider getting a flu "jab".
Specific population groups such as children and Pacific Island and Maori people should also be targeted for vaccination.
An estimated 18 percent of the population caught the virus, with the highest rate of infection during 2009 in school age children -- where one in three children were affected. Virologists found that infection among school children was about 10 times higher than expected.
Almost half of the people infected showed no obvious symptoms.
Auckland University medical school head Professor John Fraser said this was one of the extraordinary features about the 2009 H1N1 influenza strain highlighted by the Environmental Science and Research study, which was commissioned by the Health Ministry.
It was "most extraordinary" that 45.2 percent of the positive tests were from people with no symptoms.
This was likely to be the main reason for the high infection rate, because people were transmitting the virus without knowing that they were infected.
Also unusual were the extremely high rate of infection in just a few months during the winter of 2009, and the fact that there were far fewer deaths than expected.
Prof Fraser said the key unanswered question remained to what degree the presence of antibodies in a person's blood sample necessarily indicated they had some protection from the virus.
Nearly half of all New Zealand children aged five-19 - the group most vulnerable to infection - now have antibodies to the 2009 H1N1 strain after only a single flu season.
"This substantial 'herd immunity' should slow the progress of future waves -- but only if those antibodies are protective," Professor Fraser said.
"People should not be complacent," he said, noting the other 50 percent of children remained at risk.
People should have a vaccination because that was the only way to ensure the community is well protected.
The "nightmare scenario" would be the return of a mutant strain with a similar transmission rate.
