From Stuff :
" Close to a third of New Zealanders were immune to swine flu by March this year, according to a major new scientific study.
The study commissioned by the Ministry of Health and conducted by Environmental Science and Research (ESR) estimated that of the 1.26 million people who were immune, about 480,000 people had some immunity prior to last year's pandemic wave, mostly in the older age groups.
An additional 780,000 were estimated to have been infected last year. The highest rate of infection during 2009 was in school-age children, who were infected at the rate of one in three, a finding which surprised researchers. Almost half of those infected showed no obvious symptoms.
New Zealand’s MOH"