# 4622
Last November it was announced that New Zealand would conduct a major seroprevalence study of their population to see how many people had pre-existing immunity to the novel H1N1 virus, how many were infected over the winter, and how many remained susceptible to the virus.
For details on that announcement see New Zealand To Conduct Seroprevalence Studies.
Today we’ve got the results of this study, as summarized in this Ministry of Health (MOH) release (hat tip Sally Furniss on FluTrackers).
As you read it, remember the total population of New Zealand is about 4.2 million people. I’ll return with a little more information after the report.
Media Release
4 June 2010
Pandemic Influenza H1N1 2009 (swine flu) – Update 188
Study shows full effect of Pandemic Influenza H1N1 in New Zealand in 2009
A major new scientific study, estimates that 1.26 million or 29% of New Zealanders were immune to the pandemic influenza virus by March this year. But a large proportion of the population remain vulnerable.
Since the testing was completed many New Zealanders have also been immunised against pandemic influenza, however, given that there is still a large proportion of the population at risk from infection this winter, both influenza immunisation and basic measures to stop the spread of influenza remain important.
The Ministry commissioned study, conducted by ESR, estimates that of the 1.26 million people, 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 during 2009. The highest rate of infection during 2009 was in school age children - where one in three children were affected. Almost half of those infected showed no obvious symptoms.
ESR’s national influenza centre head, Dr Sue Huang, said the rate of pandemic influenza A H1N1 infection during 2009 varied greatly by age – with the most surprising finding of the study being how many young people were affected.
“The high rate of transmission and infection among school-aged children was about ten times higher than we initially estimated based on clinical surveillance.
“This study confirms that the Pandemic Influenza is able to spread rapidly through communities. One of the more interesting findings was that it was a sub-clinical illness (i.e. no symptoms) for almost half of those who were affected. For most people their symptoms were mild, however, for many it was a very serious illness requiring an admission to intensive care,” Dr Huang said.
The complete report (PDF File) is available on the New Zealand MOH website.
Seroprevalence of the 2009 influenza A (H1N1) pandemic in New Zealand
The study reflects the results of nearly 1700 blood serum samples taken between November 2009 and March 2010, along with another 521 sera collected pre-pandemic that were subjected to hemagglutination inhibition (HI) tests for antibodies to the 2009 H1N1 virus.
A titer of 40 or higher was assumed to be seroprotective, and indicative of exposure to the virus or prior immunity.
Some selected excerpts from the study include:
- The overall community seroprevalence was 26.7% (CI:22.6-29.4) for the study population.
- The seroprevalence varied with age and ethnicity. Children aged 5-19 years had the highest seroprevalence 46.7% (CI: 38.3-55.0), a significant increase from the baseline 14.0% (CI:7.2-20.8).
- This is followed by pre-school children aged 1-4 years at 29.5% (CI:21.2-38.0) an increase of 23.5% from a baseline of 6.0% (CI:0.9-11.0).
- Age group 60+ had no significant difference in seroprevalence between the serosurvey, 24.8% (CI:18.7-30.9), and baseline, 22.6% (CI:15.3-30.0).
- Pacific Peoples had the highest seroprevalence 49.5% (CI: 35.1-64.0) followed by Maori at 36.3% (CI: 28.0-44.6).
- Based on the questionnaire survey approximately 45.2% of seropositive individuals had no symptoms giving an indication of a relatively silent spread of the disease in a naive population.
This study confirms the relatively low impact that the virus had on those over the age of 60, the pandemic’s heavy burden upon children and Pacific peoples, and provides us with a surprisingly high (45.2%) rate of asymptomatic infections.
Despite the high number of those infected, nearly 70% of New Zealand’s population appears still vulnerable to the virus.
I’ve only glossed over the highlights of this report. There is much more to be gleaned here.
Follow this link to read it in its entirety.
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