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A lower than expected adult Victorian community attack rate for pandemic (H1N1) 2009

Authors

Errata

This article is corrected by:

  1. Errata: Errata Volume 34, Issue 4, 426, Article first published online: 3 August 2010

Correspondence to:
Dr John Carnie, Chief Health Officer, Department of Health, Victoria, 50 Lonsdale Street Melbourne Victoria 3000 Australia fax: 03 9096 9176; e-mail: john.carnie@health.vic.gov.au

Abstract

Objectives: To determine the community seropositivity of pandemic (H1N1) 2009 influenza in order to estimate immunity and the community attack rate.

Methods: Selected clusters of participants (n=706) in the ‘Victorian Health Monitor’ (VHM), from whom blood samples were taken between August and October 2009, were tested opportunistically for antibodies to pandemic (H1N1) 2009 influenza virus. A titre of ≥1:40 was chosen as the cut-off for recording seropositivity. The proportion (95% CI) of seropositive participants, aged 18 to <65 years of age, were computed for groups of census collection districts (CDs) across metropolitan Melbourne.

Results: The observed pandemic (H1N1) 2009 seropositivity rates for all CDs tested in metropolitan Melbourne was 16.0% (95% CI:12.9-19.1%); in northern Melbourne subset was 14.4% (95% CI:12.4-16.3%); and in eastern subset was 16.2% (95% CI:9.7-22.6%). The pre-pandemic (H1N1) 2009 positivity rate was estimated at 6%.

Conclusion: Given this study's estimate of 16.0% seropositivity in adults in metropolitan Melbourne, and given the WHO laboratory's estimate of 6% pre-pandemic positivity, the estimated adult community attack rate was 10% for metropolitan Melbourne.

Implications: This community attack rate is lower than anticipated and suggests that levels of immunity to Pandemic (H1N1) 2009 might be lower than anticipated. Although limited by a low response rate of 34%, this study suggests low adult seropositivity, which may be useful for public health professionals when encouraging the community to get vaccinated.

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