Pandemic (H1N1) 2009 Influenza Community Transmission Was Established in One Australian State When the Virus Was First Identified in North America

Abstract

BackgroundIn mid-June 2009 the State of Victoria in Australia appeared to have the highest notification rate of pandemic (H1N1) 2009 influenza in the world. We hypothesise that this was because community transmission of pandemic influenza was already well established in Victoria at the time testing for the novel virus commenced. In contrast, this was not true for the pandemic in other parts of Australia, including Western Australia (WA).MethodsWe used data from detailed case follow-up of patients with confirmed infection in Victoria and WA to demonstrate the difference in the pandemic curve in two Australian states on opposite sides of the continent. We modelled the pandemic in both states, using a susceptible-infected-removed model with Bayesian inference accounting for imported cases.ResultsEpidemic transmission occurred earlier in Victoria and later in WA. Only 5% of the first 100 Victorian cases were not locally acquired and three of these were brothers in one family. By contrast, 53% of the first 102 cases in WA were associated with importation from Victoria. Using plausible model input data, estimation of the effective reproductive number for the Victorian epidemic required us to invoke an earlier date for commencement of transmission to explain the observed data. This was not required in modelling the epidemic in WA.ConclusionStrong circumstantial evidence, supported by modelling, suggests community transmission of pandemic influenza was well established in Victoria, but not in WA, at the time testing for the novel virus commenced in Australia. The virus is likely to have entered Victoria and already become established around the time it was first identified in the US and Mexico.

Pandemic Influenza H1N1

“…34 Finally, differences in infectivity or susceptibility between population groups can lead to differences in estimates of the reproduction number if the disease is largely confined to one group. 35 For example, estimates of the reproduction number from early Japanese data (with 84% child cases) 16 were considerably higher than estimates over an extended period of time with 67% child cases, 10 suggesting a higher reproduction number in children.…”

“…Large numbers of imported cases can distort estimates of the reproduction number, 19,30,31 especially when infected travelers are not typical of community-acquired cases (for example, the age structure of imported vs. local cases 32 ). Changes in reporting rates can lead to inaccuracies in estimates of the reproduction number, 10,14,33 and when the reproduction number is close to one, the stochastic nature of the early stages of an outbreak can cause it to be overestimated. 34 Finally, differences in infectivity or susceptibility between population groups can lead to differences in estimates of the reproduction number if the disease is largely confined to one group.…”

Clinical and epidemiological profile of patients with severe H1N1/09 pandemic influenza in Australia and New Zealand: an observational cohort study

“…This has implications for surveillance systems and for infection-control measures in hospitalised patients. This study does not encompass the full duration of the epidemic which was waning in several states (notably Victoria and New South Wales) at the commencement of the study period 9 17 18. Although several hospitals provided maternity and paediatric services, these patient groups are likely to be under-represented in this series.…”

Comparison of the pandemic H1N1 2009 experience in the Southern Hemisphere with pandemic expectations

“…[11][12][13][14][15][16][17] We compared surveillance results from our data analysis and the published data on morbidity, mortality and R with the expectations from previous pandemics as described above. …”

“…13 The mean effective reproduction number (R) during the peak of transmission was estimated for pH1N1 in 2009 WA as 1.2-1.4 13 and for NZ as 1.2-1.5, [14][15][16] although earlier estimates of a higher R had also been reported in NZ. 16,17 In NZ and WA the estimated effective reproduction number was initially around 1.6-2.0, but rapidly declined to 1.2-1.4.…”