Distinguishing intrinsic from extrinsic factors underlying firing rate saturation in human motor units.

During voluntary contraction, firing rates of individual motor units (MUs) increase modestly over a narrow force range beyond which little additional increase in firing rate is seen. Such saturation of MU discharge may be a consequence of extrinsic factors that limit net synaptic excitation acting on motor neurons (MNs) or may be due to intrinsic properties of the MNs. Two sets of experiments involving recording of human biceps brachii MUs were carried out to evaluate saturation. In the first set, the extent of saturation was quantified for 136 low-threshold MUs during isometric ramp contractions. Firing rate-force data were best fit by a saturating function for 90% of MUs recorded with a maximum rate of 14.8 ± 2.0 impulses/s. In the second set of experiments, to distinguish extrinsic from intrinsic factors underlying saturation, we artificially augmented descending excitatory drive to biceps MNs by activation of muscle spindle afferents through tendon vibration. We examined the change in firing rate caused by tendon vibration in 96 MUs that were voluntarily activated at rates below and at saturation. Vibration had little effect on the discharge of MUs that were firing at saturation frequencies but strongly increased firing rates of the same units when active at lower frequencies. These results indicate that saturation is likely caused by intrinsic mechanisms that prevent further increases in firing rate in the presence of increasing synaptic excitation. Possible intrinsic cellular mechanisms that limit firing rates of motor units during voluntary effort are discussed.

The effects of physiological adjustments on the perceptual and acoustical characteristics of simulated laryngeal vocal tremor.

The purpose of this study was to determine if adjustments to the voice source [i.e., fundamental frequency (F0), degree of vocal fold adduction] or vocal tract filter (i.e., vocal tract shape for vowels) reduce the perception of simulated laryngeal vocal tremor and to determine if listener perception could be explained by characteristics of the acoustical modulations. This research was carried out using a computational model of speech production that allowed for precise control and manipulation of the glottal and vocal tract configurations. Forty-two healthy adults participated in a perceptual study involving pair-comparisons of the magnitude of "shakiness" with simulated samples of laryngeal vocal tremor. Results revealed that listeners perceived a higher magnitude of voice modulation when simulated samples had a higher mean F0, greater degree of vocal fold adduction, and vocal tract shape for /i/ vs /ɑ/. However, the effect of F0 was significant only when glottal noise was not present in the acoustic signal. Acoustical analyses were performed with the simulated samples to determine the features that affected listeners' judgments. Based on regression analyses, listeners' judgments were predicted to some extent by modulation information present in both low and high frequency bands.

Beyond injecting drug use: investigation of a Victorian cluster of hepatitis C among HIV-infected men who have sex with men.

OBJECTIVES: To examine increased notifications of hepatitis C virus (HCV) in men who have sex with men (MSM) infected with HIV in Victoria, and evaluate HCV transmission risk factors other than injecting drug use. DESIGN, SETTING AND PARTICIPANTS: Case series through retrospective review of all HCV cases in Victoria from 1 April 2010 to 30 June 2011, with clinical and laboratory data examined in likely MSM to identify a co-infected cohort. Patients with newly acquired HCV with HIV co-infection were invited to complete a questionnaire exploring novel risk factors for HCV transmission (non-injecting drug use, sexual practices with increased likelihood of trauma, and presence of genital ulcers). Sequencing was performed to determine the local molecular epidemiology of HCV co-infection. MAIN OUTCOME MEASURES: Demographics of newly co-infected MSM, traditional versus novel risk factors for HCV acquisition, prior knowledge of potential for sexual transmission of HCV, and association between viral sequences. RESULTS: Thirty-one patients with HIV were identified from 3365 notifications of hepatitis C. The median age was 42 years, and median time from HIV to HCV diagnosis was 22 months. Most patients were asymptomatic, with abnormal liver function tests prompting HCV testing. Interviews with 14 patients identified a high prevalence of novel risk factors and limited knowledge of HCV risk. Two clusters of matching viral sequences were identified. CONCLUSIONS: Novel HCV transmission routes have emerged in Victoria. These data reinforce the need for targeted testing and prevention strategies among HIV-infected MSM.

Intrahousehold transmission of pandemic (H1N1) 2009 virus, Victoria, Australia.

To examine intrahousehold secondary transmission of pandemic (H1N1) 2009 virus in households in Victoria, Australia, we conducted a retrospective cross-sectional study in late 2009. We randomly selected case-patients reported during May-June 2009 and their household contacts. Information collected included household characteristics, use of prevention and control measures, and signs and symptoms. Secondary cases were defined as influenza-like illness in household contacts within the specified period. Secondary transmission was identified for 18 of 122 susceptible household contacts. To identify independent predictors of secondary transmission, we developed a model. Risk factors were concurrent quarantine with the household index case-patient, and a protective factor was antiviral prophylaxis. These findings show that timely provision of antiviral prophylaxis to household contacts, particularly when household members are concurrently quarantined during implementation of pandemic management strategies, delays or contains community transmission of pandemic (H1N1) 2009 virus.

Public health response to imported case of poliomyelitis, Australia, 2007.

Australia, along with 36 other countries in the Western Pacific Region, was declared free of poliomyelitis by the World Health Organization in October 2000. Yet, the persistence of wild poliovirus in the 4 remaining polio-endemic countries-Afghanistan, India, Nigeria, and Pakistan-poses a risk for its importation into all countries declared polio free. We describe the public health response and outcomes resulting from the importation of a wild poliovirus infection in Melbourne, Australia, in July 2007. This response, based on an assessment of the risk for transmission, included offering vaccination with inactivated polio vaccine to the contacts and placing the index patient in isolation and the household contacts in quarantine until consecutive fecal specimens were negative for poliovirus by culture. The experience gained from the polio importation event in Australia may assist other polio-free countries to prepare for, and respond to, a similar event. No secondary clinical cases resulted from this importation.

Effectiveness of parental cocooning as a vaccination strategy to prevent pertussis infection in infants: A case-control study.

BACKGROUND: During a pertussis epidemic in 2009, the Department of Health, Victoria, Australia, implemented a cocoon program offering parents of new babies a funded-dose of pertussis-containing vaccine. We assessed vaccine effectiveness (VE) of the program in reducing pertussis infection in infants. METHODS: Using a matched case-control design, infants aged <12 months that were notified with pertussis between 1 January 2010 and 31 December 2011, and born during the time that the cocoon program was in place, were identified. Controls were matched by area of residence and date of birth. Telephone interviews we conducted to ascertain parents' vaccination status, and if vaccinated, timing of vaccination receipt relative to the birth of their baby. Odds ratios (ORs) were calculated for the association between vaccination and pertussis infection, with VE calculated as (1 - OR) × 100%. RESULTS: The study recruited 215 cases and 240 controls (response rates 67% and 25% of eligible participants, respectively). Vaccination of both parents after delivery of the infant and ≥28 days prior to illness onset reduced pertussis infection by 77% (Vaccine Effectiveness [VE] = 77% (confidence interval [95% CI], 18-93%). After adjusting for maternal education, presence of a sibling within the household, and the infants' primary course vaccination status, the adjusted VE was 64% (95% CI, -58-92%). CONCLUSIONS: Although not reaching statistical significance, our results demonstrated that cocoon immunisation - where both parents are vaccinated in the post-partum period - may offer some protection again infant pertussis infection. Cocoon immunisation could be considered in circumstances where antenatal vaccination of the mother has not occurred.

Discriminating simulated vocal tremor source using amplitude modulation spectra.

OBJECTIVES/HYPOTHESIS: Sources of vocal tremor are difficult to categorize perceptually and acoustically. This article describes a preliminary attempt to discriminate vocal tremor sources through the use of spectral measures of the amplitude envelope. The hypothesis is that different vocal tremor sources are associated with distinct patterns of acoustic amplitude modulations. STUDY DESIGN: Statistical categorization methods (discriminant function analysis) were used to discriminate signals from simulated vocal tremor with different sources using only acoustic measures derived from the amplitude envelopes. METHODS: Simulations of vocal tremor were created by modulating parameters of a vocal fold model corresponding to oscillations of respiratory driving pressure (respiratory tremor), degree of vocal fold adduction (adductory tremor), and fundamental frequency of vocal fold vibration (F0 tremor). The acoustic measures were based on spectral analyses of the amplitude envelope computed across the entire signal and within select frequency bands. RESULTS: The signals could be categorized (with accuracy well above chance) in terms of the simulated tremor source using only measures of the amplitude envelope spectrum even when multiple sources of tremor were included. CONCLUSIONS: These results supply initial support for an amplitude-envelope-based approach to identify the source of vocal tremor and provide further evidence for the rich information about talker characteristics present in the temporal structure of the amplitude envelope.

Uptake of a government-funded pertussis-containing booster vaccination program for parents of new babies in Victoria, Australia.

INTRODUCTION: An epidemic of Bordetella pertussis in Victoria, Australia, led to the implementation of a Government-funded vaccination program for parents of new babies. The rationale was to protect unimmunised infants from infection by vaccinating parents with a pertussis-containing vaccine. This is known as cocooning. AIM: To estimate uptake of the vaccine among parents of new babies, two-and-a-half years after the program was implemented. METHODS: A state-wide cross-sectional survey of parents was conducted to ascertain vaccine uptake, and to identify where and when the vaccination took place. Surveys were administered between 15 February and 14 March 2012, inclusive. RESULTS: Of 6308 surveys distributed, 2510 completed surveys were returned (response rate 40%). Ninety-five surveys completed outside the study period were excluded, leaving 2415 available for analysis. Overall, 1937 (80%) mothers and 1385 (70%) fathers were vaccinated in relation to the birth of their most recent child. A majority of mothers were vaccinated in hospital (62%). Most fathers were vaccinated by a general practitioner (72%). The most common point at which mothers were vaccinated was before their child turned two weeks of age (65%). Fathers' vaccination time-point varied more widely: during pregnancy (25%); before their child turned two weeks of age (29%); and when their child was between two and eight weeks of age (28%). CONCLUSION: Results of this survey indicated excellent uptake of the vaccine among both mothers and fathers under the Government-funded cocooning program. The findings are suggestive of an effective communications program designed to raise awareness of the risks of pertussis, and to promote availability of the funded vaccination program. The results may contribute to policy implementation of adult immunisation programs such as cocooning.

Immunity to poliomyelitis in Victorians.

OBJECTIVES: In view of WHO's goal for poliomyelitis eradication by 2005 and the possible introduction of IPV into the Australian Standard Vaccination Schedule, this investigation was conducted to assess current immunity to poliomyelitis across the Victorian community. METHODS: 1,775 sera collected from three population samples within the Victorian community between 1990 and 1995 were tested for neutralising antibody titres against each poliovirus type in accordance with WHO recommended procedure. RESULTS: In infants over three months and adults under 40 years, 76-100% of people in each age group were seropositive to all poliovirus types, with 90-100% seropositive to type I, 94-100% seropositive to type II and 80-97% seropositive to type III. Of the very small number of adults over 40 years tested (n = 13), 85% were seropositive to each of types I and II, and 62% to type III. 92% of vaccination histories taken and checked were confirmed, and reported immunisation rates were significantly below seropositive rates. CONCLUSIONS: According to poliovirus antibody seroprevalence, current immunity to poliomyelitis appears sufficient for herd immunity. When compared with vaccination histones, significantly more people demonstrated immunity to poliomyelitis than the number who reported having been vaccinated against it, indicating the possible role of intestinal vaccine strain poliovirus spread in maintaining high immunity levels. IMPLICATIONS: The current Australian Standard Vaccination Schedule has successfully provided protective levels of poliomyelitis immunity for the Victorian community in the absence of wild virus since at least 1972. This must be considered in assessing the future direction of Australia's poliomyelitis immunisation program.

Increases in vaccination coverage for children in child care, 1997 to 2000: an evaluation of the impact of government incentives and initiatives.

OBJECTIVE: To compare vaccination coverage of children in child care before (1997) and after (2000) implementation of government immunisation strategies including parent/providers incentives and surveillance of vaccination uptake. METHODS: Cross-sectional parent surveys of vaccination coverage for children (<3 years old regularly attending child care) in 47 child care centres and 19 councils operating family day care in metropolitan Melboume. RESULTS: Data were collected for 1,578 (72%) children in 1997 and 1,793 (72%) in 2000. In 2000, 93% were completely immunised, a 9% (95% CI 6%-11%, p<0.001) increase from 1997. Less than 1% of children were unimmunised (0.8% in 1997, 0.5% in 2000). For those >2 years, 94% were completely immunised before their second birthday in 2000 compared with 80% in 1997. Immunisation levels were 10% (95% Cl 6-12) higher in 2000 than in 1997 for those receiving child care benefits compared with a 7% (95% Cl 3-10) increase for families not receiving benefits. In 1997, 8 (17%) child care centres and 4 (21%) councils reported > or = 90% children completely immunised increasing to 33 (70%) and 16 (84%) in 2000 respectively. Fewer families reported delaying immunisations because of minor illness in 2000 (27%) compared to 1997 (44%, p<.001). Updating immunisation data by child care coordinators increased from 51% in 1997 to 98% in 2000. CONCLUSION: A substantial increase in immunisation uptake has been achieved for this population of young children attending child care. This study provides evidence that the increase in vaccination rates is attributable to some extent to increased surveillance of immunisation rates and both parent and provider incentives to immunise.

Prevalence of influenza immunisation in Australia and suggestions for future targeting of campaigns.

OBJECTIVES: Reports on the results of a national survey conducted in Australia, in 2000. The objectives were to determine national estimates of influenza vaccination coverage for each state and territory of Australia, to obtain information related to attitudes towards and influences on immunisation decisions and explain the factors involved with failure to immunise. METHOD: The survey was conducted using the Computer Assisted Telephone Interview (CATI) system. The overall participation rate for the survey was 88.6% and the final number of completed interviews across Australia was n = 10,505. RESULTS: Two target groups, those aged 65 years and over and those "at risk" of influenza aged between 40 and 64 years were defined. The overall immunisation rates in these two groups were 74% and 32% resp. The rate of immunisation among females generally exceeded that of males. A multivariate model provided the best joint set of explanatory variables for not getting immunised. These include sex, income, general practitioner recommendation, and general perceptions regarding the influenza injection. CONCLUSION: This study identified important issues in the decisions of people to immunise. It also highlighted the need to target the findings in effective immunisation policies and strategies to improve health outcomes for those at risk of adverse influenza events.

Nasal and oral inspiration during natural speech breathing.

PURPOSE: The purpose of this study was to determine the typical pattern for inspiration during speech breathing in healthy adults, as well as the factors that might influence it. METHOD: Ten healthy adults, 18­45 years of age, performed a variety of speaking tasks while nasal ram pressure, audio, and video recordings were obtained. Inspirations were categorized as nasal-only, oral-only, simultaneous nasal and oral, or alternating nasal and oral inspiration. The method was validated using nasal airflow, oral airflow, audio, and video recordings for 2 participants. RESULTS: The predominant pattern was simultaneous nasal and oral inspirations for all speaking tasks. This pattern was not affected either by the nature of the speaking task or by the phonetic context surrounding the inspiration. The validation procedure confirmed that nearly all inspirations during counting and paragraph reading were simultaneous nasal and oral inspirations, whereas for sentence reading, the predominant pattern was alternating nasal and oral inspirations across the 3 phonetic contexts. CONCLUSIONS: Healthy adults inspire through both the nose and mouth during natural speech breathing. This pattern of inspiration is likely beneficial in reducing pathway resistance while preserving some of the benefits of nasal breathing.

Acoustic characteristics of simulated respiratory-induced vocal tremor.

PURPOSE: The purpose of this study was to investigate the relation of respiratory forced oscillation to the acoustic characteristics of vocal tremor. METHOD: Acoustical analyses were performed to determine the characteristics of the intensity and fundamental frequency (F0) for speech samples obtained by Farinella, Hixon, Hoit, Story, and Jones (2006) using a respiratory forced oscillation paradigm with 5 healthy adult males to simulate vocal tremor involving respiratory pressure modulation. The analyzed conditions were sustained productions of /a/ with amplitudes of applied pressure of 0, 1, 2, and 4 cmH2O and a rate of 5 Hz. RESULTS: Forced oscillation of the respiratory system produced modulation of the intensity and F0 for all participants. Variability was observed between participants and conditions in the change in intensity and F0 per unit of pressure change, as well as in the mean intensity and F0. However, the extent of modulation of intensity and F0 generally increased as the applied pressure increased, as would be expected. CONCLUSION: These findings suggest that individuals develop idiosyncratic adaptations to pressure modulations, which are important to understanding aspects of variability in vocal tremor, and highlight the need to assess all components of the speech mechanism that may be directly or indirectly affected by tremor.

Physiologic and acoustic patterns of essential vocal tremor.

OBJECTIVES/HYPOTHESIS: This article describes a case study of physiologic and acoustic patterns of essential vocal tremor (EVT). Simulations of vocal tremor were used to test hypotheses regarding measured acoustic patterns and expected physiologic sources. STUDY DESIGN: This is a case study of EVT using an analysis by synthesis approach. METHODS: Oscillations of vocal tract and laryngeal structures were identified using rigid videostroboscopic examination. Acoustical analyses of sustained phonation were completed using the methods previously described in the literature and custom-written MATLAB functions. Simulations of the client's vocal tremor were created using a computational model. RESULTS: The client exhibited vocal fold length changes and oscillation within the laryngeal vestibule during sustained phonation at a comfortable pitch and loudness. Despite the involvement of vocal fold length changes, a low average extent of fundamental frequency (F0) modulation (ie, 5.3%) and high average extent of intensity modulation (ie, 23.0%) were measured. Simulations of vocal tremor involving modulation of F0 demonstrated that this source of tremor contributes to frequency-induced intensity modulation, although there was a greater extent of F0 modulation than intensity modulation. CONCLUSIONS: The greater extent of intensity than F0 modulation in one client with EVT exhibiting predominant vocal fold length changes contrasted with the lower extent of intensity than F0 modulation in simulated vocal tremor involving F0 modulation. These findings demonstrate that other potential sources of intensity modulation outside the larynx should be determined during the evaluation of clients with vocal tremor.

The spread of influenza A(H1N1)pdm09 in Victorian school children in 2009: implications for revised pandemic planning.

BACKGROUND: Victoria was the first state in Australia to experience community transmission of influenza A(H1N1)pdm09. We undertook a descriptive epidemiological analysis of the first 1,000 notified cases to describe the epidemic associated with school children and explore implications for school closure and antiviral distribution policy in revised pandemic plans. METHODS: Records of the first 1,000 laboratory-confirmed cases of influenza A(H1N1)pdm09 notified to the Victorian Government Department of Health between 20 May and 5 June 2009 were extracted from the state's notifiable infectious diseases database. Descriptive analyses were conducted on case demographics, symptoms, case treatment, prophylaxis of contacts and distribution of cases in schools. RESULTS: Two-thirds of the first 1,000 cases were school-aged (5-17 years) with cases in 203 schools, particularly along the north and western peripheries of the metropolitan area. Cases in one school accounted for nearly 8% of all cases but the school was not closed until nine days after symptom onset of the first identified case. Amongst all cases, cough (85%) was the most commonly reported symptom followed by fever (68%) although this was significantly higher in primary school children (76%). The risk of hospitalisation was 2%. The median time between illness onset and notification of laboratory confirmation was four days, with only 10% of cases notified within two days of onset and thus eligible for oseltamivir treatment. Nearly 6,000 contacts were followed up for prophylaxis. CONCLUSIONS: With a generally mild clinical course and widespread transmission before its detection, limited and short-term school closures appeared to have minimal impact on influenza A(H1N1)pdm09 transmission. Antiviral treatment could rarely be delivered to cases within 48 hours of symptom onset. These scenarios and lessons learned from them need to be incorporated into revisions of pandemic plans.

Low seroprevalence of Murray Valley encephalitis and Kunjin viruses in an opportunistic serosurvey, Victoria 2011.

OBJECTIVE: To assess evidence of recent and past exposure to Murray Valley encephalitis virus (MVEV) and West Nile clade Kunjin virus (KUNV) in residents of the Murray Valley, Victoria, during a period of demonstrated activity of both viruses in early 2011. METHODS: A cross-sectional serosurvey using two convenience samples: stored serum specimens from a diagnostic laboratory in Mildura and blood donors from the Murray Valley region. Specimens were collected between April and July 2011. The main outcome measure was total antibody (IgM and IgG) reactivity against MVEV and KUNV measured using an enzyme immunoassay and defined as inhibiting binding of monoclonal antibodies by >50%, when compared to negative controls. Evidence of recent exposure was measured by the presence of MVEV and KUNV IgM detected by immunofluorescence. RESULTS: Of 1,115 specimens, 24 (2.2%, 95% CI 1.3-3.0%) were positive for MVEV total antibody, and all were negative for MVEV IgM. Of 1,116 specimens, 34 (3.1%, 95% CI 2.0-4.0%) were positive for KUNV total antibody, and 3 (0.27%) were KUNV IgM positive. Total antibody seroprevalence for both viruses was higher in residents born before 1974. CONCLUSIONS: Despite widespread MVEV and KUNV activity in early 2011, this study found that seroprevalence of antibodies to both viruses was low (<5%) and little evidence of recent exposure. IMPLICATIONS: Our findings suggest both viruses remain epizootic in the region and local residents remain potentially susceptible to future outbreaks.

The clinicopathologic spectrum of benign mass lesions of the vocal fold due to vocal abuse.

Benign masses of the vocal fold related to phonotrauma are clinically classified into polyps, nodules, Reinke's edema, and cysts. Despite the apparent distinctiveness of the clinical nomenclature, low inter- and intraobserver diagnostic agreement has been reported. Excepting cysts, which are epithelial lined, histologic examination of the remaining lesions has shown a variety of overlapping features insufficiently specific for the clinical diagnoses. This study reviews the clinicopathologic characteristics among these benign lesions of the vocal fold. A total of 78 nonneoplastic lesions of the vocal fold were reviewed by 2 pathologists for the presence of epithelial hyperplasia, basement membrane thickening, edema, vascular proliferation, and extracellular "amyloid-like" fibrin. In 46 cases with prebiopsy stroboscopic images, 2 otolaryngologists classified each lesion as polyp, nodule, Reinke's edema, cyst, or other. They agreed in 43% (n = 20, 13 polyps, 5 nodules, 1 Reinke's edema, 1 other) and disagreed in 57% (n = 26). There was no histologic feature that reliably distinguished among the lesions. In addition, reactive stromal cell atypia was present in 14 cases. Cysts were distinctive, as all were epithelial lined. The clinicopathologic classification of benign laryngeal lesions is neither clinically reproducible nor histologically unique. Treatment will continue to be individualized based on clinical judgment.

Evaluation of Campylobacter infection surveillance in Victoria.

Campylobacter infection is a notifiable infectious disease in Victoria and with more than 6,000 cases notified annually, it is the second most commonly notified disease after chlamydia. The objectives of Campylobacter infection surveillance in Victoria are to monitor the epidemiology of Campylobacter infection, identify outbreaks, initiate control and prevention actions, educate the public in disease prevention, evaluate control and prevention measures, and plan services and priority setting. An evaluation of the system was undertaken to assess performance against its objectives, identify areas requiring improvement and inform a decision of whether Campylobacter infection should remain a notifiable infectious disease. The surveillance system was assessed on the attributes of data quality, timeliness, simplicity and acceptability using notifiable infectious diseases data and interviews with doctors who had failed to notify, and laboratory and public health staff. The evaluation found that the system collects core demographic data with high completeness that are appropriately reviewed, analysed and reported. In 2007, 12% of Campylobacter isolates were subtyped and only one to 3 outbreaks were identified annually from 2002 to 2007. Fifty-four per cent of cases were notified by doctors and 96% by laboratories, although nearly half of laboratory notifications were not received within the prescribed timeframe. Half of the surveyed non-notifying doctors thought that Campylobacter infection was not serious enough to warrant notification. The Campylobacter surveillance system is not fully satisfying its objectives. Investment in the further development of analytical methods, electronic notification and Campylobacter subtyping is required to improve simplicity, acceptability, timeliness and sensitivity.

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

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 > or = 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.

Pandemic (H1N1) 2009 influenza community transmission was established in one Australian state when the virus was first identified in North America.

BACKGROUND: In 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). METHODS: We 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. RESULTS: Epidemic 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. CONCLUSION: Strong 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.