RESEARCH The rates of hospitalization and death were deﬁned relative to the CAR by using a case-hospitalization rate and case-fatality rate. We used age-speciﬁc case-hospitaliza- tion rates (0–19 years = 1.875%, 20–64 years = 2.5%, >65 years = 5%) and case-fatality rates (0–19 years = 0.75%, 20–64 years = 1%, >65 years = 2%). There are no reliable estimates of when a future pandemic might occur. We used a base-case delay to a pandemic of 5 years. Cost Estimates As recommended by Australian pharmaceutical fund- ing guidelines, we focused on direct healthcare costs (28) and performed our base-case analysis from a healthcare system perspective. In scenario analysis we considered a broader societal perspective, which included lost produc- tion costs. Costs and effects were discounted at 5% annu- ally (28). All costs are reported in 2005 Australian dollars. Intervention Costs In the base-case model, we assumed a stockpile pur- chase price for pharmaceuticals of $12 per vaccine dose and $32 per antiviral course; a range of values was considered in sensitivity analysis. The limited shelf-life of the pharma- ceuticals requires the renewal of stockpiles for prepandemic vaccine every 3 years and antiviral drugs every 5 years. The number of times stockpiles were replaced was based on the expected time to a pandemic. We assumed partial replace- ment of the stockpile annually on a continuous basis. An annual storage cost for vaccines ($1, refrigeration) and anti- viral drugs ($0.5, no refrigeration) was included. We assumed that vaccination (and initial antiviral drug distribution) would be administered in mass clinics at a cost of $11.60 per course/dose (29). An administration cost for antiviral treatment was not included because this treatment would be given as part of a primary care visit for inﬂuenza illness. However, for strategies that included antiviral treat- ment, the percentage of clinical cases seeking medical care was increased to 80%. Healthcare Unit Costs Hospitalization costs were based on analysis previ- ously conducted by our group, which reviewed records of patients hospitalized for inﬂuenza and pneumonia in Australia (30). We estimated age-speciﬁc hospitalization costs by multiplying the average cost per day by the aver- age length of stay for that age group (31). Expenses for emergency department visits for inﬂuenza not requiring hospitalization were estimated by the Australian Ambula- tory Classes emergency department presentation cost for “Other respiratory diseases with procedure” (32). The cost of a general practitioner visit for inﬂuenza ($33.32) was based on a general practitioner survey of consultation for inﬂuenza-like illness (30). Production Costs (Societal Perspective Only) The costs of lost production were valued by using the human capital approach. Lost production was only valued for those employed in paid work (33). The cost attached to lost work days was based on average weekly earnings (34). Clinical inﬂuenza patients were assumed to have 2.6 days absent from work (35). We assumed that those <15 years of age would require 1 adult caregiver when sick. We used length of stay to estimate lost production for hospitalized patients. Results Clinical Outcomes The base-case analysis used an R value of 1.7, which 0 led to a CAR of 31.1% in the overall population in the ab- sence of any intervention. The assumption of greater mix- ing in children meant that this group experienced the high- est CARs, with 38.1% in persons 0–19 years of age, 30.4% in persons 20–64 years of age, and 20.4% in persons >65 years of age. In the absence of any intervention, the base- case model produced an overall population hospitaliza- tion rate of 782.3/100,000 persons and a mortality rate of 312.9/100,000. Strategies incorporating a population pre- pandemic vaccination program resulted in a low CAR and, consequently, a low number of hospitalizations (strategy 3 = 136.8/100,000; strategy 4 = 79.4/100,000) and deaths (strategy 3 = 54.7/100,000; strategy 4 = 31.8/100,000). The antiviral drug treatment strategy did not affect the CAR but signiﬁcantly reduced the number of hospitaliza- tions (strategy 2 = 450.0/100,000) and deaths (strategy 2 = 180.0/100,000). Several parameters were inﬂuential in determining the CARs (Figure 2). In prepandemic vaccination strategies, R0 (Figure 2, panel A), VE (Figure 2, panel A), and vaccine coverage (Figure 2, panel B) played major roles in deter- mining whether a large outbreak was prevented or simply mitigated. The CAR rose as R0 increased and declined as VE improved and coverage increased, with sharper tran- sitions occurring as the number of secondary cases that a single case infects approached 1. The CAR for prepan- demic vaccination strategies also increased markedly when vaccination was delayed until after a local outbreak had commenced (Figure 2, panel C). The number of deaths pre- vented by antiviral treatment rises as R0 increases (Figure 2, panel D). This increase occurs because the incidence of preventable disease is larger for higher values of R0. The effect on prepandemic vaccination strategies is similar, provided the strategy is largely successful in containing the pandemic. However, for R0 values >1.7, when this is no longer the case, prepandemic vaccinations strategies pre- vented fewer deaths (Figure 2, panel D). 226 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 16, No. 2, February 2010