RG4→5 = 0.36, and RG5→6 = 0.35; none of the 35 simulated epidemics in the 6,000-person ring town scenario 3 lasted longer than six generations after the attack generation.

Clinical disease expression and transmission. In accordance with the values proposed by the working group expert advisors, cases were assigned one of three different clinical disease expressions: ordinary smallpox, modified smallpox, or hemorrhagic smallpox. Among all of the smallpox cases that occurred in these 35 simulated epidemic runs, 57 percent were ordinary, 40 percent were modified, and 3 percent were hemorrhagic cases. However, these proportions varied according to the epidemic generation (Figure 3-11A). In G0, the proportions were 76 percent ordinary, 20 percent modified, and 5 percent hemorrhagic. In G1 and subsequent generations, the proportions were 46 percent ordinary, 51 percent modified, and 3 percent hemorrhagic. These changes in clinical disease expression by epidemic generation reflect the effects of post-exposure vaccination of contacts, which rendered a number of cases to become modified that would otherwise have been ordinary cases (Fenner et al., 1988). We also calculated the epidemic reproductive rate for each clinical disease type for each epidemic generation. The initial disease-type epidemic reproductive rates, or RG(ordinary)0→1, RG(modified)0→1, and RG(hemorrhagic)0→1, were respectively measured to be 1.70, 1.57, and 6.19. The overall initial epidemic reproductive rate RG(all cases)0→1 value of 1.88 is a composite of these values. Subsequent epidemic generational values of R for ordinary, modified, and hemorrhagic smallpox were also calculated and are displayed in Figure 3-11B. After G0, epidemic generational values of RG for modified smallpox were consistently higher than for other types of smallpox at the same generation of the epidemic.

Sensitivity of results to day of withdrawal. We examined the sensitivity of our results to a number of our model assumptions. Most notably, we found that our results were very sensitive to the assumption of the period of time that infected individuals who did not go to the hospital or withdraw to their home on the second day of fever would circulate in the community. These individuals were a small proportion of cases that were not ill enough to withdraw from the community until later in their course of illness. The base model assumes that all individuals will have withdrawn from the community (to the hospital) on day 4 of fever. In our sensitivity analysis, we varied this day from the day fever begins to day 5 of fever. This model change is roughly equivalent to varying the proportion of transmission that occurs before and after symptoms begin, a factor that other investigators have suggested is very important for the controllability of an infectious disease (Fraser et al., 2004). The number of cases that resulted in our model of 6,000 persons under scenario 2 varied dramatically as a function of this parameter from a mean of 2,981 cases in our base model assumptions to a mean of 174 cases if these individuals withdrew from the community on the day their fever began.

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