Effectiveness assessment of countermeasures against bioterrorist smallpox attacks in Japan using an individual-based model

Abstract

Objectives: This study is aimed at the evaluation of countermeasures in terms of the total number of patients and deaths, the number of vaccine doses used, and the time for eradication as examined through simulations of the outbreak of smallpox following a bioterrorist attack in the center of Okayama City. Three countermeasures were considered: traced vaccination (TV), mass vaccination (MV), and school closure.

Methods: An individual-based model was adopted, in which every individual is assigned their own personal information, behavioral pattern, and interactions among social settings to simulate the situation on a realistic basis. The influence of residual immunity obtained by past vaccination reflected age-dependent immunity to smallpox in Japan.

Results: MV performed within a 2-day period at the same time in 10 school districts had the highest effectiveness in reducing the total number of patients and deaths among all simulations. Performing both TV and MV simultaneously was much more effective than performing TV or MV individually. The decrease in the number of patients with TV or MV in combination with school closure was faster than that by TV or MV without school closure.

Conclusions: According to the simulations results, it was advisable to carry out MV, or both TV and MV, simultaneously with school closure as countermeasures against a smallpox epidemic initiated by a bioterrorist attack.

Fig. 1

Model scheme for transmission dynamics of smallpox

Fig. 2

Age distribution of Okayama City in 2006 [15] a male, b female

Fig. 3

Progression of the number of smallpox patients in scenario 1. The black line shows the mean number of patients in 300 trials. The dark gray and light gray zones show the interquartile range and the whole range, respectively

Fig. 4

Progression of the number of smallpox patients in scenarios 2–4. a–c The transmission in scenarios 2, 3, and 4, respectively. The black line shows the mean number of patients in 300 trials. The gray zone shows the 95% confidence interval

Fig. 5

Progression of the number of smallpox patients in scenarios 5–7. Solid, broken, and dashed lines show the mean number of patients in 300 trials in scenarios 5, 6, and 7, respectively

Fig. 6

Progression of the number of smallpox patients in scenarios 8–10. Solid, broken, and dashed lines show the mean number of patients in 300 trials in scenarios 8, 9, and 10, respectively

Fig. 7

Progression of the number of smallpox patients in scenarios 4, 7, and 11. Solid, broken, and dashed lines show the mean number of patients in 300 trials in scenarios 4, 7, and 11, respectively

Fig. 8

Progression of the number of smallpox patients in scenarios 1 and 12. Solid and broken lines show the mean number of patients in 300 trials in scenarios 1 and 12, respectively

Fig. 9

Progression of the number of smallpox patients in scenarios 3 and 13. Solid and broken lines show the mean number of patients in 300 trials in scenarios 3 and 13, respectively

Fig. 10

Progression of the number of smallpox patients in scenarios 8 and 14. Solid and broken lines show the mean number of patients in 300 trials in scenarios 8 and 14, respectively