Simulation Examines Effects Of Smallpox Attack

A powerful computer simulation graphically illustrating the effects of a bioterrorism outbreak of smallpox in a hypothetical American county has been released by experts at the Brookings Institution-Johns Hopkins Center on Social and Economic Dynamics and the Johns Hopkins Bloomberg School of Public Health.

Based on the computer model, they developed an alternative strategy for vaccinating against smallpox compared to the strategy recently announced by President Bush.

Dr. Joshua M. Epstein, a senior fellow at the Brookings Institution, and Dr. Donald S. Burke, a professor of international health and epidemiology at the Johns Hopkins Bloomberg School of Public Health, will demonstrate and explain the project, and answer questions, at a briefing Monday, Dec. 23 at 10:00 a.m. at the Brookings Institution. A version of the computer simulation can be viewed on the Brookings website, at www.brookings.edu. By running the simulated smallpox outbreak computer model multiple times with varying assumptions, the Brookings/Johns Hopkins team developed a strategy for containing the disease while minimizing adverse side effects of vaccinations. They exploit the scientific fact that epidemics are nonlinear threshold phenomena and can be made to simply die out under certain preventive vaccination strategies.

“If there is a confirmed bioterror release of smallpox, the U.S. government must provide vaccine. Politically, there is no alternative,” the report states. “This is the easy part of the policy problem. The deeper and politically tougher question is what to do before any release to contain the epidemic and ease the burden of further vaccination if necessary (and the attendant risks of indiscriminate immunizations).”

In the county-level model, the following mix of preemptive (pre- attack) and reactive (post-attack) policy measures achieves those goals:

Preemptive:
— Vaccinating all hospital workers

— Providing revaccinations on a voluntary basis to all healthy persons previously vaccinated against smallpox (before routine vaccination was halted in the 1970s).

Reactive:
— Isolating all confirmed smallpox cases in hospitals.

— Tracing and vaccinating all persons living in the same household as smallpox cases.

“In our model, this package of measures offers the public an excellent chance that a bioterror smallpox attack will be quenched and limited in its severity and sharply reduces the logistical burden and public health risk of further vaccination,” the report states. The authors do not assess the risk of a bioterror attack but say that “given a credible bioterrorist threat, this combination of measures can serve as the basis for a smallpox containment strategy.”

With this combination of policy measures implemented, the model was run 100 times (with different random effects each time). Under this policy package, 100 percent of the runs resulted in fewer than 70 cases (21 deaths); 75 percent of the runs saw fewer than 45 cases (14 deaths); and fully half (50 percent) of the runs yielded fewer than 35 cases (11 deaths). This reflects a high level of containment, compared to the runs without any vaccinations, isolation, or other intervention.

In these “no intervention” cases, all 800 people in the simulated county would become infected with smallpox, and roughly 240 would die. (Smallpox has a case fatality rate of roughly 30 percent). The authors explicitly note a variety of model uncertainties, practical considerations, and important extensions of the research.

The Brookings/Johns Hopkins experts have demonstrated their computer simulations and presented their conclusions to members of the Council of Economic Advisers, National Institutes of Health, National Academy of Sciences, and other government officials, as well as the New York Academy of Medicine, and the American Society for Microbiology.

The computer simulation model was calibrated with data based on 49 outbreaks of smallpox in Europe from 1950 to 1971. The simulation creates an artificial county containing two towns of 400 people each. Each town contains 100 households, made up of two working adults and two school-aged children. Each town has one school and one workplace. Ten percent of the adults commute from one town to the other for work. There is one common hospital and one morgue in the county.

The Brookings/Johns Hopkins smallpox computer simulation is different from usual epidemic models which assume that every infective has contact with every potential victim.

Instead, the Brookings/Johns Hopkins model depicts each of the 800 county residents as an individual dot on the screen and explicitly depicts the differing social interactions and contacts of each adult and child in school, the workplace, and in a hospital. Individuals in the model are also colored to represent their disease state in the progression of smallpox.

The project was carried out with the support of the Sloan Foundation and the Walker Foundation.