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Targeted smallpox vaccination is best, but data still lacking, researcher writes

Targeted vaccination of people potentially exposed to smallpox during a bioterrorist episode, together with an appropriately low level of vaccination before an attack, provides the best protection against smallpox, according to an analysis by U-M Prof. Jim Koopman in the Nov. 15 issue of the journal Science.

But questions remain because of the lack of relevant data and because there is scientific modeling work left to be done, Koopman, professor of epidemiology at the School of Public Health, writes in a Perspectives article in the journal.

In the absence of ongoing cases of smallpox to study, Koopman writes that scientists must use the best possible mathematical models to hypothesize what might happen if bioterrorists were to unleash the highly contagious disease, which the CIA suspects is held by four unauthorized countries including Iraq and North Korea.

Assessing the pros and cons of mass vaccinationsthe administering of the vaccine to anyone who comes in to get itversus targeted vaccinations by health department personnel going to those who must be vaccinated in order to stop spread, Koopman evaluates the infection transmission models used in two recent studies of how smallpox might move from person to person.

A continuous population model treats people as being blended together; if any part of the population is infected, transmission to the susceptible part of the population will occur. A discrete individual model instead looks at who is infected, and how and to whom they might pass along the disease.

Koopman favors the discrete individual approach because its computations take into account chance events that often end chains of disease transmission. Because most chains of smallpox die out locally, continuous population models don't reflect reality; they assume that as long as infectious and susceptible people co-exist, they will come into contact and the disease will continue to spread.

Regardless of which approach they use, scientists build their models on limited data, Koopman writes. He suggests crafting smallpox models by analogy to influenza outbreaks, and collecting more data to support more realistic modeling. Specifically he suggests analyzing influenza virus DNA from infected individuals with known exposures to understand how influenza flows through a population.

The debate is not simply one of logistics and where scarce resources should be deployed first. Estimates from the 1960s, when the smallpox vaccine was last mass administered, show that about 15 out of every million people will experience life-threatening side effects, and one or two are likely to die from the effects of the vaccine. Meanwhile, those who have been immunized recently carry a live virus called vaccinia and can infect vulnerable unvaccinated people around them.


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