Targeted vaccinations could be effective intervention against deliberate attack.(smallpox )

2002 DEC 11 - (NewsRx.com & NewsRx.net) -- Targeted vaccination of the close contacts of infected individuals following a smallpox outbreak could rival the effectiveness of mass vaccination, given a sufficiently high level of immunity within the population, according to a new study by biostatisticians at Emory University.

Since targeted vaccinations would lead to fewer adverse vaccine reactions and would prevent more cases of smallpox per dose of vaccine, a targeted strategy would be desirable given an equally effective outcome, said the investigators. The existing immunity could be a combination of residual immunity from smallpox vaccinations received prior to 1972, new vaccinations of first responders, and voluntary vaccinations of other individuals, they suggested.

M. Elizabeth Halloran, MD, DSc, and Ira M. Longini Jr., PhD, professors of biostatistics at Emory University's Rollins School of Public Health, along with colleagues Azhar Nizam and Yang Yang, constructed a model that simulated the spread of smallpox deliberately introduced by infected individuals moving through a community. The research is published in the November 15, 2002, issue of Science.

The model is based on the investigators' previous experience with modeling flu epidemics. It assumes that people interact primarily within known contact groups, including their own household, schools or daycare centers, their neighborhood and their community. The model differs from other recent models of smallpox epidemics that assume random mixing of individuals within a large, homogeneous population, with the conclusion that mass vaccination is the only way to sufficiently control an outbreak.

The Emory model simulated a range of scenarios for smallpox epidemics, based on different probabilities of how smallpox might spread, whether or not there was residual immunity from prior vaccinations, at what stage individuals might stay home during different stages of their disease, and the effectiveness of smallpox vaccines in different age groups.

The investigators conducted 200 simulations for each proposed intervention within their model, using a statistical community of 2000 people based on the age distribution and approximate household sizes reported in the U.S. Census 2000. Each simulated community consisted of four neighborhoods, one high school, one middle school, two elementary schools, small play groups, and day care centers. Households had between 1 and 7 people, and 33% of households consisted of single adults. The model assumed that in a deliberate smallpox attack, either 1 or 5 individuals already infected with smallpox would circulate through the community at the beginning of the infectious stage of their disease.

Halloran and Longini believe their model is more realistic than "deterministic" models that assume all individuals within a large, homogenous population interact randomly with others, concluding that an epidemic always follows the same course. The Emory study considered many different variables, including where transmission was likely to be highest (first within households, then schools, then neighborhoods, and lastly the community at large) and which individuals were likely to have residual immunity.