Compared to the targeted approach, mass vaccination before any smallpox introduction or immediately thereafter was more effective at preventing and containing epidemics--when no prior immunity existed. (The researchers assumed that any leftover immunity would be half as protective as immunity from a fresh vaccination.)
With such leftover immunity, however, the effectiveness of both types of vaccination efforts after smallpox introduction increased, the authors report. Targeted vaccination became the more effective approach if vaccination was delayed until multiple cases had been detected.
Under all the scenarios, targeted vaccination prevented more cases per dose of vaccine.
"Our findings suggest that increasing the level of immunity in the population, possibly by vaccinating first responders or allowing people to be vaccinated if they so choose, would make a post-attack effort more effective," Halloran said. "Also, at increased levels of immunity in the population, targeting close contacts of smallpox cases may become competitive with mass vaccination as the strategy of choice"
"In this model people interact in known contact groups such as households, schools, and neighborhoods. In addition, transmission occurs more probably in closer contact, such as in households than in neighborhoods," said co-author Ira Longini of the Rollins School of Public Health at Emory University. "This aspect captures the general belief that smallpox is spread more effectively by close contact and allows the targeted strategy to have a better chance of success."
Historically, smallpox infection rates are higher in children than in adults, according to Halloran. Thus, the model includes more opportunities for children to spread the virus.
If 80 percent of the entire population were vaccinated (mass vaccination) once the 15th smallpox case o
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Contact: lonaga@aaas.org
lonaga@aaas.org
202-326-7088
American Association for the Advancement of Science
14-Nov-2002