8 August 2005

Human Bird Flu Outbreak "Containable"

by Kate Melville

Despite suspicions of human-to-human transmission, the H5N1 strain of the avian flu virus seems, at least so far, to be restricted to birds and swine in Southeast Asia. But fears that a genetic exchange or successful mutation could make highly contagious person-to-person transmission possible has prompted some researchers to crunch the numbers on how a human outbreak might pan out.

Computer models from the Models of Infectious Disease Agent Study (MIDAS) were used to simulate what might happen if human-to-human transmission began in Thailand. The results of the study, which found that a combination of public health measures could stop the spread of an avian flu outbreak at its source, were published in Nature and Science.

"The pressing questions are if and how we can contain an outbreak of avian flu at the source before it becomes a pandemic," said biostatistician and study author Ira M. Longini, Jr., from Emory University. To try and find answers, two computer models were employed, differing in the specific scenarios they simulated and the intervention strategies they tested. The model described in Science simulated 500,000 people living in rural Southeast Asia and used information about how people move about within their communities.

Containment strategies included giving antiviral medication to people in the same social networks, vaccinating before an outbreak with a vaccine that is not well matched to the strain that emerges, quarantining the houses or neighborhoods of infected people, and combinations of these approaches.

The model detailed in Nature simulated 85 million people living in Thailand and bordering regions of neighboring countries. It tested the effectiveness of giving courses of antiviral treatment to everyone, socially or geographically targeting who received them, and combining these drug-sparing approaches with other measures, such as restricting travel.

Both models were based on detailed data for Thailand, such as population densities, household sizes, age distribution and distances traveled to work. The models also included information about the flu virus, such as the possible contagiousness of an infected person.

In both models, the researchers found that preventing a pandemic would require a combination of carefully implemented public health measures introduced soon after the first cases appear. They suggest that an international stockpile of 3 million courses of flu antiviral drugs, combined with other interventions, could contain a pandemic. Treating infected individuals and everyone in their vicinity, along with closing schools and workplaces, could have more than a 90 percent chance of stopping the spread of a pandemic virus, they reported. Both models demonstrated that the need for additional public health measures greatly increased as the virus became more contagious. "Each measure can have a significant effect, but it can't contain spread on its own," said Neil M. Ferguson, a co-author and computational biologist at Imperial College in London. He added that targeted antiviral treatment was a crucial component of all combined strategies.

The researchers stressed that the most important factor for successful containment is the early detection of the first cases and the rapid implementation of public health measures. But this may be problematic given the communications infrastructure of the rural communities where the outbreak would likely start. The researchers admit that while implementing a containment strategy may be challenging, it was feasible and it could contain an avian flu pandemic at its source. The researchers plan to continue to refine their simulations, leading to more accurate predictions in the future.

More Information on MIDAS