A warning heeded yields good news on Ebola

Countries in West Africa – primarily Sierra Leone, Guinea and Liberia – are battling the most severe outbreak of Ebola since the virus was discovered in 1976. According to the most recent World Health Organization report, more than 20,700 people have been infected and more than 8,200 have died since the epidemic was declared in March.

In Liberia, cases have dropped from a peak of more than 300 new confirmed infections per week in August and September to eight new confirmed cases and 40 probable cases in the five days leading up to Jan. 2, according to the WHO

Mathematical models are proving instrumental in guiding the public health response against Ebola-virus disease and monitoring the effectiveness of control interventions, according to Dr. Gerardo Chowell, an expert on modeling infectious diseases from Georgia State University. In a commentary accompanying The Lancet paper, Chowell praised the analysis by Vespignani, Halloran and researchers at the Bruno Kessler Foundation and the Institute for Scientific Interchange in Italy, the University of Florida, the University of Washington and Harvard University, for using a model that captures “spatial heterogeneity,” detecting patterns of infection that homogeneous-mixing models have not been able to detect.

Improving the projection of infectious-disease patterns

A homogeneous model, Halloran explained, assumes that all members of a population mix equally with everyone else and can contract an infection from any of them. A more finely detailed heterogeneous model assumes that infections spread at different intensities at different times and within certain contact networks. Heterogeneous models are becoming more common for projecting infectious-disease patterns, but the difficulty getting data from the three countries makes such models difficult to construct, she said.

The new model incorporated population figures, geography and the locations of households, hospitals and Ebola treatment units in Liberia. It took into account that Ebola is spread by direct contact with blood or bodily fluids of a person who is sick with Ebola or with contaminated objects such as bedding or syringes; infectious individuals would most likely be confined to the home or a health-care setting with contacts limited to caregivers.

Up to mid-August last year, the researchers estimated that 38.3 percent of infections were acquired in hospitals, 30.7 percent in households and 8.6 percent at funerals. In the early stages of the epidemic, the moving and mixing of Ebola-infected hospital patients with patients not infected helped drive the spread of the disease. The availability of Ebola treatment units separated the infected from the uninfected and drastically decreased hospital transmission, the analysis found. Safe burials and the distribution of household protection kits also helped bring down transmission rates.

In his commentary, Chowell noted that other interventions not explicitly modeled may have also played a role, including the use of rapid diagnostic kits in Ebola treatment units, which reduced the delay from presentation to isolation, and changes in population behavior due to mass education campaigns.

The analysis was a project of the Models of Infectious Disease Agents Study, or MIDAS, network. Halloran heads a MIDAS center based at the Hutch, one of three such sites in the nation that use computational and mathematical models to advise the U.S. government about the spread of diseases including polio, dengue fever, tuberculosis, influenza and Ebola.

The long-term solution: a preventive vaccine

The group meets frequently with policy makers, including the U.S. Centers for Disease Control and Prevention and the National Institutes of Health, to discuss how modeling can help in making intervention decisions. The next step is to apply the new model to Guinea and Sierra Leone, where Ebola cases and deaths remain higher than in Liberia, Vespignani said. Sierra Leone ran behind Liberia in getting treatment centers open, he said, but he is “more and more optimistic” that the same interventions, if rigorously applied, will work to contain the outbreak there as well.

The long-term solution, he added, is a preventive vaccine. There’s no proven treatment for the infection other than supportive care. Clinical trials testing experimental drugs began on Wednesday, and a potential vaccine is in the pipeline. The model can also be used to help design clinical trials to test vaccines, according to Vespignani and Halloran.

Developing a vaccine is critical because even if this outbreak is stamped out, the Ebola virus can “hide” in animal reservoirs, the researchers said.

“Our goal is to eliminate this outbreak,” Halloran said. “The next time there’s an Ebola outbreak, everybody will be better prepared. This one just got out of hand because the initial response was too slow.  Modeling can help us figure out what we need to do to eliminate the outbreak.”