EbolaModels of Ebola spread cannot model people’s behavior

In the early stages of what the Centers for Disease Control and Prevention (CDC) has called the world’s first Ebola epidemic, the World Health Organization (WHO), Doctors Without Borders, and other health-aid organizations worked to limit the spread of the disease by convincing patients to report their symptoms to doctors or let aid workers quarantine their homes and villages. The strategies deployed have had little positive effect on the two most affected countries, Sierra Leone and Liberia. The WHO recently reported that the number of cases has nearly doubled in the last three weeks. As of 25 September, the (CDC) recorded 6,263 cases of Ebola, resulting in 2,917 deaths.

Dr. Martin Meltzer, a senior health economist at the CDC and head of the Health Economics and Modeling Unit, began building the CDC’s Ebola model, called EbolaResponse, on 4 August. The model will help understand how to reduce the rate of transmission to an average of less than one infected person per infectious person. According to Government Executive, similar models have been used for stifling previous epidemics.

Meltzer’s model relies on the number of confirmed cases and the current rate of transmission to determine how many new cases will emerge if current conditions persist. According to a paper published in the CDC’s Morbidity and Mortality Weekly Report, Meltzer’s model predicts 8,000 reported cases in Sierra Leone and Liberia by 30 September if health interventions fail to make a significant impact. Sierra Leone and Liberia could then experience 550,000 reported cases by January 2015, or 1.4 million if corrected for underreporting. A growing number of infected persons are not reporting their symptoms for fear of quarantine.

Once predictions have been made, Meltzer uses the model to understand the best way to reduce the spread of Ebola by testing different hypothetical situations. One experiment suggests that if 70 percent of patients were in an effective quarantine setting such as a hospital, treatment unit, or home by 22 December, the epidemic in Liberia and Sierra Leone “would almost be ended by January 20, 2015.” Currently, only 10 percent of patients are in such settings. The model does not account for organization and infrastructure conditions on the ground. “Appreciate that not all hospitals/Ebola treatment units and certainly not all households with a patient ‘at home with effective quarantine/ isolation’ will be entirely secure,” Meltzer wrote to GovExec. “We can expect some transmission to occur at such locales, but hopefully on average, less than 1 person infected per infectious person.” The rate of transmission in an effective hospital quarantine is .12, and .18 in an effective at-home quarantine, but ineffective isolation results in a 1.8 transmission rate.

Meltzer’s model cannot predict the most effective way to convince patients to report their cases to health authorities, but reporting is vital to effective quarantine and reducing transmission rates.

“The trouble is to get people to believe that going to the hospitals is in their best interest,” said Meltzer. “We’ve got to get people to understand that. You can go around to villages and cities and slums all you want and say, ‘If you’re ill, go to the hospital.’ Why should anybody believe? We can’t model that.”