A Model of Ebola Transmission and Its Suppression

 From NEJM

A Model of Ebola Transmission and Its Suppression

Stephen G. Baum, MD reviewing Yamin D et al. Ann Intern Med 2014 Oct 28.

Isolation of critically ill patients within 4 days after symptoms develop is key to truncating the epidemic.

The current Ebola outbreak in three West African countries is unprecedented in both the number of people infected and the case fatality rate. Without a vaccine or a curative drug, hopes for arresting the epidemic rest on hygiene and isolation measures. To quantify what might be necessary for effective control, researchers developed a stochastic model based on incidence, case fatality rates, and contact tracing from the early days of this outbreak in Liberia, as well as viral load data for survivors and nonsurvivors of an outbreak in Uganda in 2000–2001.
The resulting assumptions for the model were that Ebola transmission increases with viral load. For survivors, the load is greatest 4 days after symptom onset. For nonsurvivors, it is greatest late in the disease and immediately after death; throughout the course of infection, it may be 100-fold that of survivors. Based on these data-driven assumptions and the likely number of contacts for infected individuals at the various stages of illness, the probability of infecting at least one other person was 32% for survivors and 67% for nonsurvivors. The authors estimated that isolation of 75% of critically ill individuals within 4 days of symptom onset has a high probability of eliminating the disease.