Isolation of the most severely ill patients with Ebola within the first few days of symptoms may be the key to ending the epidemic. A new mathematical model of the Ebola epidemic in Liberia reveals that a shift in the focus of public health efforts from case isolation and hygienic burial of the dead to the isolation of infected individuals before they progress into critical condition and the late phase of illness may slow disease transmission.

Dan Yamin, PhD, from the Yale School of Public Health in New Haven, Connecticut, and colleagues report the results of their stochastic transmission model in an article published online October 28 in the Annals of Internal Medicine. The investigators used contact tracing data from Liberia for their model. They could only use available data and acknowledge that the actual case fatality rate from the ongoing Ebola outbreak is still uncertain.

The team model included empirical contact information, as well as viral load data, to estimate the separate contributions of disease progression and case fatality on disease transmission. They also used the model to test interventions aimed at the most infectious patients. "Our findings indicate that the number of secondary cases resulting from an infected individual varies with the phase of disease progression (early versus late) and outcome (survival versus nonsurvival) of infection, such that nonsurvivors and, in particular, nonsurvivors 4 days after symptom onset are most responsible for perpetuating the epidemic," the authors write.

They estimated viral load on the basis of data from the 2000-2001 Uganda outbreak, which indicated that viral load among nonsurvivors was substantially higher than viral load among survivors.

The investigators calculated the R0 for the current Ebola outbreak in Liberia as 1.73 (95% confidence interval [CI], 1.66 - 1.83). They found a substantial stratification between the R0 of survivors and nonsurvivors (0.66 [95% CI, 0.10 to 1.69] for survivors and 2.36 [95% CI, 1.72 - 2.80] for nonsurvivors). Nonsurvivors experience clinical symptoms of bleeding diathesis in the late phase of infection, which may contribute to their higher transmissibility.

The Ebola outbreak in Liberia has resulted in at least 3500 infections and 2000 deaths in the last 3 months. The investigators point out that, clinically, survival cannot be predicted at the outset of symptoms. Because of this, they suggest that resources are best spent in isolating individuals who are gravely ill.

The authors have disclosed no relevant financial relationships.

Ann Intern Med. Published online October 28, 2014. Full text