Determinants of Transmission Risk During the Late Stage of the West African Ebola Epidemic

Abstract

Understanding risk factors for Ebola transmission is key for effective prediction and design of interventions. We used data on 860 cases in 129 chains of transmission from the latter half of the 2013-2016 Ebola epidemic in Guinea. Using negative binomial regression, we determined characteristics associated with the number of secondary cases resulting from each infected individual. We found that attending an Ebola treatment unit was associated with a 38% decrease in secondary cases (incidence rate ratio (IRR) = 0.62, 95% confidence interval (CI): 0.38, 0.99) among individuals that did not survive. Unsafe burial was associated with a higher number of secondary cases (IRR = 1.82, 95% CI: 1.10, 3.02). The average number of secondary cases was higher for the first generation of a transmission chain (mean = 1.77) compared with subsequent generations (mean = 0.70). Children were least likely to transmit (IRR = 0.35, 95% CI: 0.21, 0.57) compared with adults, whereas older adults were associated with higher numbers of secondary cases. Men were less likely to transmit than women (IRR = 0.71, 95% CI: 0.55, 0.93). This detailed surveillance data set provided an invaluable insight into transmission routes and risks. Our analysis highlights the key role that age, receiving treatment, and safe burial played in the spread of EVD.

Keywords: Ebola; Guinea; multiple imputation; regression analysis; risk factors.

Figure 1.

Characteristics of the chains data, Ebola epidemic, Guinea, 2014–2016. A) Time series of the daily incidence in Guinea. Light gray is total incidence, and dark gray area shows cases included in this analysis. B) Distribution of the number of generations per chain. C) Example of a chain with 9 generations. Squares symbolize women, and circles symbolize men. Lighter shade is under 35 years of age, and darker shade is over 35 years of age.

Figure 2.

Characteristics of transmission, Ebola epidemic, Guinea, 2014–2016. A) Serial interval through time (overall mean, 12.3 days). Number of cases in each group: 61, 163, 61, and 23, respectively. B) Distribution of the transmitter status of the cases depending on the age of the cases. C) Distribution of the transmitter status depending on the route of transmission.

Figure 3.

Distribution of secondary cases, Ebola epidemic, Guinea, 2014–2016. A) Distribution of the number of secondary cases per case, fitted to a negative binomial distribution (line and shaded areas). B) Fits of the first or the subsequent generations only (total: mean, 0.89 (standard deviation, 0.31; 95% confidence interval: 0.25, 0.37); first generation: mean, 1.77 (standard deviation, 0.88; 95% confidence interval: 0.53, 1.23); subsequent generations: mean, 0.70 (standard deviation, 0.25; 95% confidence interval: 0.19, 0.30)).