Ebola superspreading
Authors: Christian L Althaus
Journal: The Lancet Infectious Diseases (2015)

Ousmane Faye and colleagues1Faye O Boëlle P-Y Heleze E et al.Chains of transmission and control of Ebola virus disease in Conakry, Guinea, in 2014: an observational study.Lancet Infect Dis. 2015; 15: 320-326Summary Full Text Full Text PDF PubMed Scopus (168) Google Scholar recently described the chains of transmission for 152 individuals infected with Ebola virus diseases in Guinea. The resulting transmission trees provide unique insights into the individual variation in the number of secondary cases generated by an infected index case. A better understanding of this variation provides crucial information about epidemic spread, the expected number of superspreading events, and the effects of control measures.2Lloyd-Smith JO Schreiber SJ Kopp ME Getz WM Superspreading and the effect of individual variation on disease emergence.Science. 2005; 438: 355-359Google Scholar The number of secondary cases in the transmission trees is highly skewed, with 72% of individuals not generating further cases (figure). Fitting a negative binomial distribution to the data (appendix) provides maximum-likelihood estimates of the mean (0·95, 95% CI 0·57–1·34) and the dispersion parameter (k=0·18, 95% CI 0·10–0·26). The mean corresponds to the basic reproduction number (R0) of the overall population. The estimated value of k, which is substantially smaller than 1, suggests that the distribution of the individual reproduction number is highly overdispersed.2Lloyd-Smith JO Schreiber SJ Kopp ME Getz WM Superspreading and the effect of individual variation on disease emergence.Science. 2005; 438: 355-359Google Scholar The value for Ebola virus disease is similar to that estimated for severe acute respiratory syndrome (k=0·16).2Lloyd-Smith JO Schreiber SJ Kopp ME Getz WM Superspreading and the effect of individual variation on disease emergence.Science. 2005; 438: 355-359Google Scholar This finding suggests that superspreading events for Ebola virus disease are an expected feature of the individual variation in infectiousness.3Volz EM Pond SL Phylodynamic analysis of Ebola virus in the 2014 Sierra Leone epidemic.PLoS Curr. 2014; (published online Oct 24.)https://doi.org/10.1371/currents.outbreaks.6f7025f1271821d4c815385b08f5f80eCrossref PubMed Google Scholar I simulated stochastic trajectories of Ebola virus disease outbreaks starting from one infected index case (figure). To this end, I drew the number of secondary cases for each case from the fitted negative binomial distribution (appendix). The time from disease onset in one case to disease onset in the next case was drawn from the reported gamma-distributed serial interval with a mean duration of 15·3 days.4WHO Ebola Response TeamEbola virus disease in West Africa—the first 9 months of the epidemic and forward projections.N Engl J Med. 2014; 317: 1481-1495Crossref Scopus (1221) Google Scholar Although most outbreaks rapidly become extinct, some epidemic trajectories can reach to more than 100 infected cases. This finding is particularly remarkable because R0 is less than 1, and shows the potential for explosive outbreaks of Ebola virus disease. R0 during the early phase of the Ebola virus disease epidemic in Guinea has been estimated to be roughly 1·5.5Althaus CL Estimating the reproduction number of Ebola virus (EBOV) during the 2014 outbreak in West Africa.PLoS Curr. 2014; (published online Sept 2.)https://doi.org/10.1371/currents.outbreaks.91afb5e0f279e7f29e7056095255b288Crossref PubMed Google Scholar The transmission trees from Faye and colleagues were generated from data obtained between February and August, 2014, when the reproduction number was fluctuating around unity.1Faye O Boëlle P-Y Heleze E et al.Chains of transmission and control of Ebola virus disease in Conakry, Guinea, in 2014: an observational study.Lancet Infect Dis. 2015; 15: 320-326Summary Full Text Full Text PDF PubMed Scopus (168) Google Scholar, 4WHO Ebola Response TeamEbola virus disease in West Africa—the first 9 months of the epidemic and forward projections.N Engl J Med. 2014; 317: 1481-1495Crossref Scopus (1221) Google Scholar That scenario is similar to the present situation in parts of west Africa where the incidence is declining but new outbreaks still occur. The observed variation in individual infectiousness for Ebola virus disease means that although the probability of extinction is high, new index cases also have the potential for explosive regrowth of the epidemic. This online publication has been corrected. The corrected version first appeared at thelancet.com/infection on May 19, 2015 This online publication has been corrected. The corrected version first appeared at thelancet.com/infection on May 19, 2015 I received funding through an Ambizione grant from the Swiss National Science Foundation (project 136737). I declare no competing interests. Download .pdf (.34 MB) Help with pdf files Supplementary appendix Chains of transmission and control of Ebola virus disease in Conakry, Guinea, in 2014: an observational studyIn Conakry, interventions had the potential to stop the epidemic, but reintroductions of the disease and poor cooperation of a few families led to prolonged low-level spread, showing the challenges of Ebola virus disease control in large urban centres. Monitoring of chains of transmission is crucial to assess and optimise local control strategies for Ebola virus disease. Full-Text PDF CorrectionsAlthaus CL. Ebola: the real lessons from HIV scale-up. Lancet Infect Dis 2015; 15: 507–08—The title of this Correspondence should have been 'Ebola superspreading'. This correction has been made to the online version as of May 19, 2015. Full-Text PDF