Potential Impact of Sexual Transmission on Ebola Virus Epidemiology: Sierra Leone as a Case Study

PLoS Negl Trop Dis. 2016 May 2;10(5):e0004676. doi: 10.1371/journal.pntd.0004676. eCollection 2016 May.


Background: Sexual transmission of Ebola virus disease (EVD) 6 months after onset of symptoms has been recently documented, and Ebola virus RNA has been detected in semen of survivors up to 9 months after onset of symptoms. As countries affected by the 2013-2015 epidemic in West Africa, by far the largest to date, are declared free of Ebola virus disease (EVD), it remains unclear what threat is posed by rare sexual transmission events that could arise from survivors.

Methodology/principal findings: We devised a compartmental mathematical model that includes sexual transmission from convalescent survivors: a SEICR (susceptible-exposed-infectious-convalescent-recovered) transmission model. We fitted the model to weekly incidence of EVD cases from the 2014-2015 epidemic in Sierra Leone. Sensitivity analyses and Monte Carlo simulations showed that a 0.1% per sex act transmission probability and a 3-month convalescent period (the two key unknown parameters of sexual transmission) create very few additional cases, but would extend the epidemic by 83 days [95% CI: 68-98 days] (p < 0.0001) on average. Strikingly, a 6-month convalescent period extended the average epidemic by 540 days (95% CI: 508-572 days), doubling the current length, despite an insignificant rise in the number of new cases generated.

Conclusions/significance: Our results show that reductions in the per sex act transmission probability via abstinence and condom use should reduce the number of sporadic sexual transmission events, but will not significantly reduce the epidemic size and may only minimally shorten the length of time the public health community must maintain response preparedness. While the number of infectious survivors is expected to greatly decline over the coming months, our results show that transmission events may still be expected for quite some time as each event results in a new potential cluster of non-sexual transmission. Precise measurement of the convalescent period is thus important for planning ongoing surveillance efforts.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Africa, Western / epidemiology
  • Body Fluids / virology*
  • Convalescence*
  • Ebolavirus / genetics
  • Ebolavirus / isolation & purification
  • Ebolavirus / physiology
  • Epidemics
  • Hemorrhagic Fever, Ebola / epidemiology*
  • Hemorrhagic Fever, Ebola / transmission*
  • Hemorrhagic Fever, Ebola / virology
  • Humans
  • Incidence
  • Models, Biological
  • Models, Statistical
  • Monte Carlo Method
  • RNA, Viral / isolation & purification
  • Semen / virology*
  • Sexually Transmitted Diseases, Viral / epidemiology*
  • Sierra Leone / epidemiology
  • Virus Replication / physiology


  • RNA, Viral

Grants and funding

JLA received post-doctoral support from the University of Bern and a grant from the French National Research Agency (ANR JC "STORY"). CLA received funding through an Ambizione grant from the Swiss National Science Foundation (project 136737). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.