Risk factors for transmission of Ebola or Marburg virus disease: a systematic review and meta-analysis

Background The Ebola virus disease outbreak that started in Western Africa in 2013 was unprecedented because it spread within densely populated urban environments and affected many thousands of people. As a result, previous advice and guidelines need to be critically reviewed, especially with regard to transmission risks in different contexts.
Methods Scientific and grey literature were searched for articles about any African filovirus. Articles were screened for information about transmission (prevalence or odds ratios especially). Data were extracted from eligible articles and summarised narratively with partial meta-analysis. Study quality was also evaluated.
Results 31 reports were selected from 6552 found in the initial search. Eight papers gave numerical odds for contracting filovirus illness, 23 further articles provided supporting anecdotal observations about how transmission probably occurred for individuals. Many forms of contact (conversation, sharing a meal, sharing a bed, direct or indirect touching) were unlikely to result in disease transmission during incubation or early illness. Amongst household contacts who reported directly touching a case, the attack rate was 32% (95% CI 26-38%). Risk of disease transmission between household members without direct contact was low (1%; 95% CI 0-5%). Caring for a case in the community, especially until death, and participation in traditional funeral rites were strongly associated with acquiring disease, probably due to a high degree of direct physical contact with case or cadaver.
Conclusions Transmission of filovirus is unlikely except through close contact, especially during the most severe stages of acute illness. More data are needed about the context, intimacy and timing of contact required to raise the odds of disease transmission. Risk factors specific to urban settings may need to be determined.