WHO and CDC recommend that individuals who are potentially infected with Ebola virus should be quarantinedÂ for 21 days. Where does this numberÂ come from? Charles Haas at Drexel UniversityÂ asked the same question, and provides an answer.
The quarantineÂ period forÂ an infectious disease is based onÂ the incubation period,Â the time before symptoms of an infection appear. For Ebola virus, the incubation period is 2-21 days after infection. During this time it is believed that individuals infected with the virus are not contagious, but they could produce small amounts of virus. Whether or not a patient is contagious during the incubation period depends on the virus.
It is clearly important to determine the correct quarantine period for Ebola virus to prevent chains of infection. The longer the quarantine period imposed, the less risk of infecting others. However the cost of enforcing quarantine must be balanced with the cost of releasing exposed individuals (illustrated). According to Haas,Â the optimal quarantine time should be at the intersection of the two curves.
To determine how the Ebola virus quarantine period was set at 21 days, Haas examinedÂ the incubation periods calculated for previous outbreaks. In aÂ study of theÂ 1976 Zaire outbreak, the mean timeÂ between exposure and disease for 109Â cases of person-to-person spread was calculated at 6.3 days withÂ a range of 1 to 21 days. Mean incubation times for the 1995 Congo outbreak (315 cases) and the 2000 Uganda outbreak (425 cases) were 5.3 and 3.35 days, respectively. Two otherÂ analyses of the 1995 Congo outbreak gave mean incubation times of 10.11 and 12.7 days. WHO has estimated a mean incubation period for the first 9 months of the current west African outbreak as 11.4 days, with an upper limit (95% confidence) of 21 days.
Haas concludes that the 21 day quarantine value is derived from a ‘reasonable interpretation’ of outbreak data, but it might not be long enough. He estimates that there is a risk of between 0.2% and 12%Â of developing Ebola virus infection after 21 days.
The current outbreak shouldÂ allow collection of data for revising and updating the 21 day quarantine period for Ebola virus infection.