_MEDICINE Epidemiology

Quarantine in Question
A Drexel study suggests that the standard 21-day quarantine for individuals who might have been exposed to Ebola might not be long enough.

_Charles Haas

Charles Haas is the department head and LD Betz Professor of Environmental Engineering, and director of Drexel’s Environmental Engineering program in the College of Engineering.


Sierra Leone health officials check passengers transiting at the border crossing with Liberia in Jendema on March 28, 2015. The authorities in Sierra Leone started enforcing on March 27 a three-day lockdown to curb the spread of Ebola, with the entire population ordered to stay at home.

Twenty-one days. That’s the length of time an individual who has been exposed to the Ebola Virus should remain in quarantine. It’s a number set forth by the Centers for Disease Control and Prevention (CDC), but is it enough?

A new study suggests the 21-day quarantine period might not be sufficient to completely prevent spread of Ebola.

College of Engineering Professor Charles Haas looked at the murky basis for our knowledge about the virus, namely previous outbreaks in Africa in 1976 (Zaire) and 2000 (Uganda), as well as the first nine months of the most recent outbreak.


The CDC’s 21-day incubation period appears to be based on data from outbreaks in 1976 and 2000, which show less variance than data from recent outbreaks.

In both cases, data gathered by the World Health Organization reported a 2–21 day incubation period for the virus — meaning that after 21 days, if the individual hasn’t presented symptoms, it is likely that they are not infected or contagious. This is likely the reason for the Centers for Disease Control and Prevention’s 21-day quarantine period.

In his study, Haas looked more broadly at data from other Ebola outbreaks, such as the Congo in 1995 and recent reports from the outbreak in West Africa, where the range of deviation in the disease’s incubation periods was between 0.1 and 12 percent, according to Haas. This means that there could be up to a 12 percent chance that someone could be infected even after 21 days.

“While the 21-day quarantine value, currently used, may have arisen from reasonable interpretation of early outbreak data, this work suggests reconsideration is in order,” Haas says.


The more contagious and potentially deadly a disease, the higher the cost of making a mistake by prematurely releasing an exposed individual.