Photo by Jhon Paz/Xinhua Press/Corbis
The quarantined family members of Thomas Eric Duncan are set to be released on Oct. 19, after having spent 21 days in isolation — the amount of time widely considered to be the maximum duration for Ebola incubation. But is that really long enough? New research from Drexel University, published in PLOS Currents: Outbreaks, suggests the answer may be no.
According to study author Charles Haas, the current 21-day recommendation likely stems from data for the 1976 Ebola outbreak in Zaire. “In that outbreak, the latest case to convert did so at 21 days,” he said. One problem: The analysis from this outbreak included only 109 cases. “I wouldn’t say [the analysis] has been lacking — I would say they had a small data set,” he said. “And the WHO and CDC apparently never updated their recommendations with data sets from succeeding outbreaks.”
When Haas included data from other Ebola cases — for example, the 1995 outbreak in Congo, as well as 4,010 confirmed and probable cases from the first nine months of the current outbreak in West Africa — he made a startling discovery: There’s up to a 12 percent chance that the incubation period could extend beyond the 21-day mark.
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In fact, the Ebola incubation period can vary widely: The average was about six days for the Zaire outbreak, while it's closer to 12 days this time around. “Each individual case has an [individual] incubation time, determined by the dose that somebody was exposed to,” Haas said. “If you were helping a person with their daily hygiene needs — maybe assisting with intubation or other medical procedures without adequate coverage — you would have more intense contact. Increasing intensity would increase the dose, and you would expect that to decrease the incubation time.”
So how long is long enough? “I believe it should be greater than 21 days,” Haas told Yahoo Health; in the paper, he noted that “21 days may not be sufficiently protective to public health.” Based on data from the Congo outbreak, the necessary period could be as long as 31 days.
However, crunching numbers shouldn’t be the sole basis for an updated recommendation, cautioned Haas. “I think that number should be derived by people coming from a scientific background who have looked at the data, plus people who have knowledge of the economics and practicalities of putting a quarantine order into effect.” In other words, an updated quarantine time should balance the likelihood of infection beyond 21 days with the cost of extending the isolation period.