FiveThirtyEight looks at Ebola's 21-day incubation, critiques study suggesting longer incubation

In Dallas this week, 43 people who came in contact with Thomas Eric Duncan after he contracted Ebola were released after 21 days of quarantine. In Madrid, people who were exposed to an infected nurse could be released from isolation soon if they get through 21 days without showing symptoms. The paramedics who brought Craig Spencer, the New Yorker diagnosed with Ebola this week, to the hospital will have their temperatures taken twice a day for 21 days.

Why 21 days? And is 21 days enough?

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Haas ran that data through models created by other scientists to convert the raw counts of days into a probability curve. He showed that each data-model pair paints a slightly different picture of how long the virus can lie dormant in the body before making its carrier sick. The models broadly agree that the average case emerges in much less time than 21 days; the average time is somewhere between three and 13 days. That can translate into big disagreements about the tail end of the distribution: Anywhere from 0.1 percent to 12 percent of people got sick more than 21 days after exposure, by Haas’s calculations.

But did they really? Outliers could arise simply because sick people forgot when they were exposed. “I think there is some recall bias or respondent bias,” Gerardo Chowell, an associate professor at Arizona State University’s School of Human Evolution and Social Change, said in a Skype interview. “When you are in the field, people forget things. People don’t know exactly where they were when.” He pointed out that there are plenty of problems with data collected during an outbreak.2

Chowell’s studies of incubation times in earlier Ebola outbreaks in Congo and Uganda, cited in the Haas paper, show short incubation times, and he thinks the true maximum period may be less than 21 days. “That said, I think the policy of 21 days is good,” Chowell said. “I don’t think we need to change that or extend it.”

Haas doubts recall bias alone could explain cases with incubation periods longer than 21 days. He points out that some analyses didn’t use interviews at all, instead extrapolating incubation times from other data.

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Chowell thinks a negative blood test before 21 days could be sufficient evidence to end isolation early. “I bet you, if you don’t find a virus in two weeks after exposure, you could release them quickly,” he said. “Then let them go with peace of mind.”

Chowell’s brother Diego, as it happens, is studying the potential benefit of testing asymptomatic carriers. Diego Chowell is a graduate student at ASU’s Center for Personalized Diagnostics. “The faster you can diagnose an individual infected with Ebola virus, the faster you can isolate the patient and then you can stop further transmission,” he said in an email.

Haas is open to the idea of early release with negative tests. He wants more evidence, though, before accepting that a negative test even at 21 days is enough. “There is more data that is needed on the efficacy of these tests — with aggressive and appropriate testing, it is entirely possible that 21 days is OK,” Haas said.

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