Calculating incubation periods – or how long it takes for contact with an infected person to result in disease – is not exact. And that’s the case with the Ebola virus.
“It’s retrospective,” said Dr. Stephen Gluckman, director of global medicine at the University of Pennsylvania. Still, he said, carefully considering how long this strain of Ebola can take to incubate before expressing itself – and therefore becoming contagious — “is a conversation that should be happening right now.”
He’s referring to recent research by Drexel professor Charles Haas. Haas, a professor of environmental engineering, does a lot of cost-benefit analysis of contagious disease. He says the current 21-day rule for incubation – and thus quarantine — is not always ironclad.
Haas looked at past outbreaks to discover that, “at 21 days, there’s still anywhere from a 0.1 and 12 percent chance that an individual may subsequently have an Ebola infection that occurs.”
Data from the current outbreak support a wider range as well. For the first nine months of the current outbreak, “roughly 5 percent of people reported having incubation times greater than that [21 days],” said Haas. That number comes from the World Health Organization and the New England Journal of Medicine.
Getting the incubation time about 95% right is standard practice, and advising a 21-day quarantine is still a “a very very good position,” Gluckman said.
Taking possible contagion seriously past the 21-day period could mean different things in different places, he said. In West Africa where the disease is having a big impact, “if you could lower the transmission rate by 95 percent, you have made a significant dent.”
By contrast, the impact in the United States will likely be much smaller – and the cost of a longer quarantine lower – so “one could argue that we’re going for better than 95 percent.”
To date, there have been two confirmed cases of Ebola transmission in the United States. Ebola is only contagious when a person has symptoms.