It takes up to 21 days for someone who has been exposed to Ebola to develop symptoms, according to the Centers for Disease Control and Prevention and World Health Organization. And 21 days is the recommended quarantine period for people who come into direct contact with Ebola patients.
But is 21 days enough?
According to Charles N. Haas, a researcher at Drexel University, it isn't.
Last week, the WHO reported that in the current outbreak, victims developed symptoms 11 days after exposure, on average, but about 5 percent (or one in 20 people) infected with Ebola have developed symptoms after 21 days.
While 21 days may have been a "reasonable interpretation of early outbreak data," Haas wrote in the journal PLOS Currents, "this work suggests a reconsideration is in order and that 21 days may not be sufficiently protective to public health."
According to Haas' calculations, as many as 12 percent (or one in eight people) became sick after 21 days.
Hawaii Rep. Tulsi Gabbard, citing Haas' work, called on the CDC earlier this week to extend the recommended quarantine period to 42 days — the amount of time WHO waits until declaring a country, like Senegal and Nigeria, Ebola-free.
“I strongly recommend that the CDC follow the WHO guidelines of a 42-day incubation period," Gabbard said in a press release. "It is far better to err on the side of caution."
But the CDC isn't budging.
“The incubation is still 21 days,” CDC spokesman Tarik Jasarevic wrote in an email to FiveThirtyEight.com.
Earlier this week, Texas health officials announced 43 people who had possible contact with Thomas Eric Duncan, the Liberian man who died from Ebola in a Dallas hospital on Oct. 8, are now cleared from monitoring. An additional 120 people, mainly health care workers who treated Duncan, are still being watched for symptoms. If no other Ebola cases are confirmed, all monitoring would be complete on Nov. 7.
“We’re breathing a little easier,” Dallas Mayor Mike Rawlings said during a news conference. "But we’re still holding our breath a fair amount
There have been more than 9,936 cases of Ebola since the outbreak began in West Africa in March, according to WHO estimates. There have been 4,877 deaths as a result of the disease.
Gerardo Chowell, an associate professor at Arizona State University who studied the 1976 Ebola outbreak in Zaire and 2000 outbreak in Uganda, told FiveThirtyEight.com he thinks the incubation period for Ebola is probably less than 21 days.
“That said, I think the policy of 21 days is good,” said Chowell, whose studies are cited in the Haas paper. “I don’t think we need to change that or extend it.”