As the 48 patients exposed to the first U.S. Ebola patient near the end of their 21-day incubation period on Sunday, Oct. 19, a new study is questioning whether that period is sufficient to keep the public safe.

"Twenty-one days has been regarded as the appropriate quarantine period for holding individuals potentially exposed to Ebola Virus to reduce risk of contagion, but there does not appear to be a systemic discussion of the basis for this period," lead researcher Charles Haas, an environmental engineering professor at Drexel University, wrote in the study paper.

In the study, published in the Oct. 14 issue of the journal PLOS: Outbreak, Haas speculates that official health authorities such as the Centers for Disease Control and Prevention (CDC) have based the current incubation period on transmission rates in Zaire, in 1976, and Uganda, in 2000. In these countries, the maximum incubation period was 21 days based on available data, the study reveals.

Twenty-one days is the standard time that health authorities currently use to monitor a person who may have been exposed to Ebola but is not exhibiting symptoms.

“I got curious where the 21-day figure was coming from,” Haas told “I started to look at the literature, and thought that it would shed light on it, and I came up with the previous outbreak studies.”

“There is a suggestion in the literature that the nature of the contact, whether it’s profound or lighter contact, might influence the incubation time,” Haas added.

This rule holds true for any infectious disease, Haas said.

“That’s not specifically about Ebola but more [about] the general incubation time of microorganisms— the idea that in order to get infection in the disease, you need multiplication of the microorganisms in the body,” he said. “The lower the initial dose of the microorganisms you have, the longer it will take for them to accumulate to a critical level.”

In the research, Haas found that according to statistics from the recent outbreak in West Africa— where the disease has run rampant in Sierra Leone, Liberia and Guinea since March— as well as in 1995 in Congo, there’s up to a 12 percent chance that someone could be infected after the 21-day incubation period. The range of deviation from the incubation period was between .1 and 12 percent during those outbreaks. In other words, from 0.1 to 12 percent of the time, an individual case will have a greater incubation time than 21 days.

“While the 21-day quarantine value currently used may have arose from reasonable interpretation of early outbreak data,” Haas wrote, “this work suggests a reconsideration is in order, and that 21 days may not be sufficiently protective to public health. Further, outbreaks such as the current West Africa EBOV are presenting an opportunity for careful collection of data sufficient to revise and update (perhaps in an adaptive fashion) such recommendations.”

A representative from the CDC was not available to comment on whether the 21-day incubation period is flexible for potential Ebola patients who are currently being monitored.  

David Dausey, dean of the school of health professions and public health at Mercyhurst University in Erie, Pa., and a Yale University trained epidemiologist, argues that an incubation or isolation period for a potential Ebola patient should depend on that person’s level of contact with the infection.

“Certainly you have to know the range of their exposure,” Dausey told  “The incubation period is determined scientifically, but there’s still a possibility for error.”

“Anytime we come up with benchmarks for the incubation periods, it’s always based on what we think we’re observing out in the field,” Dausey added. “Obviously, there’s room for error— we may not have seen every case or collected data on everything. That range can have some flexibility.”

Dausey wrote a column published earlier this month for the Washington Post urging the federal government to impose a commercial airline travel ban from passengers coming from those countries where Ebola is not yet under control. He suggested sending aid to suffering areas by way of military aircraft instead.

“To me, you have got to separate potentially sick people from well people,” Dausey told

The current Ebola outbreak has spread to Spain and the United States, where there have been one and two reported Ebola transmissions within these countries’ borders, respectively.

In Spain, a nurse became infected with Ebola while caring for two priests who succumbed to the disease. In the U.S., two health care workers in Dallas became infected while caring for Liberian national Thomas Eric Duncan, who died of the virus Oct. 8.

The CDC is currently monitoring more than 70 health care workers from Texas Health Presbyterian Hospital, where Duncan was treated before his death, and launched an investigation into how Nina Pham, 26, and Amber Vinson, 29, could have contracted the disease despite wearing personal protective equipment.

Health officials are also using contact tracing to examine whether Pham had close contact with anyone while exhibiting symptoms. Vinson reportedly flew on a plane from Cleveland to Dallas while she had a low-grade fever on Monday, a day before she was quarantined. The CDC is working to contact her family members and all 132 patients on Frontier Airlines flight 1143.

Ebola spreads through direct contact with bodily fluids, such as blood, sweat, tears, semen and urine. A patient must be exhibiting symptoms to transmit the disease.

Sunday marks the end of the 21-day incubation period for the 48 people currently being monitored by the CDC. These people reportedly had contact with Duncan while he was symptomatic. 

In West Africa, the virus has killed nearly nearly 4,500 people.

According to World Health Organization (WHO), 95 percent of confirmed Ebola cases have an incubation period in the range of 1 to 21 days, while 98 percent have an incubation period that falls in the 1 to 42-day interval, which means 2 percent— or 1 in 50 people— have a longer incubation period than 42 days.

WHO does not declare an Ebola outbreak over in a country until no new cases are reported within a 42-day period.  

According to the organization, people infected with Ebola present symptoms seven to eight days after exposure on average.