Originally published October 16 2014
New study reveals that up to 12% of Ebola cases are asymptomatic past 21 days
by David Gutierrez, staff writer
(NaturalNews) As many as 12 percent of Ebola patients may not show symptoms for more than 21 days after infection, thereby rendering current quarantine practices unacceptably short, according to a new analysis conducted by pathogen transmission expert Charles Haas of Drexel University, and published in the journal PLOS Currents: Outbreaks.
A disease's incubation period is the time that passes between initial infection and when a person begins to show symptoms. Because previous studies have suggested that Ebola has a maximum 21-day incubation period, many health institutions including the U.S. Centers for Disease Control and Prevention (CDC) have recommended 21-day quarantines of those potentially exposed to Ebola. If they do not show symptoms after that time, they are presumed to be safe.
"When someone has been exposed to a contagious disease and it is not yet known if they have caught it, they may be quarantined or separated from others who have not been exposed to the disease," the CDC says. "For example, they may be asked to remain at home to prevent further potential spread of the illness. They also receive special care and observation for any early signs of the illness."
"This work suggests a reconsideration is in order and that 21 days may not be sufficiently protective to public health," Haas wrote.
Recent data show wider spreadThe problem, Haas notes, is that the assumption of Ebola's 21-day incubation period has never been subjected to rigorous scientific assessment.
"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," he said.
The 21-day incubation period is based on studies of the 1976 Ebola outbreak in Zaire, the 2000 outbreak in Uganda and preliminary data from the first 9 months of the current outbreak. In all three cases, World Health Organization (WHO) data showed a 2-21 day incubation period for the virus. But when Haas also examined data from the 1995 Congo outbreak and more recent data from the current outbreak, he found that the actual incubation period deviated between 0.2 percent and 12 percent from the 2-21 day range.
This means that a quarantine period of 21 days might miss up to 12 percent of people who have actually been infected with the virus, Haas noted.
"While the 21-day quarantine value, currently used, may have arisen from reasonable interpretation of early outbreak data, this work suggests reconsideration is in order and that 21 days might not be sufficiently protective of public health," he said.
Risk-benefit analysisSetting the guidelines for any quarantine period requires more than epidemiological data, Haas noted. One thing that governments or health agencies must consider is the tradeoff between the costs of enforcing a longer quarantine versus the infection risks of a shorter one. The more contagious and deadly a disease, the higher the costs of erring on the short end.
"Clearly for pathogens that have a high degree of transmissibility and/or a high degree of severity, the quarantine time should be greater than for agents with lower transmissibility and/or severity," Haas said. "The purpose of this paper is not to estimate where the balancing point should be, but to suggest a method for determining the balancing point."
The new data also suggest that understanding of Ebola's incubation period remains incomplete. It is possible, for example, that the incubation period varies depending on the intensity or nature of the contact that led to the initial infection, Haas suggested. For that reason, he urges that the current outbreak be used to gather more data on the incubation period of Ebola, and that quarantine guidelines be revised correspondingly.
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