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The 21-day incubation window for Ebola virus to show symptoms could be insufficient, a new study claims.

WHO guidelines state that individuals exposed to the virus should be monitored 21 days post exposure. If no symptoms are reported during the quarantine window, an individual is deemed not infected or is said to be non-contagious. New study by a Drexel University professor claims up to 12 percent chance of an infection after the 21-day period.

"A 21 day period for quarantine may result in the release of individuals with a 0.2 - 12% risk of release prior to full opportunity for the incubation to proceed," study's author Charles Haas wrote.

"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," said Haas, according to Zee News.

Haas said the possibility of Ebola incubation being longer than 21 days is derived from analysis of past outbreaks including 1976 Zaire outbreak, 1995 Congo outbreak, Uganda outbreak in 2000 and the first nine months of the current outbreak.

Up on recovery, patients cannot infect others though the virus can be present in semen for up to three months.  

According to Forbes, the Haas's study showed that in most cases of infections, the virus shows symptoms within 21 days. However given that the current outbreak has killed over 4,000 people and threatens to infect many more, one of the ways it could be effectively tackled is by increasing the isolation period for survivors to ensure the virus does not spread.

"It is suggested that a detailed cost-benefit assessment, including considering full transmission risks, needs to occur in order to determine the appropriate quarantine period for potentially exposed individuals," Haas said in the study which was published in PLOS Current Outbreaks.