FYI, the difference between the 21 day vs. the 42 day quarantine seems to be a statistical standard deviation. WHO is just warning the world that there is a 5% chance that Ebola may still be viable in the population with 21 day quarantine. The 42 day quarantine limits the probability to less than 2%. Question is are we comfortable with a 5% probability of that some one may still begin to show symptoms after 21 days.
http://www.who.int/mediacentre/news/ebola/14-october-2014/en/
The period of 42 days, with active case-finding in place, is twice the maximum incubation period for Ebola virus disease and is considered by WHO as sufficient to generate confidence in a declaration that an Ebola outbreak has ended.
Recent studies conducted in West Africa have demonstrated that 95% of confirmed cases have an incubation period in the range of 1 to 21 days; 98% have an incubation period that falls within the 1 to 42 day interval. WHO is therefore confident that detection of no new cases, with active surveillance in place, throughout this 42-day period means that an Ebola outbreak is indeed over.
I’m in favor of 99.9999 certainty when dealing with large populations....