` :::: Thanks for the link.  Interesting reading. I was only ever aware of ``42`` in context of a country being declared Ebola free eg. Nigeria. I had assumed it was just double the maximum 21 day incubation, in case someone either recovers or dies without being diagnosed you have double the maximum and 3-4 times the mean.  And indeed this very reference supports my theory stating ``42 .... is twice the maximum incubation period``.  But then goes on to up-end the 21 days by saying ``98% have an incubation period that falls within the 1 to 42 day interval.`` and 95% 21 days.  What is interesting is these exact numbers appear in NEJM but the conclusion drawn in same paper is that there is no need to monitor beyond 21 days.  What I don't know is whether the 95% probability stems from the lack of confidence or accuracy in the raw epidemiological data, or is a clinical/biological fact. They don't say.  Nigeria was very effective in quashing it in Lagos (20 million people) after the highly infectious index case by being very aggressive in monitoring a large number of contacts (more aggressive than the US as far as I read) ... but only up to 21 days. My understanding from reading some of these epidemiology studies going back to 1976 is that the mean is around 8-10 days depending on outbreak, and that actually if you are symptom free for 2 weeks past last contact you most likely will not get it but the third week is for insurance.  I have not read of any proven confirmed case with symptom onset longer than 21 days past last contact.  They studied this in great detail in the initial outbreaks back to 1976 including a nurse that had a needle stick so they knew exactly when she was infected.  My impression was the raw data is case-by-case and they delete or otherwise lower weightings on unknown outcomes.  IMO any implication in WP that you could possibly incubate longer than 21 days has to be well supported (in data and conclusion by several sources) since it would definitely cause new consternation.  `