Father gets chickenpox, but doesn't infect his two children. How is this possible?
My brother in law got chickenpox, yet somehow he didn't infect my two nephews, even though they are living together. According to wikipedia, varicella has an infection rate of 90%:
Varicella is highly communicable, with an infection rate of 90% in close contacts.
en.wikipedia.org/wiki/Chickenpox
He got varicella over a week ago and the children are completely healthy, even though they have not had the disease yet nor are they vaccinated against it.
How is this possible? Is the infection rate actually lower, than 90%? Is an outcome like this usual or plausible?
edit: they did end up getting sick after all.
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If there was close contact, if the 90% rate is accurate, and if occurrence is independent in related individuals, then you would expect 0.10 * 0.10 = 1% of contacts with 2 potentially vulnerable people to result in neither person infected.
1% sounds rare, but rare events happen all the time, and 1% isn't even particularly rare. If you know 100 families, you'd expect this outcome to happen on average in 1 of them.
That's not very unusual and is clearly plausible just from the information you have at hand. As @DeNovo mentioned in a comment, it is also likely that the spread is not independent, because the children share several characteristics: they are related, so they share:
any genetic component to vulnerability
any characteristics of the father's illness such as the level of virus replicating in the father's lungs
perhaps the level of actual contact with the father and how well he may be effectively quarantined from the others
Those factors could make the joint probability across the two children closer towards the 10% rate for a single individual: once you know if the first child is infected or not infected, you can make a better guess about the second child based on all the possible shared characteristics of the children or the infected person.
To add to @BryanKrause 's answer re: rare events happen all the time, the children are not out of the woods yet. The mean incubation time for a primary VZV infection (the clinical syndrome known as chicken pox) is 14 days, but often lasts up to 21 days (see Murray Medical Microbiology, Ch. 53). The father is infectious while shedding virus, usually via the lungs. This correlates with the period of time a patient is febrile. I wouldn't say the father didn't infect his children until he has been afebrile for 21 days.
Apart from not getting infected due to pure chance (as mentioned already) there is one highly probable explanation (explanation, not overall chance).
People get, but don't show it:
asymptomatic infection
Asymptomatic infection is unusual, but some cases are so mild, they go unrecognised. The primary viraemic phase is followed by a secondary viraemia to the skin and the mucosal surfaces.
Chickenpox (varicella zoster) - Clinical Review, GP-online, 2014
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