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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.

https://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.

user1721135
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    Adults with primary VZV infection (chickenpox syndrome) are usually VERY sick, often with interstitial pneumonia, and often require hospitalization. Are you certain he didn't have secondary VZV infection (shingles syndrome)? – De Novo Feb 18 '19 at 23:40
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    How infectious you are also depends on your behavior. Your BIL will be much less infectious if he’s using proper hygiene like coughing/sneezing into his elbows, washing hands after using a tissue or scratching itches, keeping distance while talking etc. Statistics can’t account for individual cases. – Michael Feb 19 '19 at 07:55
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    If this were in the Puzzles SE, I'd say the answer is in this sentence... 'My brother in law got chickenpox, yet somehow he didn't infect my two nephews,'. It could be your other brother, so he's not a close contact of HIS nephews... I got the wrong end of the stick here. – AJFaraday Feb 19 '19 at 09:03
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    @DeNovo that still not a given. Especially if their brother in law is young. I've contracted chicken pox in my 20s, and while it wasn't as light as it could be if I was a kid, I've only dealt with a approx. a week of high temperature and blisters (not really itchy). I had colds that made me feel worse. – Dan M. Feb 19 '19 at 11:12
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    I don't think it's shingles, but even if, why would it matter? – user1721135 Feb 19 '19 at 11:31
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    @user1721135 because shingles is a different syndrome, with its own, (not as well studied) transmission risk. This is a different question: "my brother in law got shingles, yet somehow he didn't infect my two nephews." – De Novo Feb 19 '19 at 14:05
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    @DanM yes, there is certainly variability. I'm glad your case wasn't very bad! – De Novo Feb 19 '19 at 14:07
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    How sure are you the nephews haven't caught it? Chickenpox is less severe in the young, and natural variability sometimes means it's almost asymptomatic. My aunt's case of chickenpox when she was quite young manifested as a single bump on her arm, with no other symptoms; the bump probably would have gone unnoticed if other members of the family hadn't come down with chickenpox at the same time. It's entirely possible the nephews caught it first, had asymptomatic cases, and infected the father. – ShadowRanger Feb 19 '19 at 15:25
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    Is there a reason all answers assume the kids haven't been vaccinated and are therefore immune? Did i miss something in the description? – Tim Nevins Feb 19 '19 at 16:44
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    @Tim Nevins: yes you missed this sentence in the description: “they have not had the disease yet nor are they vaccinated against it.” – Laurent R. Feb 19 '19 at 17:52
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    I agree with @ShadowRanger: my plan for giving my kids lifelong natural immunity (so that they wouldn’t have to rely on a vaccine launched before long term studies on immunity persistence and the potential need for lifelong boosters were completed - back in the 00s) was exposure while very young, and if no objective signs showed, get a titer around age 14-15, and vaccinate if the titers were negative. In this way I was able to tell many families that their child did have lifelong natural immunity, despite no recollection of objective disease, because some kids are virtually asymptomatic. – Laurent R. Feb 19 '19 at 18:07
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    @LaurentR.: I missed that too somehow, and my first reaction was to answer "because this is 2019 not 1980, and kids don't get chickenpox anymore"... – R.. GitHub STOP HELPING ICE Feb 20 '19 at 02:48
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    What's missing from this question is the age of the children. When I was very young, 1 or 2, my older brother caught chicken pox from his day care centre. He had a slight temperature and many spots. I had no temperature and no more than 5 spots over my entire body. My father had a high fever and joint pains, and was bed-ridden. Age is an important factor to mention! – CJ Dennis Feb 20 '19 at 05:04
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    "Nor are they vaccinated" is there are chickenpox vaccine? I had no idea. – Aethenosity Feb 20 '19 at 07:32
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    Yes, but not in the UK for some reason. The NHS is hardcore antivaxx (jk). – user1721135 Feb 20 '19 at 08:21
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    For anyone interested: the children did get infected after all. – user1721135 Mar 13 '19 at 23:38
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    Thank you for updating us. Great question! Looks like you chose the right answer to accept since @DeNovo was the only one to point out the other shoe could still drop. – Carey Gregory Mar 14 '19 at 01:44

3 Answers3

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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.

Bryan Krause
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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.

De Novo
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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

Community
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LаngLаngС
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    Yes, this is an important additional piece of information. The 90% figure is almost certainly based on serology, not the appearance of a rash. Thank you for adding this! – De Novo Feb 20 '19 at 13:53