Re: Daycare illnesses (how often does a child get sick?

kqr
Fri, 24 Apr 2026 06:14:25 GMT

The recent post on whether common diseases build immunity reminded me of the data collection I made when my children were young. It supports the idea that immunity does not build, but very weakly because n=2 and limited period of data collection. Here's a reproduction of the original article with light edits for the LW crowd.

Our first child was born mere weeks before covid​-19 came to our part of the world. As a result, they were effectively locked down for the first 1.5 years of their life until they started going to daycare. This had the effect that for those 1.5 years, they were not sick a single time.

At some point they turned three, and in total, had been sick 22 times until then. The average duration was 4.4 days, meaning us parents lost about 16 % of our working time caring for them when they were sick. Since we shared that burden fairly evenly, that’s 8 % for each person, or in practise only getting 37 hours of work out of a 40 hour work week.

If we plot this as a survival curve, it looks like this:

I feel like there are three sections on the curve:

0–30 days between illness: hazard rate 2.4 % per day.

30–55 days between illness: hazard rate 1 % per day.

55–100 days between illness: hazard rate < 0.1 % per day.

The last one is definitely a separate phenomenon: that was over the summer break. I suspect the first two are periods with more and less infections going around in society, so e.g. early winter will have a 2.5 % daily probability of getting sick, while mid-spring has an 0.8 % daily probability. These are of course very unscientific observations, but they may serve as a good starting point for inquiry.

Our second child has not grown up as secluded, and already been sick multiple times before starting daycare. I was curious whether their hazard rate would be lower when they started daycare (because acquired immunities) or if the curve above is sort of the background rate.

Here is the corresponding (significantly less dense) data for the second child, plotted together with the first.

The pre-exposed child has been sick more frequently than the isolated one!

This difference is nowhere near significant: the log-odds difference is 0.27, but the standard error is a whopping 0.40!

If it was significant, it would maybe be an interesting cue as to how much individual variation affects hazard rate – two siblings, meaning similar genetics and upbringing, still have a hazard rate difference of about one percentage point. That would translate to one being sick 16 more days of the year than the other!

Discuss