I remember looking into exactly this question when my wife and I were looking into pros and cons of daycare. One thing that I think the analysis here misses is that this is generally worst for the first 1 or 2 years, and then much less so. I don’t remember exactly what studies I was looking at back when I was researching this, but asking Claude just now “Is there a study that looks at the frequency of child illnesses by year after first enrolled in daycare?” yielded the following references:
I haven’t gone through any of the above links in detail just now, but the general message one gets from the abstracts seems to be an increase in frequency for years 1-2, then back to baseline. Some suggest some protective effect in early elementary school years (the first link, which is the Tuscon study OP mentioned; the Côté paper that the CNN article pointed to; and apparently the Hullegie et al. 2016 study OP mentioned, which wasn’t among those that Claude dug up).
The Søegaard et al. study highlighted by the OP has an interesting couple of figures 1 and 2, for boys and girls respectively. These are differences in infection rate per year for four groups, compared to children never in childcare. Since this is Denmark I’m guessing the “instition enrollment at 3 yrs” is kids who started børnehave (preschool) at age 3.
This does look like it shows some amount of immunity happening: otherwise, we’d presumably expect to see group (b) having a spike as high as group (d) at age 3 yrs. Though importantly it isn’t enough to compensate if what you care about is total number of illnesses avoided. [1]
Also, although the spikes look quite dramatic, the y-axis shows that the difference in infection rate per year is approximately 1 for the highest spike in each graph. Similarly, the abstract notes that children enrolled in childcare before age 12 months had experienced 0.5 − 0.7 more infections than peers enrolled at 3 years, cumulatively, by the time they got to age 6 years. To be sure, that certainly corresponds to more than one actual infection, since “infection” in this paper means an infection serious enough to result in an antimicrobial (usu. an antibiotic) being prescribed, but is not an enormous effect.
Regarding the beliefs and confidences listed in the post:
(Quite confident) The most common illnesses (colds and flu) don’t build immunity in general (in kids or adults) because they mutate every year
(Quite confident) The same illness has a greater risk of complications in babies vs. older children and adults
(Moderately confident) The same illness has a greater duration in babies vs. older children and adults
(Moderately confident) Illness during early development is probably more harmful than illness during adulthood
(Weak guess) Daycare environments are more conducive to disease spread than schools for older kids and the number of possible illnesses is very high; there isn’t just a limited number of things you catch once
For #1, I think my level of agreement depends on exactly what is meant by “immunity in general”. Claude’s answer to “Does catching viruses improve your immune system long term?” can be summed up as (in Claude’s words): “Surviving one virus generally makes you better at fighting that specific virus (and sometimes closely related ones). It doesn’t broadly upgrade your immune system’s ability to handle unrelated threats.” This matches my previous understanding.
However, due to the caveat about “and sometimes closely related ones”, I think this is consistent with the claims of lower rates of illness in early school ages reported by Tuscon / Côté / Hullegie, and the difference between lines (b) and (d) at age 3 in the Søegaard graphs. My understanding is that even though viruses mutate all the time, many remain “closely related” to the versions they mutated from, and this confers some protection from infection and/or severity. For example, if I remember correctly from back when I was doing a lot of reading on COVID, the consensus was that after repeated and significant mutations, protection given by a vaccination based on an older strain gave limited or no protection from infection (no longer recognized by B cells), but still gave significant protection from severe infection, since the epitopes recognized by T cells remained consistent. Something like this goes for flu viruses also (look up “heterosubtypic immunity”) and I believe common cold coronaviruses too.
That said, I am a bit baffled by the lack of any dip at all for the nursery groups below the baseline at age 6 (when Danish compulsory education starts) in the Søegaard graphs.
#2 seems true for most illnesses, and seems likely to be an underappreciated consideration. My understanding is that children under 1 year old are particularly vulnerable.
I hadn’t really thought about #3-4 and haven’t taken the time to dig up relevant literature to see if I agree, but they seem plausible: if true, then they should also inform one’s calculus.
Less certain about #5 (seems likely to be technically true, but not sure it moves me very much one way or another given my beliefs on the others and the data from the studies above).
An additional consideration is that there is some evidence that catching COVID can have long-term negative effects on the immune system, although COVID is also weird in that children fare better than adults with it overall.
So, considerations pointing in both directions. I will say that in our own case I am happy we made the decision to start our son in daycare shortly after his first birthday, particularly given that our alternative was one parent quitting a fulfilling job to stay at home (IIRC there were no available full-time nannies or au pairs in our area, or at least none at a cost we felt we could remotely afford). This would have been a financial hit that probably would have required us to take on substantial debt, and also would have been incredibly challenging. Additionally, the level of different experiences and socialization our son gets on a daily basis is well beyond what we could realistically provide on our own, and he loves it.
So for our family, daycare has been worth the illnesses. But of course we would be biased to prefer the decision we actually made, and might feel differently if we’d had a worse experience. And I won’t pretend that the first year of it was easy: due in part to her asthma, my wife got pneumonia twice. (We’ve used this as an excuse to get the daycare to allow us to wait to pick him up outdoors, rather than in the cramped coatroom in which every other child and parent breathes in from 5.30-6pm).
Regarding the reference group and the weird increase after age 14, the authors write:
We observed slight increases in the infection rates and cumulative number of infections at ages 14–19 years among children enrolled in childcare during the first 3 years of life compared with peers in homecare, which were most pronounced among girls. However, these increases were proportional between the different enrolment types (ages at enrolment and types of facilities) and thus appeared to be a phenomenon related to the reference group comprising a small number of children who remained in homecare during the first 6 years of life (0.7% of the total cohort). Thus, although we adjusted for maternal education and income, maternal smoking, maternal age at delivery, ethnicity and child’s diagnoses of chronic diseases among other factors, we cannot rule out that the observed increasing rates and cumulative number of infections in adolescence associated with childcare attendance were due to residual socioeconomic confounding.
Similarly, the abstract notes that children enrolled in childcare before age 12 months had experienced 0.5 − 0.7 more infections than peers enrolled at 3 years, cumulatively, by the time they got to age 6 years. To be sure, that certainly corresponds to more than one actual infection, since “infection” in this paper means an infection serious enough to result in an antimicrobial (usu. an antibiotic) being prescribed, but is not an enormous effect.
I wonder why you conclude this ‘is not an enormous effect’. Maybe it’s differences in antibiotic usage between countries, and you’re from a country where it’s more common? Because for me it seems like a really big deal to have like 1 in 2 kids needing an extra treatment of antibiotics before they’re 6 years old.
I think Denmark has lower general usage of antibiotics than the United States for example (I think 70% as much, if I just divide the Defined Daily Dose by the population.) And I guess the gap is bigger for smaller issues, as I expect more critical situations to always result in antibiotics usage whether you’re in the US or Denmark.
I’m not an expert in this field, nor did I check the statistics carefully, this is mainly based on the feeling I get when foreigners come to the Netherlands (60% DDD per person antibiotics usage compared to Denmark) and complain that they don’t get antibiotics when they get the sniffles.
I remember looking into exactly this question when my wife and I were looking into pros and cons of daycare. One thing that I think the analysis here misses is that this is generally worst for the first 1 or 2 years, and then much less so. I don’t remember exactly what studies I was looking at back when I was researching this, but asking Claude just now “Is there a study that looks at the frequency of child illnesses by year after first enrolled in daycare?” yielded the following references:
https://jamanetwork.com/journals/jamapediatrics/fullarticle/191522
https://pubmed.ncbi.nlm.nih.gov/2007922/
https://pubmed.ncbi.nlm.nih.gov/11296076/
https://pmc.ncbi.nlm.nih.gov/articles/PMC5588939/
a CNN article that pointed to https://pubmed.ncbi.nlm.nih.gov/21135342/
and an Emily Oster blog post which links to some other relevant studies.
I haven’t gone through any of the above links in detail just now, but the general message one gets from the abstracts seems to be an increase in frequency for years 1-2, then back to baseline. Some suggest some protective effect in early elementary school years (the first link, which is the Tuscon study OP mentioned; the Côté paper that the CNN article pointed to; and apparently the Hullegie et al. 2016 study OP mentioned, which wasn’t among those that Claude dug up).
The Søegaard et al. study highlighted by the OP has an interesting couple of figures 1 and 2, for boys and girls respectively. These are differences in infection rate per year for four groups, compared to children never in childcare. Since this is Denmark I’m guessing the “instition enrollment at 3 yrs” is kids who started børnehave (preschool) at age 3.
This does look like it shows some amount of immunity happening: otherwise, we’d presumably expect to see group (b) having a spike as high as group (d) at age 3 yrs. Though importantly it isn’t enough to compensate if what you care about is total number of illnesses avoided. [1]
Also, although the spikes look quite dramatic, the y-axis shows that the difference in infection rate per year is approximately 1 for the highest spike in each graph. Similarly, the abstract notes that children enrolled in childcare before age 12 months had experienced 0.5 − 0.7 more infections than peers enrolled at 3 years, cumulatively, by the time they got to age 6 years. To be sure, that certainly corresponds to more than one actual infection, since “infection” in this paper means an infection serious enough to result in an antimicrobial (usu. an antibiotic) being prescribed, but is not an enormous effect.
Regarding the beliefs and confidences listed in the post:
(Quite confident) The most common illnesses (colds and flu) don’t build immunity in general (in kids or adults) because they mutate every year
(Quite confident) The same illness has a greater risk of complications in babies vs. older children and adults
(Moderately confident) The same illness has a greater duration in babies vs. older children and adults
(Moderately confident) Illness during early development is probably more harmful than illness during adulthood
(Weak guess) Daycare environments are more conducive to disease spread than schools for older kids and the number of possible illnesses is very high; there isn’t just a limited number of things you catch once
For #1, I think my level of agreement depends on exactly what is meant by “immunity in general”. Claude’s answer to “Does catching viruses improve your immune system long term?” can be summed up as (in Claude’s words): “Surviving one virus generally makes you better at fighting that specific virus (and sometimes closely related ones). It doesn’t broadly upgrade your immune system’s ability to handle unrelated threats.” This matches my previous understanding.
However, due to the caveat about “and sometimes closely related ones”, I think this is consistent with the claims of lower rates of illness in early school ages reported by Tuscon / Côté / Hullegie, and the difference between lines (b) and (d) at age 3 in the Søegaard graphs. My understanding is that even though viruses mutate all the time, many remain “closely related” to the versions they mutated from, and this confers some protection from infection and/or severity. For example, if I remember correctly from back when I was doing a lot of reading on COVID, the consensus was that after repeated and significant mutations, protection given by a vaccination based on an older strain gave limited or no protection from infection (no longer recognized by B cells), but still gave significant protection from severe infection, since the epitopes recognized by T cells remained consistent. Something like this goes for flu viruses also (look up “heterosubtypic immunity”) and I believe common cold coronaviruses too.
That said, I am a bit baffled by the lack of any dip at all for the nursery groups below the baseline at age 6 (when Danish compulsory education starts) in the Søegaard graphs.
#2 seems true for most illnesses, and seems likely to be an underappreciated consideration. My understanding is that children under 1 year old are particularly vulnerable.
I hadn’t really thought about #3-4 and haven’t taken the time to dig up relevant literature to see if I agree, but they seem plausible: if true, then they should also inform one’s calculus.
Less certain about #5 (seems likely to be technically true, but not sure it moves me very much one way or another given my beliefs on the others and the data from the studies above).
An additional consideration is that there is some evidence that catching COVID can have long-term negative effects on the immune system, although COVID is also weird in that children fare better than adults with it overall.
So, considerations pointing in both directions. I will say that in our own case I am happy we made the decision to start our son in daycare shortly after his first birthday, particularly given that our alternative was one parent quitting a fulfilling job to stay at home (IIRC there were no available full-time nannies or au pairs in our area, or at least none at a cost we felt we could remotely afford). This would have been a financial hit that probably would have required us to take on substantial debt, and also would have been incredibly challenging. Additionally, the level of different experiences and socialization our son gets on a daily basis is well beyond what we could realistically provide on our own, and he loves it.
So for our family, daycare has been worth the illnesses. But of course we would be biased to prefer the decision we actually made, and might feel differently if we’d had a worse experience. And I won’t pretend that the first year of it was easy: due in part to her asthma, my wife got pneumonia twice. (We’ve used this as an excuse to get the daycare to allow us to wait to pick him up outdoors, rather than in the cramped coatroom in which every other child and parent breathes in from 5.30-6pm).
Regarding the reference group and the weird increase after age 14, the authors write:
I wonder why you conclude this ‘is not an enormous effect’. Maybe it’s differences in antibiotic usage between countries, and you’re from a country where it’s more common? Because for me it seems like a really big deal to have like 1 in 2 kids needing an extra treatment of antibiotics before they’re 6 years old.
I think Denmark has lower general usage of antibiotics than the United States for example (I think 70% as much, if I just divide the Defined Daily Dose by the population.) And I guess the gap is bigger for smaller issues, as I expect more critical situations to always result in antibiotics usage whether you’re in the US or Denmark.
I’m not an expert in this field, nor did I check the statistics carefully, this is mainly based on the feeling I get when foreigners come to the Netherlands (60% DDD per person antibiotics usage compared to Denmark) and complain that they don’t get antibiotics when they get the sniffles.
https://www.pnas.org/doi/10.1073/pnas.2411919121#supplementary-materials