When enrolling in daycare, there’s a spike of infections, then a decay afterwards, which you would expect if you started developing resistance to infections. So they are acquiring immunity, that is clear; the question is whether this acquired immunity is (on net) beneficial.
It’s also about how long lasting the immunity is. If the immunity lasts 1-2 years, you aren’t gaining that much.
If the immunity is short-lived, we would not expect the decay to persist (which is does to age 13 independent of age of entry into daycare, Figure 1 and 2). It takes about 3 years for the number of infections to decay back to baseline (no more infections than children who do not go to daycare); if immunity was short-lived, we could expect a spike as one is exposed to new pathogens, then settling to a low baseline level of infection as immunity wanes and reinfection occurs, but we don’t see a higher baseline (or if the baseline is higher, it is hard to see).
One could argue that the decrease in infections is due to age-dependent immune system development instead of an adaptive response, but then the different age-of-enrollment decay curves should overlap, which they do not. Later enrollment does lower the magnitude of the initial spike, which suggests that there might be an effect, but the non-overlapping decay curves implies that adaptive immunity is playing a role.
It’s also about how long lasting the immunity is. If the immunity lasts 1-2 years, you aren’t gaining that much.
If the immunity is short-lived, we would not expect the decay to persist (which is does to age 13 independent of age of entry into daycare, Figure 1 and 2). It takes about 3 years for the number of infections to decay back to baseline (no more infections than children who do not go to daycare); if immunity was short-lived, we could expect a spike as one is exposed to new pathogens, then settling to a low baseline level of infection as immunity wanes and reinfection occurs, but we don’t see a higher baseline (or if the baseline is higher, it is hard to see).
One could argue that the decrease in infections is due to age-dependent immune system development instead of an adaptive response, but then the different age-of-enrollment decay curves should overlap, which they do not. Later enrollment does lower the magnitude of the initial spike, which suggests that there might be an effect, but the non-overlapping decay curves implies that adaptive immunity is playing a role.