Large study from Scotland finds exposure to children is protective for adults.

They looked at 300,000 NHS workers in Scotland.

https://www.medrxiv.org/content/10.1101/2020.09.21.20196428v1

Conclusion: Increased household exposure to young children was associated with an attenuated risk of testing positive for SARS-CoV-2 and appeared to also be associated with an attenuated risk of COVID-19 disease severe enough to require hospitalisation.

This is consistent with similar findings in a smaller study in Germany back in July. https://www.medrxiv.org/content/10.1101/2020.07.20.20157149v1

The theoretical mechanism here is exposure to other viruses -- likely common cold coronaviruses -- that provide some level of cross-reactive immunity. This is how the new Scotland paper puts it.

They also point out that the apparent protective effect of exposure to young children means school closures tend to INCREASE the health risk to teachers, who are denied the benefit of exposure to children.

I've been saying this since the German paper in July. https://twitter.com/kerpen/status/1288170003209039877