Should we be surprised that “young people” make up a substantial amount of coronavirus cases?

I’ve noticed media outlets are reporting that young adults make up a significant share of coronavirus cases with an air of incredulity.

My local paper posted on Facebook that “More than 50% of coronavirus cases in Massachusetts are people under the age of 50.” Very similarly, the Pittsburgh Post Gazette writes that “more than half of Pennsylvania’s confirmed COVID-19 patients are under 50 years old.” The New York Times, for its part, reports that “nearly 40 percent of patients sick enough to be hospitalized were age 20 to 54.” *

I can’t decide if this is a psyop to get young people to take the epidemic more seriously (as numerous spring-break photos show they should) or genuine surprise. If it’s the latter, I’m not sure if that’s warranted.

In each case, the age ranges in question are massive and not very meaningful without comparison to the age distribution of the general population. For example, in Massachusetts, about 63% of the population is under the age of 50. So if the incidence of coronavirus were age-independent, we might actually expect more cases among people under 50.

I think the issue, then, is that people seem to be assuming prevalence of the virus should be age-dependent to a higher degree than we’re observing. Maybe there’s a good Bayesian case to be made for this null hypothesis; I don’t know. But I feel like laypeople — local papers included — ought to be preceding with the assumption of age-independence, especially because we still don’t have much information.

Also, what’s going on with the under-20 crowd, which makes up 23% of the population but only 2.2% of MA coronavirus cases? Is Gen Z+ holding out on us?

The typo in the y-axis comes from the MA DPH data set, which has decided there are no 19-year-olds in the Bay State.

* This isn’t as egregious as the other two examples. It’s still a huge age range: about 48% of America is between the ages of 20 and 54. But since we’re talking about the severity of symptoms and hospitalizations, it seems much more noteworthy.

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