This article from The Intercept cites some recent research studies that put some numbers behind what level of community spread would make opening schools unsafe. The basic idea is that school (especially elementary school) is pretty safe when the level of infection in the community is relatively low, because kids coming to school are not that likely to be infected. But when the level of infection in the community rises, kids coming to school are more likely to be infected and further accelerate the spread.

Even educated people in the general public have a hard time with unit conversions, and this article switches between various units within the article. Come on, guys. Anywhere, here are the numbers from a variety of sources in the article. I’ve done the unit conversions (correctly, I think, but this blog post does not constitute medical advice…)

• 36-44 per 100,000 population per week (~5-6 per 100,000 per day)
• 147 per 100,000 per week (21 per 100,000 per day)
• 35 per 100,000 per week (5 per 100,000 per day)

That seems like a pretty big range, and I am also suspicious whether the reporters have carefully checked the math, given how they jump around even within the article. But let’s assume they have it right. The threshold is somewhere between 35-147 cases per 100,000 per week. The Pennsylvania Department of Education recommends a threshold of 100 cases per 100,000 per week to consider in-person K-12 school. (Although private and parochial elementary schools have been open throughout the pandemic, and public school districts are hit or miss.) The official number for Philadelphia county at the moment (I’m writing this on January 7), which they only update once a week, is 225.9 per 100,000 per week and falling. My unofficial 7-day running average of the numbers the Philadelphia Health Department reports in its daily press releases is 235.0 per 100,000 per week and falling (but looking at a plot, I would say it’s bouncing around and not clearly rising or falling this week). Those of us with children in public school have not had the option of in-person school so far during this school year.