The COVID Report with David Blake: Inherent risk of transmission and school safety

Wednesday, Aug. 5

Most of the nation, at a state level, has a percentage of people currently infected between 1.5 percent and 0.5 percent.

The ability to safely hold school depends on the inherent risk of transmission at school, the mitigation and surveillance applied at school, and the transmission in the community.

The community transmission relates to the likelihood that someone in a given classroom will be currently infected. At a 1-percent level of infection, the probability in a room of 20 people is 19 percent. Every fifth classroom of 20 people will have someone infectious in it. If only a few of them are able to transmit, but they transmit to quite a few others in the same class, the situation will grow out of control quickly. (Note: Columbia County schools opened on Monday, Aug. 3, and already positives have been recorded at two high schools.)

Now suppose the community transmission is 0.1 percent. New Hamshire, Maine and Vermont are at this level now. The chances of hacving someone transmissible in a classroom of 20 is instead only 2 percent. If we do something like test all the students twice a week, keep students in masks, and don’t let them live together in large groups or use communal bathrooms, it should be possible to keep such a situation stable.

Most of the nations have adopted a set of restrictions that allows reasonable mobility in summer months when school is not in session without allowing transmission to grow rapidly.

School really changes the situation, and the community transmission required to make in-person school work is much lower. Unfortunately, achieving the levels needed in most states really requires about eight weeks with new restrictions. Here is a plot of R and the percent currently infected in all 50 states.


Contact Dave Blake.