Question: I only hear about the number of cases, but do we have an estimate of the number of contagious people at any time? Wouldn’t that be a helpful estimate to decide to open schools?
Answer: To my knowledge, we do not have an estimate of the number of people contagious at any given time. Instead, we generally rely on the number of daily cases, test positivity rate, and in some cases, the reproduction rate (R0) to understand whether the virus spread is growing or contracting. If we wanted to try to estimate the number of people who are infectious at any given time, we’d need to make several assumptions, including:
- At what stage of infection a person is when they are tested. Using data described in our Q&A of 9/4 #Infectivity, I suppose a fair assumption would be an infectious period of 10 days after a positive test.
- What proportion of infectious people are getting tested. To be confident that we are identifying most symptomatic and asymptomatic people, we need to be sure that we have widespread, easily accessible testing with test positivity rates <5%. If test positivity rates are higher, it means that we’re only seeing the tip of the iceberg and are not capturing the full extent of community transmission. For example, earlier in the pandemic, CDC found through seroprevalence surveys in March-May that “it is likely that greater than 10 times more SARS-CoV-2 infections occurred than the number of reported COVID-19 cases.” With testing more widespread, but test positivity still above 5% in the US as a whole (currently 5.4% 7 day rolling average), I’m not sure what the best estimate would be… For the sake of this post, I’ll make the estimate that for every positive test, there’s 1 more positive case (e.g. 1x higher than we’re finding).
- Number of people who recently tested positive. Here we want to see new daily cases.
For the US as a whole, we had 391,821 positive cases in the last 10 days. If we assume another 391,821 people are infected but not tested, then we have 783,642 infectious people in the country. You could then break this down further by smaller and smaller geographic units to get at the more community-specific estimate you’re interested in seeing. All that said, I’m not sure that this is all that helpful, because what matters SO MUCH is the degree to which these 780K+ individuals are interacting with others, how well these presumably infectious individuals are following public health protective measures, and how well non-infectious people are following public health protective measures. We know that some super-spreader events — like this recent wedding in Maine — can quickly and widely spread the virus. But how quickly the virus moves through a community is really very much about how people are interacting. This is where estimates of reproduction rates are so important!
In that vein, and as we discussed in our Q&A of 7/30 #Teachers, the school reopening indicators proposed by the American Federation of Teachers are 1) test positivity <5% and 2) reproduction rate (R0) <1. Back in late July, only 6 states met reopening criteria. I re-ran the numbers and 10 states plus DC now meet the criteria (Table 1) — California, Colorado, Connecticut, DC, Hawaii, Illinois, Maine, Massachusetts, Oregon, Rhode Island, and Vermont. Interestingly, of the 6 states that met the criteria end-July — Delaware, Maine, Michigan, New York, Ohio, and Vermont — only Main and Vermont still meet the criteria. So what does this say about schools ability to stay open if/when they do open?!
Table 1. American Federation of Teachers School Reopening