Tracy Beth Hoeg,, Monica Gandhi and Lillian Brown
First, classrooms have thankfully been found — in studies examining schools in multiplestates — to be places of limited disease transmission, compared with communities at large. The rate of transmission within schools from individuals who test positive has been estimated to be on the order of 0.5 percent to 0.7 percent (and this includes people exhibiting symptoms).
A rate that low implies that a testing regimen would need to identify roughly 200 infected people to prevent one person from transmitting the disease in school. It would take an awful lot of tests to achieve those numbers. In New York City, where more than 234,000 asymptomatic students and staff members across approximately 1,600 schools were tested last fall, the overall rate of positive tests was only 0.4 percent. That suggests that — even during a time of high community spread — about 40,000 tests among asymptomatic individuals would need to be performed to prevent one in-school transmission.
And how accurate are these tests? Rapid antigen and saliva PCR tests, which are frequently used in schools, can have a false positive rate of 1 or 2 percent. That may sound low, but statisticians know that, when testing in a setting of low prevalence of disease, even a single-digit false-positive rate can be extremely problematic.
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which pushed Dane County this week not to calculate its percentage of positive tests — a data point the public uses to determine how intense infection is in an area.
While positive test results are being processed and their number reported quickly, negative test results are taking days in some cases to be analyzed before they are reported to the state.
The department said it was between eight and 10 days behind in updating that metric on the dashboard, and as a result it appeared to show a higher positive percentage of tests and a lower number of total tests per day.
The department said this delay is due to the fact data analysts must input each of the hundreds of tests per day manually, and in order to continue accurate and timely contact tracing efforts, they prioritized inputting positive tests.
“Positive tests are always immediately verified and processed, and delays in processing negative tests in our data system does not affect notification of test results,” the department said in a news release. “The only effect this backlog has had is on our percent positivity rate and daily test counts.”
Staff have not verified the approximately 17,000 tests, which includes steps such as matching test results to patients to avoid duplicating numbers and verifying the person who was tested resides in Dane County.
All 77 false-positive COVID-19 tests come back negative upon reruns.
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