McHenry County’s COVID-19 positivity rate is staying low despite a high rate of community transmission because of frequent testing in schools, McHenry County Public Health Nursing Director Susan Karras said.
Karras said the health department frequently gets questions about the county’s positivity rate and school testing, and she wanted to address it with the county’s public health and community services committee.
The county’s seven-day positivity rate was 2.6% as of Tuesday, according to the Illinois Department of Public Health.
“That’s not really a good metric to really look at transmission because if you have a lot of testing, that drives the denominator up and the positivity rate down,” Karras said of how the positivity rate is understood.
State data shows anywhere between 1,000 and 4,000 tests are being conducted each day in McHenry County over the past two months. The daily positivity rates have come back between 1% and 5%, but the long-term picture shows the county’s average positivity rate has declined significantly since the start of the school year in mid-August.
“We saw a huge decline in the positivity rate because of the schools,” Karras said.
Over the summer, McHenry County was testing between 500 and 1,000 people a day, according to state data. The county’s average positivity rate was 8.9% on Aug. 17, just as many schools were beginning to return. It has dropped by about six percentage points since then.
Many schools in the county and around Illinois have a test-to-stay program, where students exposed to COVID-19 can return to school after taking several negative tests. Karras said this amount of testing at a frequent rate is helping keep the positivity rate down.
Despite the low positivity rate, the county’s incidence rate remains high, reaching 127.72 new cases over seven days per 100,000 people as of Sunday, according to data from the health department.
For the county to move to the moderate category in which masking no longer is recommended by the Centers for Disease Control and Prevention, the incidence rate would need to drop below 50 cases. It has not been that low since July 22.
Karras said looking at the incidence rate and overall transmission rate paints a better picture of where the county is with the virus than the positivity rate, which is affected by the high testing rates in schools.