Health officials: Focus on weekly COVID-19 rates, not daily case totals

COLUMBIA, Mo. (KMIZ)

As COVID-19 cases continue to increase, local health departments focus on weekly COVID-19 positivity rates rather than positive daily cases.

Missouri Department of Health and Senior Services officials said on Wednesday the health department doesn't focus on daily numbers because “it represents a wide variation when specimens were collected, when tests were done, and when results are reported and entered.”

To calculate the positivity rate, take the number of positive cases divided by the total number of tests for that day.

Officials said while the state is experiencing increases in several counties, the increase in cases may be reported differently as a result of a backlog in testing. 

New cases that display on the state’s dashboard on a specific day may have actually been reported on a different day previously that week.

According to Columbia/Boone County Health Department data, the county's positivity rate is at 7.2% for the week.

The health department posts its new daily cases totals on its COVID-19 information hub, but officials said it focuses more on the weekly rate as well. 

“That really is a better way to look at it if your purpose and looking at it is to determine the overall impact of the overall spread of the virus in the community,” said Scott Clardy, Columbia/Boone County Public Health and Human Services Assistant Director. “Looking at it on a weekly basis or an every seven-day basis smooths out some of those peaks and valleys and gives you a better overall idea of what's going on.”

Clardy said the number of tests that are collected each day varies, which could also have an effect on the number of positives each day. 

“You could have a day where you had 25 positives and just 50 tests,” Clardy said. ‘That's a 50% positivity rate. You'd also have a day where you had 25 positives out of 100 tests. So that's a 25% positive positivity rate.”

Clardy said every time the health department looks at the positivity rate, it could be caused by a different issue. He said it’s important to look at all the variables and how they compare week to week. The spokesman said more could be testing positive overall, or that more people are testing positive and more tests are being done.

“We look at it on a weekly basis,” Clardy said. “Because it's a better measure to take into account all those other variables that could be impacting the rate on a day to day basis.”

Donna Seidel, Cole County Health Department’s Assistant Director of Nurses said the health department also focuses on the weekly rate rather than daily case numbers. 

“Over a week, time is going to average out to be more reflective of what's really going on as opposed to doing daily, when we may get a whole bunch of cases in one day and then not in the next,” Seidel said.

Seidel said that if people are looking at the daily cases numbers they should keep in mind the positivity rate, which she said remains low in Cole County.

She said there are more people testing positive for the virus, but that it does not seem that people’s reactions are severe right now as they are very mild signs and symptoms.

The spokeswoman said there are a lot of positive tests that are close contacts or travel-related, so she said it’s important to look at the whole picture. 

“Even though our case numbers are going up,” Seidel said. “The positivity rate is going down so that is a good thing and the fact that the death rate is also declining… in Missouri overall as well.”

Both Clardy and Seidel said that a state regulation requires COVID-19 testing labs to report certain laboratory test results to DHSS. The state then gives the test results to the county health department in which the person resides.

This is where backlog issues could affect the total number of daily cases reported in some of Missouri’s counties. 

Seidel said the DHSS backlog could have an effect on Cole county’s daily results, but Clardy said this does not affect Boone County's daily case numbers. He said the county has a local lab that also reports the results to Boone County right away.