Gov. Mike Parson and the Missouri Department of Health and Senior Services recently identified two key metrics that they feel counties should use to determine the risk of COVID spread and to guide …
Gov. Mike Parson and the Missouri Department of Health and Senior Services recently identified two key metrics that they feel counties should use to determine the risk of COVID spread and to guide policy decisions by local officials:
The 2 key measures they expect us to monitor are:
1. PCR 7-day positivity rate using CDC method (number of positive PCR tests in 7 days divided by all PCR tests in last 7 days); and,
2. 7-day case rate per 100K (Number of new cases of COVID divided by the county’s population, and then multiplied by 100,000).
PCR 7-day positivity rate using CDC method
The percent positive will be high if the number of positive tests is too high, or if the number of total tests is too low. A higher percent positive suggests higher transmission and that there are likely more people with coronavirus in the community who haven’t been tested yet.
The percent positive is a critical measure because it gives us an indication how widespread infection is in the area where the testing is occurring — and whether levels of testing are keeping up with levels of disease transmission.
A high percent positive means that more testing should probably be done — and it suggests that it is not a good time to relax restrictions aimed at reducing coronavirus transmission. Because a high percentage of positive tests suggests high coronavirus infection rates — due to high transmission in the community — a high percent positive can indicate it may be a good time to add restrictions to slow the spread of disease.
The higher the percent positive is, the more concerning it is.
As a rule of thumb, however, one threshold for the percent positive being “too high” is 5 percent.
There are two ways to lower the percent positive:
• Reduce the amount of coronavirus transmission, or;
• Increase the number of people who get tested.
Fortunately, these two things often go hand-in-hand.
If a place is doing more testing — and responding appropriately to positive tests by making sure that people who might be contagious are isolated, for example — the amount of transmission should go down over time. But even without testing, measures such as stricter regulations regarding wearing masks, physical distancing, and avoiding large gatherings are all effective ways to reduce transmission.
7-day case rate per 100K
While the rate of positivity is of critical importance, it does not tell us how many people in a given area are testing positive each day or over a week. Gasconade County Health Department reports weekly the number of new cases in our county.
However, the number of new cases is difficult to compare to different areas with different populations. In order to be able to compare case rates between different states or counties, we look at the average number of newly confirmed cases in the last seven days per 100,000 residents.
Using the population size in the calculation helps us more easily compare larger and smaller counties. A larger county would be expected to have more cases because of the larger population, but expressing the rate per 100,000 residents enables a more equal analysis.
Last Tuesday on Nov. 24, (the most recent day as of this writing that we had complete data for), we had 102(*) new cases over the previous seven days.
For the purposes of these calculations, the state uses 14,706 as our county’s population. So, to do the calculation, 102 is divided by 14,706 and then multiplied by 100,000 which equals a rate of 694 new cases over the previous seven days per 100,000 people.
This allows us to compare ourselves to much larger counties such as St. Louis where they have more cases, but also larger populations.
Per the Missouri COVID website, here are some other counties’ 7-day case rates per 100K for comparison:
• Cole County 370
• St. Charles County 364
• Franklin County 352
• St. Louis 242
What should I do when I am in a place with a high percent positive and/or high case rate per 100K?
Since this means that the level of coronavirus transmission in that area is likely high, you should be very careful (wearing masks, washing your hands, maintaining physical distance, and avoiding situations that may put you at risk for getting infected or infecting others).
You should also consider getting tested if you have any symptoms, or if you have not been distancing and are likely to be in contact with people who are at risk of getting very sick if they develop COVID-19.
(*) An asterisk explained
Health officials noted the 102 cases over the previous seven days were 58 positive PCR tests and 44 positive antigen tests.
Some will note that the Missouri State Covid-19 website lists a much lower number for Gasconade County cases on a given day. For example, on Monday (Nov. 30) the state’s site listed only 50 new cases in the past seven days for a rate of 340 per 100,000 residents.
However, cases of COVID-19 diagnosed by antigen testing were not included in their calculations.
Until very recently, antigen testing was not available in many places, and so the few cases found by that method of testing did not have a significant impact in the data for most counties. However, in Gasconade County, antigen testing is performed at Hermann Area District Hospital, our schools, assisted living facilities and nursing homes. And, about 40 percent of our new diagnoses are found with antigen testing.
For counties like ours, with a large percentage of antigen testing, we have been directed by John Bos, interim bureau chief at Missouri Department of Health and Senior Services, to redo the calculations and include all new cases of COVID-19 found by diagnostic testing (both PCR and rapid antigen).
This will allow us to get a more accurate number and be able to compare our case rate with other areas, until they are able to update their website to include new cases diagnosed by antigen testing.