Red Light, Green Light
Knox County Health Department epidemiologists explain the numbers behind their coronavirus tracking system.
The Knox County Health Department on Friday released detailed information on the benchmarks it’s using to determine whether to move forward with the phased reopening of the local economy.
The Health Department is using statistical calculations to arrive at the simplified signs about how the COVID-19 response is playing out.
The benchmarks, which have been listed on the Health Department’s website since May 1, are to be used to inform decisions about the county’s response to the novel coronavirus pandemic.
The benchmarks came without an indication of how each would be evaluated, however. The Health Department solicited questions from the media about the benchmarks last Monday, and its team of epidemiologists responded with a nine-page document explaining their use on Friday.
The Health Department has posted five benchmarks on its website:
- Sustained reduction or stability in new cases for 14 days;
- Community-wide sustained and increased diagnostic testing with consistent or decreased test-result reporting turnaround time;
- A sustained or increased public health capacity;
- Healthcare system capabilities that remain within current and forecasted surge capacity;
- And a sustained or decreased COVID-19 related death rate for identified positive or probable cases.
The Health Department’s epidemiology team is led by Roberta Sturm and includes Mark Prather and Alicia Verlinde.
“In choosing benchmarks, we selected local data that is refined for the purpose of outbreak detection, mitigation and response,” the team wrote. “These metrics are meant to be a compilation of different data elements with no hard/fast upper limit or threshold. Instead, these data points are evaluated for acceptable or allowable limits based on historical data. Additionally, metrics may be refined as situations change.”
Accompanying each benchmark is a “traffic signal” icon as a visual indicator of Knox County’s progress. Green means that Knox County is meeting the benchmark, yellow indicates that the tracked statistics are moving away from attainment. “Red signifies trends are not moving toward benchmark attainment and may indicate the need for mid-phase adjustments,” the team wrote.
Mid-phase adjustments could include closing some businesses that were allowed to open on May 1 even before the conclusion of the 28-day Phase I of the reopening. The 28-day time frame for each phase was determined by doubling the 14-day incubation period for COVID-19 to provide enough time to determine how reopening economic activity affects the course of coronavirus contagion.
“Decisions about how to move through the phases or whether to institute mid-phase adjustments will not be made based on any one number or figure,” the epidemiologists cautioned.
They said they would look at multiple data points and trends, while employing public health expertise and taking into account developments in science and technology.
“Data is evaluated to determine acceptable increases. We typically look for a three-day trend in data as one day does not constitute a trend,” they wrote.
The traffic signals for all five benchmarks were green as of Sunday.
The first benchmark, a sustained reduction or stability in new cases for 14 days, is an indicator of community spread of COVID-19. The calculation is based on a rolling average of new confirmed positive cases over 14 days.
To move from green to yellow, there has to be a shift of 1.5 standard deviations above the mean for three consecutive days. Moving to red would take a three-day shift of 3 standard deviations above the mean. As of Sunday, the 14-day mean was 4.71 new cases.
Using the formula, Knox County would have had to see more than 10 new cases a day for three days to earn a yellow light and 16 new cases for three days to reach red-light status. The actual numbers of new cases over the past three days have been three, six and two.
Numbers alone won’t be the only criteria, according to the team. “An increase of new cases that are unrelated to known cases or not part of a cluster, over a period of time would be cause for concern,” the epidemiologists wrote. “Additionally, an increase in a subset of the population, such as in the elderly could be a reason to react.”
Testing is an indicator of the Health Department’s ability to detect new cases and effectively respond to outbreaks, the epidemiologists wrote. Like the new-case metric, measuring the testing levels uses a formula based on a 14-day mean of total tests. A reduction in testing levels is needed to change the traffic-light color.
Assessing the workforce available for contact tracing, which is key to containing the spread of the coronavirus, is essential for managing the response, according to the epidemiologists. A green light means no new staff are required for contact tracing, a yellow light means more Health Department staff are needed and red means non-Health Department workers need to be brought in.
“That number is dependent on the number of cases we have,” the epidemiologists explained. “If case counts increase, our Epidemiology Response team will increase personnel. If case counts decrease, the response team doesn’t need as many individuals.”
The Health Department estimates the personnel needs for case management range from four to 15 per 100,000 of the population. That translates to approximately 16-75 investigators for Knox County, along with additional personnel to monitor cases and contacts, enter data, and provide data analysis, which would be a total of approximately 100-120 people.
“However, our current caseload does not warrant 100-120 individuals on this team,” the team wrote.
As of Sunday, there were 49 active cases in Knox County, with two requiring hospitalization.
According to the Health Department’s website, there are 31 people currently assigned to the Epidemiology Response Team. Another 50 are either trained or could be quickly trained to join the team, giving the Health Department a total of 81 people to draw from.
A further cadre of 166 county employees could be used as well, giving the Health Department a 247-person pool of potential response team members.
The healthcare system capacity metric tracks the availability of all hospital beds, intensive care unit beds and ventilators in the 16-county region, with an eye toward absorbing a surge in COVID-19 cases. Again, the Health Department is using a formula based on standard deviations to a 14-day mean to assess an ever-changing capacity.
Currently, according to the Health Department website, there are 1,049 hospital beds, 72 ICU beds and 171 ventilators available for COVID-19 patients in the region.
The team is tracking the death rate, too, though fatal cases of COVID-19 in Knox County have been few. Only five people have died of complications from the disease so far in Knox County. Knox County is the third most populous county in Tennessee but tied for eighth in COVID-19-related deaths.
The low death toll means that the Health Department doesn’t have much to work with. “Fortunately,” the team wrote, “we do not have enough data to adequately demonstrate a trend at this time.”
The Health Department is not relying on infectious disease models that predict the future course of the pandemic based on a variety of factors. The Health Department is looking at several models however, including some developed at the University of Tennessee, Vanderbilt University, Columbia University and the University of Washington.
“We are not utilizing model data in our benchmarks due to the inconsistency in their findings,” the team explained, “and you will not find them referenced in our plan.”