What the positivity rate does, does not tell us about COVID-19

By , Daily Memphian Updated: July 30, 2020 11:33 AM CT | Published: July 30, 2020 4:00 AM CT

In a year when COVID-19 dramatically altered the world, the term positivity rate emerged as the barometer for measuring the pandemic’s impact.

An area’s positivity rate is calculated by the percentage of patients testing positive for coronavirus of all those tested for the virus.

Shelby County’s overall positivity rate surpassed 10% this week, a concerning number local health experts wanted to avoid on a daily basis, much less the overall rate.

Shelby County overall coronavirus positivity rate hits 10%

Unlike total COVID-19 case counts or tests performed, the positivity rate has not steadily increased from March to July. The overall positivity rate was 6.8% on June 10, the lowest during the pandemic. That means the positivity rate increased by 3.4% over the past six weeks.

Increased community transmission in Shelby County is largely attributed as the cause, but additional context is needed when discussing a metric frequently cited by public health experts and elected officials alike.

What does the positivity rate mean?

Public health experts and elected officials often use various forms of the positivity rate when discussing its impact in Shelby County.

One form is the daily positivity rate, the percentage of positive cases reported in a single day.

Over the past month, Shelby County recorded a daily positivity rate below 10% only once – 9.7% on July 11.

Unsurprisingly, that period directly coincided with Shelby County’s overall rate increasing from 7.7% on June 28 to 10.2% on Wednesday. The same span where the new case average increased significantly, and the five highest daily increases all occurred this month.

New Shelby County COVID-19 cases increased by 96% in July

Though there is an important caveat when discussing an updated positivity rate reported by the health department each morning.

For example, the daily positivity rate of 15.6% on Tuesday, July 28, was from test results from several days, said Dr. Bruce Randolph, Shelby County Health Department health officer.

There is a currently backlog in processing test results locally, so a single day’s report can have tests taken from five to seven days or even a greater period.

“I don’t put a lot of credence in the percent positivity or number of cases that we get on a daily basis,” said Dr. Scott Strome, executive dean of the University of Tennessee Health Science Center College of Medicine. “We like to follow overall trends.”

Shelby County’s overall positivity rate provides a more complete picture of the COVID-19 pandemic locally.

For months, health experts in Shelby County wanted that percentage below 10%. With enhanced community transmission, it’s surpassed that original benchmark.

Dr. Manoj Jain, infectious disease expert advising the COVID-19 task force, said Shelby County needs to eventually have the rate at 5% to limit the virus’s spread even further. The World Health Organization also recommends a positivity rate of 5%.

“Any higher (than 10%) means that more extensive spread of the virus or not doing sufficient testing,” Jain said. “In our case, it has had extensive spread, because we were doing quite a bit of testing.”

Currently, Shelby County’s overall positivity rate is 10.2% after administering nearly 197,000 COVID-19 tests.

Meaning for every 100 tests performed, about 10 patients are positive locally. Randolph previously said the health department set a 10% goal in line with national recommendations.

One of the biggest reasons the positivity rate rose in recent weeks was the growth of cases outpaced the number of tests administered.

This rise in cases occurred as Shelby County moved into Phase 2 of its Back to Business Plan on May 18 as social distancing guidelines were relaxed.

Additional restrictions were implemented in recent weeks to slow the virus’s spread including a countywide mask mandate, bar closures and restaurant operating hours reduced. 

The metric’s limitations

Like any data point, the positivity rate has its limitations.

For one, the only patients include in the positivity rate are those who tested positive or negative for coronavirus. It does not account for those who never tested for COVID-19, who may well have the virus anyway.

Also for the majority of the pandemic, only symptomatic patients or those with direct exposure were prioritized in testing.

That leaves out asymptomatic patients from being tested, however, who can unknowingly spread the virus.

The reason for that prioritization is labs are backlogged in processing test results and Shelby County COVID-19 community testing sites are at capacity. Knowing more tests results are yet to be processed, makes evaluating the pandemic solely through the positivity rate more challenging.

“Obviously, the number is going up,” Strome said. “Is that because you’re now predominantly testing individuals who are sick whereas before there was a lot of thought about testing asymptomatic, etc.? Or is it really because your percentage is actually rising? It’s probably a mix between two things.”

When evaluating COVID-19, Strome said looking at the number of patients in local intensive care unit beds is a more accurate picture of where things are. A total of 127 COVID-19 patients are in ICU beds in the Memphis area, as of Tuesday.

Number of patients hospitalized for coronavirus increases

“There’s a whole lot of reasons why folks with COVID or PUIs (person under investigation for the virus) can be in the hospital,” Strome said. “When you’re in intensive care unit, you’re sick.”

The positivity rate is one of several data points used by the Memphis and Shelby County COVID-19 joint task force. Other metrics used include trend of new cases, hospitalizations, testing capacity and contact tracing.

“In terms of looking at daily cases and hospitalizations are certainly critical measures,” Jain said. “Looking at positivity rate and the transmission number are secondary measures that help.”

Once the backlog in laboratories is addressed over the next couple of weeks, Jain said the positivity rate will become a more complete metric for Shelby County. 

“Once we have our labs back to full capacity,” Jain said. “We will increase our testing, and then we will look at our ability to hopefully see a continuous decline in the positivity rate, that’s not impacted by the limitations in testing.”

How to decrease the positivity rate?

Health experts in Shelby County agree the easiest way to lower the area’s positivity rate is reducing community transmission of coronavirus.

The three most notable ways health experts say you can slow the virus’s spread is by wearing a mask or facial covering, handwashing and social distancing.

“If we can do these diligently and even compulsively,” Jain said. “We can literally stall the spread of the virus to not just having it slow down but then begin to decrease even before herd immunity.”

Shelby County Health Department Director Alisa Haushalter addressed the importance of limiting community transmission at a July 23 press briefing.

“It’s really critical that we avoid large crowds,” she said. “That doesn’t mean just avoiding places like bars, it also means making decisions about what you do at home. What parties you may attend? How many people you have in your household? Do you have a backyard party? Do people wear masks or not? Those are things are going to be critical.”

Lowering the spread – or reproduction rate – can help health officials do more efficient contact tracing by isolating those who are positive or been directly exposed to one.

Currently, contact tracing efforts are hampered by the testing backlog that is partly caused by increased demand since transmission rose after Memorial Day weekend in May.

With no vaccine or herd immunity likely in the immediate future, current efforts underway to slow coronavirus in Shelby County are critical.

“We can’t let the foot off the gas pedal and we need to continue our efforts to contain the virus, so that we are not accelerating or increasing but plateauing and declining,” Jain said. “That will take an effort over weeks and months.”


positivity rate coronavirus Dr. Scott Strome Dr. Manoj Jain Shelby County Health Department
Omer Yusuf

Omer Yusuf

Omer Yusuf covers Bartlett and North Memphis neighborhoods for The Daily Memphian. He also analyzes COVID-19 data each week. Omer is a former Jackson Sun reporter and University of Memphis graduate.


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