The positivity rate of the test will seem lower, so “we have to change the scale in our head,” says the state epidemiologist.
(Trent Nelson | The Salt Lake Tribune) Kylie Archuleta and Joshua Brimhall perform COVID-19 testing at Farmington Health Center at the University of Utah Health on Friday, July 31, 2020.
The state of Utah is changing the way it reports to the public one of the statistics following the spread of COVID-19.
The test positivity rate is the percentage of tests that return positive for COVID-19 – and Utah health officials have explained for months that a rate of 3 to 5% indicates that the virus is under control.
It rose to 32.71% on January 7, during the state’s post-holiday growth, but recently fell by around 16%.
The new method the Utah Department of Health will use will make the percentage appear lower. This will help public health officials “see the benefits of our increased testing capacity” and make comparisons with other states more valid, Dr. Angela Dunn, the state’s epidemiologist, recently told reporters to explain the change.
In its daily report on Monday, the department listed both the previous measurement – a seven-day continuous average of 15.4%, and the new one, which has a seven-day continuous average of 7.3%.
Here’s how these methods differ.
What does the test positivity rate show?
The percentage of tests for COVID-19 that return positive is “a key indicator of the spread of COVID,” Dunn said. “In addition, it also measures the testing we do.”
How did Utah calculate it?
Since the beginning of the pandemic, Utah has used a method called “people over people,” Dunn said. This means the number of people who tested positive, divided by the number of people who were tested.
As people were tested, the state counted their first positive test in a 90-day period – or, if they never tested positive, their first negative test. Any other test results in that window were not included. Health officials are using a 90-day window, Dunn said, because this is the period of immunity for someone after catching the coronavirus.
So, under this method, a person who is tested several times in three months is counted only once – the subsequent results are “de-duplicated” to the health department’s account of the number of people who were tested in that period. 90 days.
What changes in the way Utah calculates it?
The new method is called “test after test”. It takes the number of positive tests in a day or a week and divides it by the number of tests performed in the same period of time.
“Each test reported to [Utah] The Department of Health is included in this calculation, “Dunn said.
The number of tests performed in Utah is dramatically higher now, as the tests have expanded with the addition of rapid tests, and regular testing is widespread among employees, students, nursing home residents, and others.
These tests are generally for healthy people; they have no symptoms and I don’t think they have the virus. They are tested to stay in class or to continue working.
So counting all the results, when tested by so many supposedly healthy people, will reduce the positivity rate.
The new method, Dunn said, “will allow us to see the benefit of increasing our testing capacity.”
With all these tests, people who catch the virus will know quickly, she said. Health officials can respond by following contacts and move faster to stop the spread. And the positivity rate will reflect that environment.
She said the “people over people” method made sense at the beginning of the pandemic. At the time, the Federal Centers for Disease Control and Prevention required a COVID-19 patient to take two negative tests in a row before they could be considered recovered from the virus.
A person could accumulate a lot of positive tests before getting two negative tests, artificially inflating the “test over test” calculation, she said. The CDC later changed its guide, but Utah continued to use the “people over people” method.
The CDC uses both methods and a third method, she noted.
Public health officials are following both counts, Dunn said. She said the “people over people” method reflects our growing cases.
The “test over test” formula “is really useful, because it allows us to see the growth of testing and how well it helps us reduce our cases.”
What is the difference in practical terms?
The new positivity rate statistic will be lower.
Monday’s report showed this difference, with 15.4% compared to 7.3% for a continuous average of seven days in the two methods.
The “people over people” method, Dunn said, “biases percent positivity a little higher.” The “test over test” method “distorts the percentage of positivity a little lower,” she said. “The truth is somewhere in the middle.”
“You’ll notice that the trends for both methods are actually similar,” Dunn said. Looking at the diagrams for both calculations, over time, they are practically parallel.
Is this a political move to make the numbers look better?
Dunn downplayed this perception. She noted that at least 37 other states use “test after test” – and switching to facilitation makes it easier to make comparisons between apples and other states.
If the daily figure seems lower, will people let their guard down?
“We need to change the ladder in our heads,” Dunn said.
The positivity rate after the Christmas test of over 30% “was high because that was the scale I was using,” Dunn said. “Now, 15% will be very high, 10% will be very high.”
How should people read the data?
“We know no indicator will tell us how bad it is or when it will end,” Dunn said. “We have to look at everything.”
She recommends that people follow three key statistics: the test positivity rate, the number of new cases, and the number of hospitalizations or remaining capacity.
“All these three values together give us a very beautiful picture of where we are currently in a pandemic and where we are going,” Dunn said.