The test’s positivity rate will appear 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 conduct the COVID-19 test at the University of Utah Health’s Farmington Health Center on Friday, July 31, 2020.
Utah is changing the way it informs the public about one of the statistics that track the spread of COVID-19.
The test’s positive rate is the percentage of tests that tested positive for COVID-19 – and Utah health officials have been explaining for months that a 3 to 5 percent rate indicates that the virus is under control.
It rose to 32.71% on January 7, during the state’s post-holiday peak, but has recently fallen, hovering around 16%.
The new method that the Utah Department of Health will use will make the percentage appear lower. This will help public health officials “see the benefit of our increased testing capacity” and make comparisons with other states more valid, Dr. Angela Dunn, a state epidemiologist, told reporters recently when explaining the change.
In its daily report on Monday, the department listed both the previous measurement – a seven-day moving average of 15.4%, and the new one, which has a continuous seven-day average of 7.3%.
See how these methods differ.
What does the test positivity rate show?
The percentage of tests for COVID-19 that tested positive is “an important indicator of the spread of COVID,” said Dunn. “In addition, it also measures the amount of tests we are doing.”
How was Utah calculating this?
Since the beginning of the pandemic, Utah has been using a method called “people over people,” Dunn said. This means the number of people tested positive divided by the number of people who tested.
As people took the test, 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 use a 90-day window, Dunn said, because that is the period of immunity for someone after getting the coronavirus.
Therefore, according to this method, a person who takes the test several times in three months is counted once – subsequent results are “eliminated” in the health department’s count of how many people were tested in that 90-day interval.
What is changing in the way Utah calculates it?
The new method is called “test on test”. It takes the number of positive tests in a day or a week and divides it by the number of tests conducted in the same period of time.
“Each test reported to the [Utah] Department of Health is included in this calculation, ”said Dunn.
The number of tests conducted in Utah is dramatically higher now, as tests have expanded with the addition of rapid tests and regular tests are widespread among staff, students, nursing home residents and others.
These tests are usually done with healthy people; they have no symptoms and do not believe they have the virus. They are being tested to stay in class or to continue working.
Therefore, counting all the results, when so many presumably healthy people are being tested, will reduce the rate of positivity.
The new method, Dunn said, “will allow us to see the benefit of our increased testing capabilities.”
With all these tests, people who get the virus will know quickly, she said. Health officials can respond with contact tracking and act more quickly to prevent further spread. And the positivity rate will reflect that environment.
The “people over people” method made sense at the beginning of the pandemic, she said. At that time, the Federal Centers for Disease Control and Prevention required a COVID-19 patient to undergo two consecutive negative tests before being considered recovered from the virus.
A person can accumulate many positive tests before obtaining two negative tests, artificially inflating the “test on test” calculation, she said. Subsequently, the CDC changed its orientation, but Utah continued to use the “people over people” method.
The CDC uses both methods and a third, she noted.
Public health officials observe both calculations, Dunn said. The “people over people” method, she said, “mirrors our emerging cases”.
The formula “test on test” “is really useful because it allows us to see our increase in tests and how it is helping us to decrease our cases”.
What is the difference in practical terms?
The new positivity rate statistic will be lower.
Monday’s report showed that difference, with 15.4% versus 7.3% for a seven-day continuous average under both methods.
The “people over people” method, said Dunn, “polarizes the slightly higher percentage of positivity”. The “test on test” method “polarizes the percentage of positivity a little lower,” she said. “The truth is somewhere in between.”
“You will notice that the trends for both methods are really similar,” said Dunn. Looking at the graphs for both calculations, over time, they are almost parallel.
Is this a political movement, to make the numbers look better?
Dunn downplayed that perception. She noted that at least 37 other states use “test over test” – and switching makes it easier to make comparative comparisons with other states.
If the daily number seems lower, will people let their guard down?
“We need to change the scale in our heads,” said Dunn.
The postnatal test positivity rate of more than 30% “was high because that was the scale we were using,” said Dunn. “Now, 15% is going to be very high, 10% is going to be very high.”
How should people read the data?
“We know that no metrics will tell us how bad it is or when it will end,” said Dunn. “We have to look at everything.”
She recommends that people watch three main statistics: the test’s positive rate, the number of new cases, and the number of hospitalizations or remaining capacity.
“All three of these metrics together give us a very good picture of where we are currently in the pandemic and where we are going,” said Dunn.