The test positivity rate will look lower, so ‘we need 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 on Friday, July 31, 2020 at the University of Utah Health’s Farmington Health Center.
The state of Utah is changing how it reports one of the statistics reporting the distribution of COVID-19 to the public.
The test positivity rate is the percentage of tests that are positive for COVID-19 – and Utah health officials have been explaining for months that a rate of 3 to 5 percent indicates that the virus is under control.
It rose to 32.71% during the post-holiday boom on January 7, but it has recently fallen and fallen by about 16%.
The new method the Utah Department of Health is going to use makes the percentage appear lower. This will help public health officials ‘see the benefit of our increased testing capacity’, and it will make comparisons with other states more valid, said Dr. Angela Dunn, the state epidemiologist, recently told reporters when he explained the change.
In its daily report on Monday, the department listed both the previous measurement – a rolling seven-day average of 15.4%, and the new one, which has a seven-day average of 7.3%.
Here’s how the methods differ.
What does the test positivity rate show?
The percentage of COVID-19 tests that return positive is a leading indicator of COVID distribution, ‘Dunn said. “In addition, it also measures how much testing we do.”
How did Utah calculate 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 who tested positive divided by the number of people tested.
While people were being tested, the state counted their first positive test within a 90-day period – or, if they never tested positive, their first negative test. Any other test results in the window are not included. Health officials use a 90-day window, Dunn said, because it is the period of immunity for someone after they contracted the coronavirus.
Thus, according to this method, a person who is tested multiple times in three months is counted once – the subsequent results are ‘duplicated’ in the health department’s number of people tested within the 90 day period.
What changes in how Utah calculates it?
The new method is called ‘test over test’. It takes the number of positive tests per day, or a week, and divides it by the number of tests done in the same period.
‘Every test given to the [Utah] The department of health is included in this calculation, “said Dunn.
The number of tests conducted in Utah is now dramatically higher as tests have expanded with the addition of rapid tests and regular tests are widespread among employees, students, residents of nursing homes and others.
These tests are generally from healthy people; they have no symptoms and do not believe they have the virus. They are tested to stay in class or keep working.
If all the results are counted, when so many supposedly healthy people are tested, the positivity rate will drop.
The new method, Dunn said, “will enable us to see the benefit of our increased testing capability.”
With all the testing, people who contract the virus will know quickly, she said. Health officials can respond with contact detection and move faster to stop the spread. And the positivity rate will reflect the environment.
The “people over people” method made sense at the beginning of the pandemic, she said. At the time, the federal Centers for Disease Control and Prevention required a COVID-19 patient to undergo two consecutive negative tests before he could be considered recovering from the virus.
A person can take a lot of positive tests before taking two negative tests, which artificially inflates the test-by-test calculation, she said. The CDC later changed its guideline, but Utah continued to use the “people over people” method.
The CDC uses both methods and a third one, she noted.
Public health officials are keeping an eye on both calculations, Dunn said. The method “people over people”, she reflects, “reflects our increasing cases.”
The formula “test over test” is very useful because it allows us to see our increase in tests, and how well it helps us reduce our cases. “
What is the practical difference?
The new statistics for positivity rates will be smaller.
According to Monday’s report, this was the difference, with 15.4% versus 7.3% for a seven-day moving average under the two methods.
The “people over people” method, Dunn said, “disturbs the percentage of positivity slightly higher.” The “test over test” method makes the percentage of positivity a little lower, “she said.” The truth is somewhere in the middle. “
“You’ll notice that the trends for both of these methods are actually the same,” Dunn said. If you look at the charts for both calculations, they are parallel over time.
Is it a political step to make the numbers look better?
Dunn underestimated the perception. She noted that at least 37 other states use ‘test over test’ – and switching makes it easier to do apple-to-apple comparisons with other states.
If the daily figure seems lower, will people let their hats down?
“We need to change the scale in our head,” Dunn said.
The positivity rate of more than 30% after Christmas was high because that was the scale we used, Dunn said. “Now 15% is going to be really high, 10% will be really high.”
How should people read the data?
“We know that no single measure will tell us how bad it is or when it’s going to end,” Dunn said. “We have to look after everything.”
She recommends that people look at three key statistics: the positivity rate of the test, the number of new cases and the hospitalization numbers, or the remaining capacity.
“All three of these statistics give us a very good picture of where we are currently in the pandemic and where we are going,” Dunn said.