Experts at Johns Hopkins weighed in Thursday on questions arising from seemingly contradictory metrics used to track the COVID-19 pandemic.
Dr. Jennifer Nuzzo, lead epidemiologist for the Coronavirus Resource Center at Johns Hopkins, said a lack of clear federal guidance on how to determine the positivity rate has created some confusion and resulted in her organization and others having to develop their own calculations.
“As testing has increased and states have begun to conduct more testing, the way they count cases and positive tests has also begun to differ,” Nuzzo said. “This adds to some of the variability we’re seeing in test positivity calculations across states.”
Johns Hopkins was among the first to calculate and track positivity rates, starting in April.
The positivity rate — the percentage of people who test positive for the virus in a given period — is one of a number of key metrics used to track how well a county or state is doing in keeping the virus in check and preventing health care systems from being overwhelmed.
Nuzzo and other public health experts stress that the positivity rate should not be the sole metric used in determining a state’s or local jurisdiction’s response to the virus.
The lack of a standard has resulted in the state and Johns Hopkins and other sites reporting what appear to be conflicting rates even though they are all based on data compiled by the Maryland Department of Health.
On Thursday, the state Department of Health reported the seven-day rolling positivity rate as 3.49%, a record low.
Maryland uses the number of total positive cases reported on a given day and divides that by the total number tests reported by that day — regardless of when the test was given. Del. Shane Pendergrass, D-Howard and chair of the House Health and Government Operations Committee, said the numbers on the site — positive and negative tests — do not equal the total number of tests.
The CDC recommends a positivity rate of below 5% for two weeks before states should begin easing restrictions.
An increase above 5% could raise concerns. Should the rate increase above 5%, Gov. Larry Hogan might have to reimpose tighter restrictions at a time when there is “pandemic fatigue” and as local and national economies struggle to regain their footing.
Lawmakers Wednesday said they were concerned about the inability to independently confirm the rates based on information released on the state website even though acting Deputy Health Secretary Jinlene Chan assured them that the state was consistent in its calculations.
“There is consistency,” said Pendergrass. “You consistently don’t add up on a daily basis to the number of tests, and in part that can explain why the Hopkins data is different.”
Johns Hopkins and Covidactnow.org, which is often cited by a number of county health officers, reported the state positivity rate as 5.1%.
Nuzzo backed both the calculations of the state and her organization without saying which one might be more exact.
“While we believe Maryland’s approach is sound, providing a comprehensive and accurate view of the state of testing across the U.S. Requires Johns Hopkins to use one calculation that can accommodate for these wide disparities in national reporting and we cannot customize calculations for each individual state,” she said.