By Mario Ariza
South Florida Sun Sentinel
|Oct 14, 2020 at 5:40 PM
Florida has obscured the true extent of its COVID-19 pandemic by using a misleading measure of positive cases to justify reopening schools and businesses, state data indicates.
While Florida has publicized that its “positivity rate” has regularly fallen below 5%, other health organizations are publishing data that shows the rate may be dramatically higher.
Independent experts, including Johns Hopkins University, consistently list Florida’s positivity rate at 10% or higher, twice the recommended level for widespread reopening.
Florida itself calculates another version of the rate — not widely publicized — that shows the pandemic is worse than state officials have championed. Those figures show that the rate has never fallen to the 5% threshold, the South Florida Sun Sentinel discovered.
The result: As coronavirus cases again threaten to rise in Florida, with schools and businesses fully open, it is becoming increasingly difficult to answer a simple question: What percentage of people tested positive for COVID-19 on any given day — and are we justified in going about our business without worry? Or did Gov. Ron DeSantis put people at risk when he allowed businesses to return to normal?
“Doing what we’re doing now, I think positivity numbers are just about useless, because it’s completely opaque who is getting tested and why,” said Thomas Hladish, a scientist at the University of Florida’s Emerging Pathogens Institute who has done pandemic modeling for the Florida Department of Health.
In simple terms, “positivity” measures the percentage of coronavirus tests that come back positive. It traditionally has been considered one of the best ways to determine how much the coronavirus has spread through the community.
Data scientists acknowledge that the rate can be determined in at least three different ways, all of which they say are justified. But in two of the three cases, Florida’s positivity rate would not meet the common standard for reopening society: below 5% for 14 consecutive days.
Method 1: The rate published by Johns Hopkins University measures how many people tested positive on a given day out of all people tested in the state for the first time. By that measure, the state’s positivity rate has not trended below 10% since the middle of June — twice as high as the state claims.
On Tuesday, Johns Hopkins listed the rate at 11.7%, while Florida was reporting 4.99%.
Method 2: The number of positive tests is compared with the total number of tests on one day. This is the measure recommended by the World Health Organization. The organization says this rate should be below 5% before reopening.
The Florida Department of Health publishes the rate, but only in daily reports posted on the Department of Emergency Management’s website, separate from the Department of Health dashboard that the public commonly monitors. The rate also is published only for the entire state, not individual counties.
The last time that rate fell below 5% for 14 days was four months ago, according to data from the Florida Department of Health. Over the past two weeks, it has averaged 5.87%.
Method 3: This calculation is similar, but it excludes anyone who has tested positive in the past, even though people with previous negative tests are still included. That makes the method problematic, some experts say, because the number of people retested on a daily basis can be large, thus skewing the rate downward.
Nevertheless, this is the figure that Florida publicizes each day and publishes on a county-by-county level. It’s the one DeSantis continually touts in arguing that it’s safe to reopen schools, restaurants, theme parks, museums and stadiums. The rate has remained below 5% since the second week of September, according to Department of Health data.
The three rates have broadly tracked each other for the duration of the pandemic, rising and falling at similar times. But the state’s preferred rate is consistently lower than the others and far below the rate identified by Johns Hopkins.
Jason Salemi, an associate professor at the University of South Florida’s College of Public Health, said Florida’s preferred positivity rate is “more controversial. You tend to not see it in a lot of other places.”
Other large states do not publish as much raw coronavirus data as Florida, but they appear to present their positivity rates in simpler terms.
New York, for example, publishes a chart showing the percentage of people testing positive, based on the more widely accepted method recommended by the World Health Organization. California shows a 14-day average of positivity and appears to use a similar method.
Salemi said he still finds Florida’s preferred rate useful to determine “what happened today among people who never tested positive before.” But some experts find the statistic faulty because it appears to compare two fundamentally different variables: people who tested positive versus tests that were negative.
“That is not a real percentage, because you’re using non-identical variables.” said Jones. “It’s like dividing ice cream cones by shark attacks.”
Representatives from the governor’s office, the Department of Health and the Florida Department of Emergency Management declined to explain the reason for their continued use of a possibly misleading statistic, saying simply that it was the “best” measure.
Alberto Moscoso, spokesman for the Florida Department of Health, said in July that the state counts multiple negative tests for the same person because “a negative test only reflects the individual’s status at the moment the specimen was collected.”
“Since a person who tested negative previously remains susceptible to the virus, the subsequent test is still included in the denominator of the positivity rate calculation,” he said.
Some experts are beginning to question whether positivity rates are even a useful measure.
The rate can tell you about the spread of the virus, but only under certain ideal conditions, experts say. Levels of testing must remain constant; the criteria for who gets tested must be relatively random; and you can’t mix different kinds of tests, as Florida does.
The state combines PCR tests, or molecular testing that is considered more reliable, with antigen testing, the rapid-results tests that are less accurate.
Positivity is useful to measure whether the state is doing enough testing, but not so much for measuring spread, said Jennifer Nuzzo, lead epidemiologist for Johns Hopkins’ COVID testing initiative.
“It’s often talked about as a measure of how many infections are out there, but you could have a very different number depending on how much testing you are doing,” she said.
Florida’s daily testing numbers have come down sharply from their July peak, when the state was testing between 90,000 to 100,000 people a day. Currently, the state tests between 40,000 to 70,000 people a day.
Days with a lot of testing tend to have lower positivity rates, and days with fewer tests tend to see spikes.
Hladish, the scientist at UF, advocates for randomized testing of individuals in order to figure out a true positivity rate.
“Randomly administer tests to 1,000 people a week, and you report that percentage every week,” he suggested. “It doesn’t matter whether they tested positive before or not, but you’ll have a representative idea of how prevalent the infection is in the population.”
Alexis Madrigal, deputy editor at The Atlantic, and a co-founder of the COVID Tracking Project, notes that wider use of antigen testing, which is becoming more available, will further disrupt the reliability of the measure. Recently, Florida has said it is deploying millions of rapid tests as it opens schools, and antigen tests have been widely used by professional and college sports teams.
“Antigen tests are going to just fry this whole thing. And no one is really sure what to do about that yet,” Madrigal said.
“The metrics that we’ve come to rely on like test positivity are in for a substantial weirding if antigen testing rolls out in a major way without a workable plan for getting that data back to state health departments.”
All of this might leave the public confused about what numbers are most important. Which should you watch to judge whether Florida’s coronavirus outbreak is starting to spiral out of control?
All of them, Nuzzo says.
“Something to realize is that whatever the metric is, there probably won’t be a magic number,” she said. “I tend to pay attention to the trends in the numbers, more so than the numbers themselves.”