PROVIDENCE, R.I. (WPRI) – When Rhode Island released its daily coronavirus data on Friday, the state reported a 1.7% positivity rate.
But Johns Hopkins University, whose experts have helped advise state leaders on the pandemic, looked at the same data and said Rhode Island had a 5.3% positivity rate.
So, what gives?
The discrepancy stems from the fact that there’s little agreement when it comes to calculating the so-called positivity rate — which is supposed to show the share of tests coming back positive in any given place — and the state has chosen a different methodology from the school.
“There is no internationally accepted standard for how to do these calculations,” said Jennifer Nuzzo, lead epidemiologist at the Johns Hopkins COVID-19 Testing Insights Initiative, during an interview with Target 12.
On the surface, the rates are different because the math is different. Beneath the surface, the differences have serious implications in various states — including Rhode Island — where officials have tied travel restrictions to positivity rates.
But first, the math.
Johns Hopkins epidemiologists have decided a moving average of new cases (people testing positive for the first time) as a percentage of a moving average of new tests (people testing positive and negative for the first time) is the best way to calculate positivity.
“Ideally, to track positivity, we’d be looking at the % of people tested on a given day or week that test positive,” Nuzzo explained in a tweet. “We care about PEOPLE more than tests because we are trying to answer the question: ‘Are we doing enough testing to find the infections that are out there?'”
In Rhode Island, health officials calculate the rate differently. They measure new cases (people testing positive for the first time) as a percentage of total tests (including retests of individuals who’ve previously tested positive) for a given day.
As a result, Rhode Island’s rate over the last couple months has typically ranged between 2 to 3 percentage points lower than the positivity rate reported by Johns Hopkins.
Nuzzo said the difference doesn’t really mean much from a broader public health perspective, suggesting state leaders should instead concentrate on whether the rate is rising or falling.
She also warned against putting too much stock into the positivity rate by itself, saying it should always be considered in concert with other metrics, such as daily case numbers, case-finding efforts and most importantly, as of late, how quickly test results are returned.
“For me the bigger issue at this point is not positivity, but how long it’s taking to get test results back,” Nuzzo said. “Who cares what your positivity is if turnaround time is a week or more?”
For better or worse, however, the measurement of the positivity rate has real-life policy implications in states like Rhode Island that have tied positivity rates to travel restrictions.
Any traveler coming to Rhode Island from a state with a positivity rate above 5% — a recommended threshold established by The World Health Organization — must show proof of a recent negative test or else self-quarantine for 14 days on arrival.
According to Johns Hopkins, Rhode Island’s 5.3% positivity rate on Friday exceeded that level — and the state relies on Johns Hopkins to determine which states qualify for the travel restrictions. (As of Monday morning, the state’s rate had fallen again to just under 5%.)
Asked about the numbers Friday, Rhode Island health officials pushed back, claiming the university’s underlying data – compiled by The COVID Tracking Project – is wrong.
“We’ve reached out to The COVID Tracking Project several times to express concerns,” R.I. Department of Health spokesperson Joseph Wendelken said. “The Rhode Island numbers they are using are inaccurate.”
The COVID Tracking Project, a widely cited volunteer organization created by the Atlantic magazine, did not respond to a request for comment Friday. But it appears the organization is excluding a portion of Rhode Island’s daily tests, worsening its standing in nationwide comparisons.
Unlike some other states, Rhode Island last month started differentiating between the total number of tests it has administered and the total number of individual people who have received tests.
The gap between the two numbers largely stems from so-called “surveillance testing” of high-risk populations and workers – such as nursing home residents and hospital employees – who get tested multiple times, regardless of symptoms.
The COVID Tracking Project publicly reports the smaller number for Rhode Island, unlike other states that don’t differentiate or in some cases even disclose which numbers are being reported.
Health officials raised similar concerns last month when The New York Times released a report suggesting Rhode Island wasn’t testing enough people. The newspaper cited The COVID Tracking Project as the primary data source for the analysis.
“The real issue is that they are doing the math with numbers that are not accurate,” Wendelken said. “We will be following up again.”
State officials also disagree with how the Tracking Project calculates daily cases, claiming it inaccurately accounts for revisions made to the state’s historic data.
Last Monday, for example, The COVID Tracking Project reported 291 new cases in Rhode Island, which represented the combined total since the prior Friday, as the state doesn’t report COVID-19 data over the weekend. For Saturday and Sunday, the nonprofit reported zero cases in Rhode Island.
When the state reported its coronavirus numbers that Monday, the nearly 300 new cases were spread out across three days. (WPRI.com’s COVID-19 tracking page accounts for daily revisions across deaths, cases and hospitalizations.)
Despite the discrepancies, a Target 12 analysis of the Johns Hopkins method of calculating positivity using Health Department numbers instead of COVID Tracking Project data shows the state would still have exceeded the 5% threshold on Friday.
And while state officials may not agree with The COVID Tracking Project’s accounting methods, there’s little evidence to suggest it will spur changes to how they enforce their travel policy.
“If other states identify problems with the way their data is reported, [the Health Department] will work with them to understand the problem and potentially make adjustments to our COVID-19 travel policies as appropriate,” another Health Department spokesperson, Annemarie Beardsworth, wrote in an email.
When asked about the fact that various states – including Rhode Island, Massachusetts and Connecticut – had tied travel restrictions to the positivity rates, Nuzzo said she could understand why state leaders would want to hitch policy to seemingly hard metrics.
But she warned that “it’s never desirable to rely on one data point to make high-stakes decisions.”
After talking with Target 12, Nuzzo took to Twitter, where she posted a string of tweets further breaking down how people should be thinking about positivity.
“Many states are using positivity as a benchmark for reopening or for implementing control measures, such as interstate travel restrictions,” she tweeted. “This may overstate what positivity is telling us.”
So, in summary: positivity is an important, but imperfect metric. It can inform our interpretation of case numbers and adequacy of testing efforts. But no single measure can tell us everything. Test positivity is not an end.— Jennifer Nuzzo, DrPH (@JenniferNuzzo) July 31, 2020
A note about the data: Beginning Aug. 3, Target 12 started publishing a chart showing Rhode Island’s method of calculating the positivity rate alongside the method used by Johns Hopkins University. The Johns Hopkins rate might vary slightly from what’s reported on its website because Target 12 uses Health Department data to calculate the number; Johns Hopkins uses third-party data compiled by The COVID Tracking Project.
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