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As rate of positive coronavirus tests increases in Massachusetts, some experts urge caution

The percentage of individuals whose tests for COVID-19 come back positive has surpassed 3 percent on one metric.
The percentage of individuals whose tests for COVID-19 come back positive has surpassed 3 percent on one metric.Jessica Rinaldi/Globe Staff

As COVID-19 outbreaks ravaged much of the country over the summer, Massachusetts was largely spared. But one week into fall, epidemiologists see signs that the virus is once again on the rise here, with some warning that the state should at least press pause on plans to further loosen restrictions.

Statistics that track the spread of the coronavirus have shown a concerning trend in recent weeks, experts said: The 554 new cases identified on Sept. 23 were the most on a single day since May; daily new case totals surpassed 500 for three days in a row for the first time in many months; more patients are hospitalized now than at any point since mid-August.

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But another metric included in the state’s daily data report tracking the pandemic is even more revealing, epidemiologists said. The percentage of individuals whose tests for COVID-19 come back positive has surpassed 3 percent in recent days, more than tripling from a low of 0.9 percent a month ago.

“I have to say that I’m getting concerned about Massachusetts in a way that I have not been . . . in months,” said Dr. Ashish Jha, dean of the Brown University School of Public Health.

The apparent uptick comes as the state continues its efforts to ease social distancing restrictions placed on residents and business owners in the spring. An order permitting restaurants to serve parties of up to 10 indoors and seat patrons at bars took effect on Monday, a step that two epidemiologists urged Governor Charlie Baker to reconsider.

Several metrics give cause for concern, Jha said, but it’s the test positivity rate that he called “a yellow light . . . a cautionary light.” Infectious disease experts have said that number is particularly valuable in assessing the presence of the virus in a community, but tricky to track since it can be calculated multiple ways with vastly different results. It is one of six key indicators that the state has said it uses to guide its decision-making.

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The state calculates positivity rate in two ways, and in its closely watched daily COVID-19 data dashboard highlights a measure that simply divides the number of positive tests by the number of total tests administered on that day. That measure has held steady at roughly 1 percent for weeks.

But a relatively recent surge in the repeated testing of asymptomatic people, such as students on college campuses, has drastically increased the overall number of negative tests each day. Removing these repeated tests from the equation shows the positivity rate has climbed since late August, reaching 3.4 percent in recent days.

Experts said the latter calculation better reflects the scope of the pandemic in Massachusetts today.

“Both [metrics are] somewhat informative, but the individuals one is more indicative of what’s happening now, probably,” said Marc Lipsitch, an epidemiology professor at Harvard T.H. Chan School of Public Health, and a member of Baker’s COVID-19 commission.

The most precise measure of the pandemic would be the exact percentage of people in a community who are infected with COVID-19 at any given time. But because testing everyone in the state daily is not possible, public health experts use positivity rates to approximate community-wide infection rates based on the testing data that are available.

Lipsitch explained that both methods the state uses to calculate positivity rate are mathematically sound. But including thousands of repeated negative tests of the same people can make the rate less responsive to changes in infection levels.

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“From a disease transmission perspective, from a risk perspective, this calculation that they’re leading with [on the COVID-19 dashboard] is likely a substantial underestimate," said Samuel Scarpino, a Northeastern University epidemiologist. “If you have 10,000 tests a day coming from the colleges and universities who all have a very low percent positivity, then it’s very easy to end up with a biased positivity for the state."

How positivity rates are calculated and publicized can impact decision-making by government officials as well as ordinary people, Scarpino said, especially as COVID-related restrictions continue to ease.

In addition to the state’s move to loosen indoor dining guidelines, Massachusetts was also recently exempted from Maine travel restrictions. In a statement last week saying that Massachusetts residents would no longer have to test negative for COVID-19 before visiting Maine or quarantine upon arrival, Maine Governor Janet Mills congratulated Massachusetts on its progress mitigating the spread of the virus.

“If it’s essentially the case that those numbers the state is pointing out are giving the public a false sense of security and also potentially contributing to an increasingly relaxed set of measures around COVID . . . that’s an issue that needs to be addressed by the state," Scarpino said.

In a statement, the state did not directly address questions about why it chooses to highlight the lower positivity rate, and whether the uptick in the rate that does not include thousands of repeated tests is cause for concern.

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“The Command Center closely monitors state and community level COVID-19 data, including for the Commonwealth’s highest risk municipalities, and continues to look for trends across multiple weeks of data to ensure a snapshot is in fact a trend," said Tory Mazzola, a spokesperson for the state’s COVID-19 command center, in an e-mail. "The Commonwealth’s 7-day average positive test rate has remained below 1 percent for 34 days.”

A subsequent message seeking elaboration was not returned.

In separate interviews Monday, Scarpino and Jha were adamant that rising positivity is a sign that the state must proceed with caution, particularly with indoor dining, where they said the governor should at least pause, if not reverse, reopening.

“If I were advising the governor, I’d be pushing really hard against reopening things further," said Jha, who until recently was the director of the Harvard Global Health Institute.

He noted that while Massachusetts’ overall positivity rate was still low compared to other states, the steady climb was worrying. “The issue is that it is rising. The higher the percent positive, the more cases you are missing out there.”

“A reasonable argument would be that we should give it a couple of weeks and see how things go. But if they continue to tick up, then you’ve got to act," he said. “My general approach on this virus has been that it punishes you if you fall behind.”

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Dasia Moore can be reached at dasia.moore@globe.com. Follow her on Twitter @daijmoore.