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Rising positivity rate signals growing community spread of virus

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COVID-19 swab test. File photo
COVID-19 swab test. File photo

MADISON (WKOW) -- One metric scientists use to track how COVID-19 is spreading is the percent positive number. It's trending up statewide, and that could lead to more people being infected.

There are two main ways percent positivity can be calculated. One is comparing the number of people who test positive to the total number of people who get tested. However, this method doesn't account for people who tested positive earlier in the pandemic and again more recently.

The other way of calculating the number is by comparing the number of tests that return positive results to the total number of tests taken. If someone tests positive or negative more than once, all their test results are included in the number. This is the more commonly used analysis.

In communities across the state, the percent positive by test is well above the level health experts want to see.

"We want it to be at one percent or lower," infectious diseases expert Dr. Ajay Sethi said. "At five percent or lower, [the] government can start easing some restrictions, but, clearly, we're not at that level yet.

As of Friday, Wisconsin's seven-day average was 14.2%. Some parts of the state are seeing even higher positive rates.

From Nov. 28 through Dec. 4, an average of 36.8 percent of tests in Rock County returned positive results each day.

Sethi said high numbers like this indicate a significant level of community spread.

"It's very dangerous out there," he said. "The chances of coming across somebody who unknowingly has that infection is very high right now. That's exactly why there's a recommendation to really only go out [if] you're doing something that's essential."

Sethi said in addition to looking at raw data for percent positivity, it's also important to look at trends because they can signal what to expect in coming days.

In Wisconsin, the seven-day average has been increasing for the past week, which Sethi says is bad news.

He also says circumstances like dropping temperatures and the upcoming holiday season don't give him hope the trend will reverse itself soon.

"We're in the middle of winter, and I don't foresee conditions where we're going to see less spread of COVID," he said. "Everybody's moving indoors, and that's where COVID spreads the most."

A number of factors, including the amount of testing being done, can influence percent positivity. If testing is more scarce, a high positivity rate can be attributed to only the sickest people getting tested. When testing is more available, as it is in many area communities, a high positivity rate is indicative of more community spread.

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Caroline Dade

Reporter/Multimedia Journalist, 27 News

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