What The COVID-19 Pandemic Looks Like In Wisconsin: Maps And Charts

Tracking The Impacts Of The Novel Coronavirus Around The State
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A transmission electron micrograph shows color-enhanced SARS-CoV-2 virus particles isolated from a patient.

Editor's note: This article was originally published on April 1 and has since been updated, with the most recent changes made on Oct. 1. Data visualizations are updated once daily as new numbers become available.

 

Understanding when and where COVID-19 is spreading around Wisconsin is a matter of constantly moving targets. Tracking data related to the pandemic can help clarify this torrent of information. Here are a series visualizations that depict the impacts of COVID-19 across the state.

First, knowing the trajectory of the outbreak in Wisconsin can help public health officials discern to what degree actions designed to slow its spread are working. It can also help healthcare workers and hospital systems better prepare for surges in patients.

Key pieces of information include the number of new infections and deaths reported on a daily basis. If the number of new infections reported rises rapidly day-to-day, that's a potentially worrying sign that the outbreak is growing quickly and threatening hospital capacity. A rise in daily deaths would also be a negative indicator.

The goal of public health officials is to see the number of newly reported cases and deaths fall over time. Because the time between exposure and the onset of symptoms can be up to two weeks in the case of COVID-19, and death in serious cases can follow long after infection, any discernible effect of public health measures would be delayed.

 
 

New COVID-19 infections and deaths are initially reported at the county level, and these numbers can provide a more detailed look at outbreaks in different places around Wisconsin.

The number of confirmed cases and deaths related to the disease varies widely among the state's 72 counties. At least one resident of each has been diagnosed with COVID-19, but the total number of cases runs into the thousands in multiple counties. Deaths attributed to COVID-19 have been reported in most of the state, but are likewise concentrated in a handful of counties. Tracking the daily number of cases and deaths by county reveals more localized trends in the pandemic, with outbreaks experienced at different times in different places.

 
 

The total number of people in Wisconsin who have been infected and have died due to COVID-19 helps provide a broad look at the state's trend. If cases and deaths are growing rapidly by the day, plots of total infections and fatalities over time would show that plainly with a steep upward trajectory.

 
 

If public health measures that are designed to slow the spread of COVID-19 have an effect, and the number of new cases begins to fall, a plot of total infections over time would become less steep, or flatten. Hence the ubiquitous term "flattening the curve." in discussions about COVID-19.

 
 

A common way to visualize the dynamics of an outbreak and potential effects of strategies designed to slow its transmission is to track a 7-day moving average of newly confirmed cases and deaths. The trend line of these moving averages is more sensitive to fluctuating daily numbers, and can help indicate when the outbreak has peaked, or is declining or surging.

 
 

The total number of confirmed infections in Wisconsin is likely much smaller than the true number of infections. Many people who are infected may not show symptoms, and others with mild or moderate symptoms may not be tested due to limited test availability. Still, understanding how many people diagnosed with COVID-19 out of the total number who are tested can provide a snapshot of the disease’s prevalence in relation to other commonly circulating respiratory diseases with similar symptoms, such as influenza.

Public health officials have worked to expand COVID-19 testing capacity in Wisconsin in order to provide a more complete picture of the number of people infected with the disease.

A metric related to these efforts is the percent of tests that come back with positive results, which is known as a test-positivity rate. While the rate is subject to daily fluctuations, its longer-term trajectory can reveal the severity of Wisconsin's outbreak. A higher rate can indicate that testing strategies are missing many sick individuals. Instead, officials want to see the rate go down over time, which could indicate the state's outbreak is coming under control.

There are multiple ways to calculate test-positivity rates. One common method is to divide the number of people who test positive by the number of people who are tested. Another method is to divide all positive tests by the total number of tests with results. The rates based on these two methods may diverge over time as a growing number of people receive repeat tests. Additionally, the number of tests performed each day is highly variable, which can lead to daily fluctuations in positivity rates. To better understand trends in positivity, rates are calculated based on moving 7-day periods.

 

Tracking the test-positivity rate at a more local level is important for local policymaking related to public health guidelines for schools and group gatherings. These guidelines tend to call for fewer than 5% of test results coming back positive. Due to often wide differences in the number of tests reported per day at a local level, particularly in places with smaller populations, the 7-day and 14-day averages of the test-positivity rate provide a more reliable figure for public health officials to monitor.

 
 

A more complex COVID-19 metric is the case fatality rate. This measure reflects the percentage of confirmed cases that have resulted in death. However, it doesn't reflect the outcomes of unconfirmed COVID-19 cases, and is higher than the disease's actual death rate.

 

As the pandemic continues over time, the day-to-day case and death counts at the county level result in broad regional patterns of where COVID-19 has spread widely, where it is not as prevalent and where its deadly effects are most severe. A county-level map of total infections and deaths can help shine light on these important geographic differences.

 
 

Looking at county-level data could be deceiving without some knowledge of a local outbreak's magnitude compared to the size of a local population. Ten infections in a large, densely populated metro area with multiple hospitals is a world apart from 10 infections in a tiny community without a single health clinic.

That's why it is useful to evaluate local outbreaks relative to the size of local populations. This is commonly known as a per capita rate — in this case, cases or deaths are adjusted to per 100,000 residents.

 
 

More information about COVID-19 and how it is affecting the state can be found in a WisContext FAQ that provides explanations to common questions about the disease and in a series assembling Wisconsin Public Media reporting about the crisis, as well as in coverage by PBS Wisconsin and Wisconsin Public Radio.

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