What more at-home COVID-19 testing means for Wisconsin’s pandemic surveillance

As home antigen testing for the coronavirus becomes more accessible, are daily COVID-19 case counts becoming less reliable? Is this change in testing practices possibly masking virus transmission trends? If not, how could health officials monitor the disease to ensure that hospitals and public health departments are prepared for future outbreaks?

These questions are at the forefront as the United States enters a phase of the pandemic in which another highly contagious subvariant, omicron BA.2, is spreading amid minimal public health restrictions and movement of people. resources to respond to COVID-19.

The average number of daily cases has long provided a snapshot of the movement of the disease in communities. Test providers are required to report positive COVID-19 results to public health authorities. In Wisconsin, these reports eventually reach the Wisconsin Department of Health Services, which in turn calculates the daily statewide case counts which it updates five days a week.

But with rapid home tests becoming much more widely available since late 2021, an unknown but potentially large number of positive test results are going unreported. While this dynamic may pose a challenge to public health officials tracking COVID-19, the challenge is not insurmountable. That’s according to Ajay Sethi, an epidemiologist at the University of Wisconsin-Madison.

“The fact that we have home testing is a good thing,” Sethi said. “While this may compromise our ability to have a good record of cases that are in the community, we don’t necessarily want to abandon this very important way people can test and act, so we need to find a workaround.”

In fact, several workarounds are taking shape to ensure COVID-19 surveillance remains adequate despite a future with fewer reported test results. These include sampling community sewage for genetic material from the SARS-CoV-2 virus and tracking the number of people seeking health care for covid-like illness, similar to a strategy long used to follow the ebb and flow of the seasonal flu.

Another more limited workaround is to systematically test the same group of people in a community on a regular basis – a strategy known as sentinel surveillance which can provide a snapshot of transmission occurring in the wider community.

Even with more people testing positive through home testing, Sethi said the average number of daily cases remains a useful, if not perfect, measure for predicting new outbreaks and their worsening.

“I don’t ignore the case data just because it’s not representative, mainly because it was never representative of everyone who should get tested,” Sethi said. He added that the changing testing landscape highlights the importance of tracking multiple disease metrics, including test positivity and daily hospitalizations.

Indeed, the latest local COVID-19 risk assessment tool offered by the Centers of Disease Control and Prevention takes into account both local case rates and hospitalizations. On the other hand, this CDC measure — called “community levels” — allows for scenarios in which testing may not adequately capture high rates of local transmission. Hospitalization trends alone can trigger increased risk assessment under the new tool if they are serious enough.

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