Preliminary results from the second wave of a UW–Madison study estimated 6.8% of Wisconsin residents have evidence of recent infection of COVID-19.

The latest round of testing of Survey of the Health of Wisconsin participants, included 1,070 people. The study is looking for the presence of antibodies, which indicate if a person has been infected with COVID-19 in the recent past, even if they did not experience symptoms.

In June, the Wisconsin Department of Health Services (DHS) partnered with the University of Wisconsin School of Medicine and Public Health’s SHOW and the Wisconsin State Laboratory of Hygiene to launch a surveillance study, the Past Antibody COVID-19 Community Survey. The study tracks the prevalence of past infections of COVID-19 across the state of Wisconsin.

The antibody study showed that 1.6% of survey participants had evidence of antibodies when tested in July and early August. Between October and December, the number was up to 6.8%.

During this second statewide survey, we found the prevalence of antibodies went up between 4- to 5-fold across the state, according to Kristen Malecki, associate professor of population health sciences and director of SHOW.

The greatest increases came in the north and northeastern regions where very few individuals tested positive this summer, she said.

Kristen Malecki
Kristen Malecki

Prevalence of antibodies again varied by health region ranging from 2.4% in the south and 9.2% in the southeast, according to the latest report.

Additional focus

This second sample included special efforts to reach out to underrepresented minority populations and past Latinx participants living in the Milwaukee area.

“Many of these participants are more likely to have jobs that require them to work outside the home or are on the frontline of health care and other industries,” Malecki said. “But, this is a special population sample, and is not reflective of the entire Latinx population in the state of Wisconsin.”

Among this specific group of individuals, the second round of testing showed that more than 25% of the Latinx participants in the sample population tested positive for COVID-19 antibodies.

How long antibodies remain

While it is unclear how long individuals will have detectable levels of antibodies after an exposure, Malecki said, the study found that 33% of participants testing positive for antibodies over the summer had a negative antibody test in the fall.

Antibodies appear within two to three weeks after infection and evidence suggests that COVID-19 antibodies may disappear within three to six months after infection. Therefore, the antibodies measured reflect infections that occurred at least one month before participation in the study.

The survey is designed to test a statewide, representative group of people three times. These preliminary results reflect the rise of infections that occurred in the summer and early fall, according to Kristen Malecki, director, SHOW.

“These preliminary study results provide better insight to how infections went up between early summer and fall in Wisconsin, they do not reflect the recent surge in cases,” Malecki said. “Numbers also suggest the number of state residents who are still vulnerable to infection is significant.”

A third wave is planned later in the winter. The same group of individuals are invited to participate at all three waves, including people who have already been tested in past waves. It is important to note that the PACCS survey is ongoing and these results are preliminary. The final results from all three waves can be expected in summer 2021.

“While there is still much to learn about the long-term effects of COVID infection these numbers are helpful for state and local health officials working hard to reduce the impact of COVID-19 across the entire state,” Malecki said.