Though newly confirmed cases of COVID-19 surged in Dane County recently and set records statewide in four of the past nine days, the proportion of Madison-area people who test positive for viral antibodies grew only slightly the past four weeks.
About 2.6% of people tested at four Madison-area health systems since mid-June had antibodies to the coronavirus, which indicates a previous infection. That’s up from 2.4% of those tested for antibodies in May through mid-June. The total since antibody testing began at the health care organizations in May is 2.5%.
Unlike nasal swab tests that diagnose active infection of COVID-19, antibody tests are blood tests that look for immune system proteins that fight infection and linger afterward, potentially protecting people from reinfection.
Antibodies typically can’t be detected until a week or two after someone is ill. Given that new cases in Dane County started picking up only late last month, it’s not surprising the percentage of positive antibody tests in the Madison area hasn’t climbed much yet. Patients tested for antibodies at Madison-area hospitals and clinics also may not be a representative sampling of the general population, as the testing is not part of a structured study.
In addition, now that diagnostic testing for active COVID-19 has been widely available for a while, the mix of people who seek antibody testing may have changed, said Dr. Jeff Pothof, chief quality offer at UW Health. “People are moving their testing upstream to the diagnostic test and not relying on the antibody test as much,” he said.
Among 26,711 people who have had antibody tests at UW Health, SSM Health in Wisconsin, UnityPoint Health-Meriter and Group Health Cooperative of South Central Wisconsin since they started offering the tests in May, 665, or 2.5%, have been positive, according to data provided this week by the organizations.
The state Department of Health Services is partnering with UW-Madison’s Survey of the Health of Wisconsin to embark on a study of the prevalence of COVID-19 antibodies throughout the state. Past SHOW participants, from 10 randomly selected counties and the city of Milwaukee, will receive antibody testing quarterly over the next year.
Meanwhile, COVID-19 hospitalizations have increased statewide and in Dane County recently. There were 331 patients in hospitals with the disease Friday, up from 235 on July 4 but well below the peak of 446 patients on April 9.
Dane County had 30 patients hospitalized with COVID-19 Friday, up from 21 on July 2 but down from 46 on April 9.
Wisconsin has reported 48 COVID-19 deaths so far this month, compared with 189 deaths the first 17 days of April, 137 during the same period in May and 121 during the same period in June.
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