Two weeks from Saturday, Minnesota will mark one year since the first case of COVID-19 was confirmed in the state.
Since then, the state has recorded an additional 476,291 cases of the virus, which means roughly 8.4 percent of the state’s population has had a confirmed positive test.
That’s not the total number of Minnesotans who have had COVID-19, though. It’s likely a significant undercount for several reasons: First, testing was scarce for many months in the beginning of the pandemic, and people who were sick were told to assume they had COVID-19. Second, many people with COVID-19 may be asymptomatic; they may not know they’re sick, so they may not get tested. And third, people exposed to confirmed cases of COVID-19 may have deduced they were also likely infected, not bothering to get tested.
So what share of Minnesotans, almost one year in, have had COVID-19?
It’s impossible to know with precision, but early last summer, the Minnesota Department of Health planned to spend $1.975 million out of the money the federal government sent to Minnesota as part of the CARES Act to conduct studies based on blood tests in an effort to get a fuller picture of the virus’ spread.
Blood bank testing
One of the studies pursued by the Minnesota Department of Health involved testing samples from blood donors for COVID-19 antibodies. These blood tests are called serology tests because they involve testing the serum in a person’s blood for antibodies.
The effort started with samples from the Twin Cities and northern Minnesota collected by Memorial Blood Centers.
When the first blood samples were tested, in June of last year, 1.2 percent were found to have COVID-19 antibodies, said Stephanie Yendell, a senior epidemiology supervisor at the Minnesota Department of Health. At that time, the number of confirmed COVID-19 cases equated to less than a half a percent of Minnesotans.
Another test of samples given between mid-October and early November, when Minnesota’s case count had begun a precipitous climb, found 3.2 percent contained COVID-19 antibodies.
That might sound low given what was going on in the state at the time, Yendell said, “But if you think about it, that was right about the time when we were just hitting that ramp up. Antibodies take a few weeks to develop, and if someone is actively sick with COVID, they’re probably not a healthy blood donor.”
Now, additional blood banks have begun testing their own samples and reporting the data to MDH, one weekly and another monthly.
The most recent data available from one company as of Tuesday, from the week of Feb. 8 found 28.6 percent of samples with COVID-19 antibodies.
A not insignificant amount of that was due to vaccinations, which started in December and produce the same antibody proteins in the bloodstream the serology tests pick up if someone was infected. That company’s data showed 17.4 percent of unvaccinated samples with antibodies and 28.6 percent of the overall sample with antibodies.
As an epidemiologist, Yendell said she’s encouraged to see the data tracking closely with Minnesota’s case numbers, and now, vaccines.
“Now that we’re getting donors who are vaccinated, I’m really excited to see the percentage go up,” Yendell said.
Other serology studies being conducted in partnership with the University of Minnesota are still underway. Among them is a study testing 1,000 grocery store workers across the state for COVID-19 antibodies to learn more about the spread of the virus as well as which mitigation strategies in stores seem to work. Another is testing health care workers, who have been up close and personal with the virus for nearly a year.
A serology study of households across Minnesota, aimed at learning about the spread of COVID-19 in communities across the state, was scuttled in September when Centers for Disease Control workers surveying households were subject to racist intimidation.
There are limitations to what blood tests can tell us about COVID-19’s trajectory in the state.
First, there are population factors: In the case of the blood bank tests, blood donors are not representative of the state’s overall population. Compared to the average Minnesotan, Yendell said they tend to be older, whiter and more affluent. Confirmed cases of COVID-19, by contrast, more COVID-19 cases have been found among younger Minnesotans and Minnesotans of color are disproportionately affected by the virus, according to data from MDH.
And second, at some point, the amount of detectable antibodies present in a previously-infected person’s blood decreases over time, which means by now, many people who were infected with COVID-19 early in the pandemic may not have detectable antibodies in their blood.
“Three months is kind of the rule of thumb that we’re using for when we’re starting to see that drop off,” Yendell said. “Now that’s not, by any means, everyone, at three months, all of a sudden, your antibodies are gone. It’s just that at that point, the percentage of people who are still going to have circulating antibodies starts to decrease.”
Still, not having these types of antibodies doesn’t mean a person’s immune system is back to square one in fighting COVID-19.
“The other cells in their immune system would still be able to say ‘hey, I remember this virus, this looks really similar,’ and be able to kick up an effective immune response,” Yendell said.
At the same time, not everyone who has circulating blood antibodies can necessarily mount an effective immune response if they encounter the virus again. It might be that they get the virus with more mild symptoms, but a lot’s still being learned, Yendell said. It’s for this reasons that public health officials like to remind Minnesotans that learning you have COVID-19 antibodies doesn’t mean you’re immune to the virus.
At a population level, though, Yendell said testing for blood antibodies complements other COVID-19 surveillance efforts to help us better understand the spread of the virus in the state, putting the virus in context.
Some of these efforts include the testing of people who may be infected as well as emerging efforts by researchers at the University of Minnesota-Duluth and the Metropolitan Council to test wastewater to determine the prevalence of COVID-19 in the state or in a given community.
“It’s easy to get sucked into the sense that everyone’s been exposed and that’s not the case,” Yendell said: a lot of people have, and a lot of people haven’t and are still susceptible to the virus.