Far more Chicagoans may have been exposed to and infected by COVID-19 than previously thought.

According to the preliminary results of an ongoing study, researchers at Northwestern University found that almost 1 in 5 people they tested from 10 zip codes across the city tested positive for coronavirus antibodies.

That figure is much higher than the less than 5% of positive antibody tests recorded by other researchers. The likely reason for that is that the new Northwestern study is using a more sensitive antibody test than that used by commercial labs, says Dr. Elizabeth McNally, director of the Center for Genetic Medicine at Northwestern University Feinberg School of Medicine.

“We found that just under 20% of people in our study had antibodies to the coronavirus,” McNally said. “It’s reasonable to assume that means some degree of infection — but the key part is most of those people didn’t really have any symptoms at all.”

McNally said that the commercial test for coronavirus was developed using the  antibodies measured in the bloodstream of people who had been hospitalized.

“When you are very sick with COVID you have much higher levels of antibodies. So the threshold that they drew was based on those very high levels measured in hospitalized people,” McNally said.

But the test the Northwestern team used can measure the presence of lower levels of antibodies found in people who have been exposed to the virus but exhibit no symptoms or only mild symptoms.

President Donald Trump, after contracting the virus and being treated with a cocktail of experimental drugs — including a synthetic antibody treatment called Regeneron — has in recent days declared himself free of the virus. 

In a tweet that was labelled “misleading” by Twitter, the president claimed he was now “immune” from the virus. 

A total and complete sign off from White House Doctors yesterday. That means I can’t get it (immune), and can’t give it. Very nice to know!!!

— Donald J. Trump (@realDonaldTrump) October 11, 2020

But that statement goes beyond the science of what is known about COVID-19. McNally says the president may not be out of the woods yet.

“The president didn’t make his own antibodies — or at least probably didn’t have much chance to make his own antibodies yet because he’s still very early (in his diagnosis). The antibodies we are measuring are called IgG antibodies — they are the ones that tend to live a long time in your body — and you typically don’t make a good amount of IgG antibodies until about 14 to even 21 days after an infection,” McNally said. “The president has antibodies because he got 8 grams of antibodies injected into his bloodstream. These are synthetic antibodies that were made by Regeneron and they are now going to get an emergency use authorization to try to make those available.”

McNally said the protective effect of those synthetic antibodies does not last as long as antibodies created by the body’s own immune system, which can last for months or even years.

“(Regeneron) is for when people are not making enough antibodies and they are really quite sick,” McNally said. “The idea is you could potentially buy them (patients) some time by giving them these antibodies. So, while the president has this immunity from these antibodies that were added to him. These are antibodies that will probably just go away in a couple of weeks.”

She also notes that only about 270 people have received Regeneron treatment and that it’s not yet known how long its beneficial effects may last.

“That’s the reason why there is a lot of trepidation about the degree to which he is really protected going forward,” McNally said.

While the Northwestern researchers’ findings indicate a higher level of coronavirus exposure and spread than some other studies, McNally said they are entirely consistent with what was seen in South Korea very early in the pandemic.

“South Korea was one of the few countries that was able to conduct a fair amount of screening for the virus,” McNally said. “Not just testing when people got sick but actually screening. And when you look at their data that they were putting out in March it was very clear that there were a huge number of cases in people between the age of 20 and 40. And most of those people weren’t having symptoms.”

But they were spreading the virus.

The “$64 trillion question,” McNally says, is why some people may get very sick and even die from the virus while others exhibit few or no symptoms at all.

“That is the question we haven’t answered. If we could understand what is it that makes some people likely to get so sick from this virus then we can do a better job of really protecting those individuals,” she said.