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U of M researchers model Michigan's immunity to COVID via vaccine, prior infections

A nurse administers a COVID-19 vaccine during a drive-thru clinic.
Emma Winowiecki
Michigan Radio

Epidemiologists at the University of Michigan's School of Public Health estimate that 68% of Michigan residents have some sort of protection from COVID-19. That's thanks to both the COVID vaccine and immunity from prior infection.

That number comes from the Centers for Disease Control and Prevention's seroprevalence data, which says 28.1% of Michigan's population has been previously infected, as well as the state's vaccine coverage data. So far, about 52% of the state's total population has received at least one dose of the vaccine.

Josh Petrie is an epidemiologist at U of M's School of Public Health. He says modeling like this gives epidemiologists a rough estimate of where the population is at, in terms of immunity. 

Petrie says assuming 68% is correct, it's a good start—especially when experts have used 70% as a good number to talk about herd immunity to COVID-19. 

"We're definitely not there in terms of vaccine coverage and we're not even there yet with infection and vaccine. So I would say definitely current current trends in terms of COVID transmission in Michigan show that we we still are susceptible to outbreaks," he said.

Modeling and trying to visualize what herd immunity look like also doesn't take into account certain factors, like children being unvaccinated, Petrie says.

"But another assumption around that is that immunity is evenly distributed throughout the population, which we know is not the case since children cannot get vaccinated. So they certainly are a large group," he said. "We also have the Delta variant, which is more transmissible, which requires a higher level of immunity in the population to to stop it."

Marisa Eisenberg, another epidemiologist at the School of Public Health, says another factor to consider is that this modeling assumes that people are equally as likely to get vaccinated whether they've been previously infected or not. 

"It’s important to note that that assumption almost certainly isn’t true—most likely there are differences in vaccination levels between previously infected versus not, and definitely a person's infection risk changes if they’re vaccinated. She added, "But it gives us a rough sense of what proportion of the population might have some immunity."

Petrie says people who've been previously infected should absolutely still get the COVID-19 vaccine, and that vaccines are the best way forward to getting us closer to a state resembling heard immunity.

"You should definitely still get vaccinated if even if you've been previously infected. We do know that you get a better immune response, a larger immune response after vaccination than previous infection," he said.

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Caroline is a third year history major at the University of Michigan. She also works at The Michigan Daily, where she has been a copy editor and an opinion columnist. When she’s not at work, you can find her down at Argo Pond as a coxswain for the Michigan men’s rowing team. Caroline loves swimming, going for walks, being outdoors, cooking, trivia, and spending time with her two-year-old cat, Pepper.
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