A University of Minnesota study of COVID-19 in grocery store workers could help solve vexing questions about the true spread of the infectious disease and guide state strategies to slow it down before the vaccine is broadly available.

Public health Prof. Craig Hedberg is recruiting 1,000 grocers from across Minnesota to mail self-collected blood samples to see if they contain antibodies in response to the coronavirus that causes COVID-19. Patterns in the positive results by worker type and geographic location will offer important clues, he said.

"Understanding patterns of community spread and understanding what prevention measures seem to be effective at helping to limit that spread are going to remain important for the next months and probably the next couple years," Hedberg said.

The Minnesota Department of Health on Thursday reported 83 more COVID-19 deaths and 2,775 infections, bringing state totals to 4,658 deaths and 389,171 infections, but health officials believe that is an undercount due to the people who never sought testing because their infections produced no symptoms.

At least 15% of Minnesotans have probably been infected when including those who haven't been tested, and that number is probably 20% in some regions, said Michael Osterholm, director of the U's Center for Infectious Disease Research and Policy.

"Just look at the ongoing transmission that occurred over the course of the last month," he said.

If correct, that means 800,000 to 1.2 million Minnesotans have been infected.

Preliminary results of antibody testing during the Thanksgiving week of nearly 500 Twin Cities health care workers found a positivity rate of only 8.5%, said Ryan Demmer, an epidemiology researcher at the U School of Public Health.

However, antibodies can fade to undetectable levels three months after infection, so Demmer said some of the workers might have been infected earlier in the pandemic. He agreed with an estimated 20% total infection rate by year's end.

Antibody testing has offered checkered contributions to the nation's pandemic response. Minnesota engaged in perhaps the most ambitious effort this summer, partnering with the U.S. Centers for Disease Control and Prevention to conduct surveys and testing in 180 sites across the state. The effort ended with no results when federal surveyors withdrew from Minnesota amid claims of harassment and intimidation in some communities.

Demmer said it might not take such widespread antibody testing to come up with usable results. An accurate count is important, though, because transmission of the SARS-CoV-2 virus should die off once the nation achieves herd immunity — achieved when 80% of people have been vaccinated or have some immunity due to prior infection, he said.

"We need to get a sense of where we are at and get a sense of where the population is at heading toward herd immunity," Demmer said.

Grocery workers have shown early interest in the latest antibody study, which could offer new insights if positive tests are more common in certain parts of the state or among certain types of workers, Hedberg said. The goal is to complete recruitment and send out kits for home blood-sample collection this month.

Antibody test results won't impact the state's ongoing vaccination strategy, or priority list for who should receive the limited initial doses of the first federally approved COVID-19 vaccine made by Pfizer.

Providers in Minnesota received 46,800 doses this week and are scheduled to receive more than 200,000 doses by year's end if a second vaccine made by Moderna also gains federal safety clearance. An advisory panel Thursday recommended that the Food and Drug Administration authorize the Moderna vaccine for emergency use.

Nobody knows for sure if prior infections or positive antibody tests mean that people have immunity from second COVID-19 cases, said Kris Ehresmann, state infectious disease director.

"The recommendation is that you get vaccinated regardless of whether or not you've had COVID," she said.

The CDC advises people after COVID-19 recoveries that they don't need to isolate themselves for 90 days, which suggests that people have some level of immunity from reinfection, Ehresmann said, but its unclear how long that lasts.

Limited initial vaccine doses are being reserved for health care workers who are at higher risk of infection and long-term care residents who are at greater risk of severe COVID-19 illness due to their ages or health problems.

Ehresmann said vaccine deliveries have been smooth, though the state is getting only 33,150 Pfizer vaccine doses next week — about 45% less than planned. There has been some discussion about whether five-dose vials can be stretched into six vaccinations, but Ehresmann said federal health officials have offered no definitive guidance.

Nurses Lisa Noreen and Dan Hanson were among the first in Minnesota to be vaccinated on Thursday at Sanford Bemidji Medical Center. Vaccinations also took place at Sanford's Worthington hospital and will start at other hospitals in the next few days.

Noreen hoped for a chance after broader community vaccination to hug her grown children and visit her 103-year-old grandmother out East, while Hanson said it would allow him and his fiancée to fully celebrate their wedding with family and friends.

Hanson said he also wanted to be an example for others who might have concerns about vaccination.

"I signed up immediately," he said. "There was no way I could say no."

The current pandemic wave has shown signs of easing. The daily rate of new infections, based on a seven-day rolling average, dropped from 123 per 100,000 people on Nov. 11 to 73 on Dec. 8 in Minnesota. The number of people with COVID-19 filling Minnesota hospital intensive care beds dropped from 402 on Dec. 1 to 289 on Wednesday.

State health officials said widespread vaccination won't be available until late winter or early spring 2021, and that another wave of infections could emerge around February. They urged continued compliance with mask-wearing, social distancing, and a new emergency order that closed indoor bars and restaurants, reopened fitness clubs with limits, and restricted indoor social gatherings to no more than 10 people from two households.

Jeremy Olson • 612-673-7744