New federal research co-authored by doctors at Hennepin Healthcare offers new concerns about COVID-19 exposure risks among those out at bars, as well as at cafes or restaurants.

Surveying 314 people who sought outpatient treatment for respiratory symptoms, the researchers found those who tested positive for COVID-19 were more than twice as likely to have been places in the prior two weeks where food and drinks were served compared with those whose symptoms weren't related to the pandemic.

The two groups were similar in rates of shopping, attending churches and going to gyms, suggesting an independent risk for people who inevitably had to take off masks while eating and drinking at establishments, said Dr. Heidi Erickson, a co-author of the study and a pulmonary and critical care specialist at HCMC in Minneapolis.

"In these places where people can't necessarily wear their masks, where they're eating and drinking, it's not surprising to me at all that there would be increased transmission," Erickson said Thursday.

The study comes as health leaders try to reduce the risk of outbreaks in Minnesota's bars and restaurants — with 66 such outbreaks having been investigated and 38 establishments being publicly identified and linked to clusters of at least seven cases. Those outbreaks contributed to infections in roughly 1,200 people, who were then at risk to spread the virus elsewhere.

The study had limitations, including that it revealed an association between eating out and COVID-19, but not a cause and effect. No contact tracing was done to verify if patients with COVID-19 were infected while visiting establishments, and the study didn't determine whether patrons were indoors or outdoors or how long they spent dining.

The state on Thursday reported 15 COVID-19 deaths and 389 new infections with the corona­virus that causes the infectious disease — bringing the state's totals to 1,884 deaths and 82,249 lab-confirmed infections.

Inspection sweeps on two recent weekends found that 79 of 167 establishments had at least minor deficiencies with COVID-19 guidelines. Bars and restaurants under an emergency order by Gov. Tim Walz must operate at indoor capacities of no more than 250 people or 50% of fire code capacities and keep tables 6 feet apart.

Erickson said the study published Thursday by the U.S. Centers for Disease Control and Prevention validated the primary risk of COVID-19 through close contact with friends and relatives. Moderate risk is defined as spending 15 minutes within 6 feet of someone who is infected.

The rate of such close contact with infected people was 42% among patients in the study who tested positive for COVID-19 compared with 14% of those who tested negative. "I don't want that to get lost in the shuffle," Erickson said.

The association between dining out and positive tests was strengthened when researchers looked only at people who hadn't been in close contact with others who had COVID-19. In that comparison, people who tested positive were nearly four times as likely to have gone to bars or coffee shops as people who tested negative, and 2.8 times more likely to have gone to restaurants.

Erickson called the study a reminder "that we're all in this together and what we do as individuals in terms of our mask-wearing practices and social-distancing and quarantining — if we know we were exposed to someone with COVID — has a really significant effect."

The study, which was based on patients in 11 U.S. clinic locations, didn't account for the level of risk-mitigation efforts required in Minnesota establishments that could protect customers, said Liz Rammer of Hospitality Minnesota, which represents the state's restaurant industry.

"People who are willing to visit restaurants are probably more likely to gather in other settings," she added, so restaurants might have an association but aren't necessarily the cause of more infections.

The CDC research raises the prospect that mask-wearing and social distancing might not offer full protection if the corona­virus can circulate broadly in the air from one table to the next.

While the measles virus and others are more readily airborne, this corona­virus has shown some ability to be so as well, said Kris Ehresmann, state infectious disease director, earlier this week.

"Better air flow makes a difference in terms of reducing the likelihood of transmission," she said.

Engineers at the University of Minnesota have been studying how the strength of airflow and positioning of ventilation ducts can affect the spread of particles capable of carrying the virus in environments such as elevators and classrooms. They recently measured airflow in Orchestra Hall to assess risks there.

Some business owners are looking at solutions. Brent Frederick of Jester Concepts said Wednesday that the circulation systems in his Twin Cities restaurants are being modified to try to filter out harmful particles and reduce infection risks.

State officials have been concerned about college students going to bars and large parties because most infections found in college students so far this fall have occurred off campus. Several colleges in the state have reported clusters of cases.

Young adults are at lower risk of severe COVID-19 — with the 15 deaths reported Thursday involving 12 people age 70 and older and 11 residents of long-term care or assisted living facilities. But young, mobile people can easily carry the virus without knowing it and spread it to others who are more vulnerable.