Outbreaks centered on four bars in Minneapolis and Mankato have contributed to a surge in COVID-19 cases in young adults, which state health officials warned could undermine months of planning and recent progress in managing the pandemic.

Roughly 100 people suffered COVID-19 infections related to crowding over the June 12-14 weekend at Rounders Sports Bar & Grill and the 507 in Mankato, while more than 30 cases have been identified among people who went to Cowboy Jack's near Target Field and the Kollege Klub in Dinkytown between June 14 and June 21.

While growth of COVID-19 is inevitable until a vaccine is found for the novel corona­virus that causes it, preventable clusters could cause an escalation that could exhaust the state's medical resources and leave vulnerable people at risk, said Kris Ehresmann, state infectious disease director.

"When you have 56 cases associated with one location from one weekend, that is not managing the rate of growth," said Ehresmann, imploring businesses and individuals to take precautions "so that even as we open up, we are not putting ourselves in a position to overwhelm the system we worked so hard to strengthen."

Social media images from some of the four bars showed tight crowds and disregard for the social distancing requirements that the state imposed when it permitted a limited reopening of indoor bar and dining facilities on June 10. Ehresmann said she is consulting with health inspectors on how to compel them to comply. Anyone at the bars during those time frames should be watching for respiratory symptoms, she added.

Minnesota hasn't fared as poorly in June as mostly Southern states that are seeing upticks both in diagnosed cases of COVID-19 and also in hospitalizations for people with severe diseases.

'Trending poorly'

Minnesota on Friday reported 335 hospitalizations of COVID-19 patients, including 157 who needed intensive care, which is well below the peak of 606 on May 28 that had some metro hospitals opening up emergency surge bed capacity.

However, the COVID Exit Strategy website downgraded both Minnesota and Wisconsin to "trending poorly" this week amid the pandemic — largely due to growth in cases among young adults.

The number of lab-confirmed cases in Minnesota has increased 37% — from 25,208 on June 1 to 34,616 as of Friday — but has increased 51% among people aged 20 to 29 and 61% among children and teenagers. And 20-somethings overtook people in their 30s for the first time this week as the age group in Minnesota with the most cases.

The uptick associated with crowded locations prompted the Minnesota Medical Association, representing doctors, to call for the mandatory use of masks in public rather than just voluntary encouragement.

Following a peak of COVID-19 cases and hospitalizations in late May, case growth ebbed in Minnesota, and health officials hoped that would continue following the reopening of businesses and social activities in June, said Dr. Keith Stelter, association president.

"That has not happened," he said. "So I think at this time we have to come back to say, 'If we're going to have the economy open, then we're going to need to take other personal protective measures to prevent the spread.' Masking is the best way to do that."

The 7,045 cases among people in their 20s now represent 20% of all cases confirmed through diagnostic testing in Minnesota. (The total case number includes 30,008 people who have recovered to the point they are no longer considered infection risks.)

More infections in the young adult population might not result in more severe outcomes — only two of 1,411 total deaths in the pandemic have involved people younger than 30 in Minnesota. But state health officials worry that mobile young adults can spread the novel coronavirus to others who are vulnerable.

Risk particularly increases with age. Minnesotans 70 and older make up 12% of known cases in the state but 81% of COVID-19-related deaths.

The U.S. Centers for Disease Control and Prevention updated risk guidance on Thursday, adding obesity as a major cause of severe COVID-19 cases along with heart conditions, kidney disease, diabetes and other conditions.

Adherence to social distancing in the spring bought hospitals time to prepare with more beds and protective equipment, and now it can buy time for life-saving vaccine development, said Dr. Mark Sannes, a HealthPartners infectious disease specialist.

"We still have a vulnerable population and at some point that group is either going to have to be exposed or be vaccinated," he said. "I think we would prefer the latter."

Preventable spread of the infectious disease has other consequences. Ehresmann said some of the COVID-19 cases associated with bars in Mankato involved workers at child-care facilities, which are now having to take actions that will disrupt the lives of children and families.

"It's a sad example of how COVID works," she said.

Low rate after protests

Protests following the May 25 police killing of George Floyd do not appear to have contributed as much as feared to the rise in cases among young adults.

Health officials worried about heightened risks of the virus spreading among people singing and shouting — and even gasping and coughing when tear gas was used to disperse crowds. On the other hand, outdoor air has proved an effective deterrent to virus transmission.

Free testing clinics in Minneapolis and St. Paul found a relatively low rate of infections — roughly 1.5% of the more than 7,700 demonstrators and others tested at these sites over the prior two weeks.

Testing was recommended for anyone involved in the protests, whether they had respiratory symptoms or not. In the case of the bar patrons, Ehresmann said they most likely had symptoms as they weren't part of any mass screening recommendation and had sought out testing on their own.

Detected cases of COVID-19 via testing are the tip of the iceberg. CDC officials on Thursday estimated that every one lab-confirmed case represented 10 cases. Most infections result in mild or no symptoms, but people with mild infections are risks for spreading the virus to others.

If the CDC estimate is accurate, that means as many as 500,000 Minnesotans have already been infected — with many suffering mild or no symptoms but nonetheless carrying the virus and potentially spreading it to others. At the same time, that would mean that 90% of the population remains vulnerable and that the state has a long way to go, said Jan Malcolm, state health commissioner.

Some of the uptick in young adult cases might be due to increased COVID-19 testing, but the bar outbreaks show this is a new and growing problem as well, she added.

"A small percentage of our population has been exposed so far," Malcolm said. "That means the great majority of Minnesotans remain vulnerable to infection."