Minnesotans enjoyed a mask-free summer, but experts in aerosol and viral transmission agree that people should wear them again in high-risk situations to protect against the latest wave of COVID-19.

Not that the experts are thrilled to mask up again.

"This sucks," said Chris Hogan, a University of Minnesota researcher who has studied mask effectiveness and is editor of the Journal of Aerosol Science. "But I get it. I'm going to do it. This is a duty we have to knock this down."

Mask-wearing recommendations have emerged across Minnesota this week following federal guidance last week that they be worn in K-12 schools and in counties with high or substantial coronavirus levels. While breakthrough infections are rare, they appear more common because of a delta variant of the virus. A federal case report found 346 of 469 infected people in a Massachusetts outbreak last month were fully vaccinated.

Minnesota on Wednesday required mask-wearing for its workers in state government buildings, a day after St. Paul, Minneapolis, Hennepin County and Ramsey County required masks for workers and visitors in their buildings. Target and Cub Foods were among retailers requiring masks for workers and recommending them for customers. The University of Minnesota and the Minnesota Children's Museum are requiring them indoors as well.

While people hoped their mask-wearing days were over, they need to adapt because of the emerging breakthrough infection risk of the delta variant that requires a broader COVID-19 response to augment vaccines, said Dr. Elie Berbari, chairman of infectious diseases at Mayo Clinic. On the encouraging side, delta waves peaked fairly quickly in India and England, he added, so mask-wearing could help Minnesota endure its version with fewer severe illnesses and hospitalizations.

"Things can change quickly and recommendations that were issued in goodwill a month ago may no longer be valid," Berbari said. "We have to understand that this is a moving target and we have to readjust our reaction accordingly. The faster we adjust what we're doing, the quicker we can control these surges and get on top of this."

Mask-wearing plummeted in Minnesota following the lifting in May of a state mandate. People saying they always wear masks dropped from 79 to 10%, according to survey data published by the Institute for Health Metrics and Evaluation in Washington state.

Not much about masks has changed in that time, Hogan said. While professional grade N95 masks offer the best protection, ordinary surgical and paper masks work when worn properly, he said.

Mayo and U researchers confirmed this in study results published last week — simulating the risks of viral transmission in brief workplace encounters by placing a mannequin that was exhaling particles in front of another mannequin that was inhaling them.

Source control mattered the most, the researchers found, with a mask over the exhaling mannequin preventing 90% of particle transmission to the other mannequin even 1 foot away. Masks over both mannequins worked best, though, along with separating them by 6 feet.

While the CDC earlier this year encouraged wearing two masks, Hogan said that was mostly to eliminate gaps that could leak particles and that a good fit was most important.

"If you have a mask that fits your face well, that you're comfortable with — and I have to emphasize, covers your nose and mouth — that's what we're looking for," he said.

Hogan urged people to seek vaccines, noting that masks have limits and could be less effective against the delta variant — which tends to cause higher viral loads in people who then breathe more viral particles into the air.

Some experts doubted a return to mask-wearing would be effective without broader use of N95s. Researchers at the U's Center for Infectious Disease Research and Policy (CIDRAP) have studied proper mask-wearing levels on live TV and found that a quarter of people aren't even covering their noses with common masks.

CIDRAP director Michael Osterholm said public usage of N95s was discouraged last year when they were in short supply and needed to be conserved for hospital personnel. But he said they are available in greater supply now and should be distributed broadly — especially to front-line workers with many face-to-face contacts each day.

Even well-intentioned people fail to gain a proper fit with ordinary masks, especially if they have beards, and end up with gaps that allow viral particles to escape, Osterholm said. "It's the same thing as fixing three of the five screen doors in your submarine."

Some public health officials are hopeful that people will take mask-wearing to heart now that it is a matter of personal responsibility — compared with when it was a mandate by Gov. Tim Walz that drew political opposition.

A new mandate is unlikely because the emergency powers that Walz used to order the first one have been rescinded. The Minnesota Department of Health on Wednesday urged mask-wearing indoors by all people, regardless of vaccination status, in counties with substantial or high case rates, or in settings such as nursing homes where viral spread could be severe.

Unvaccinated people are urged to wear masks in indoor public settings, social engagements and outdoor settings where social distancing can't be maintained.

More than 3.1 million eligible people 12 and older in Minnesota have received at least a first dose of COVID-19 vaccine — a 67.5% rate that health officials would like to see rise above 80% to limit the opportunities for the virus to spread.

The state on Wednesday reported five COVID-19 deaths and 936 more coronavirus infections, raising the state's totals in the pandemic to 7,683 deaths and 615,923 known infections. COVID-19 hospitalizations in Minnesota also have tripled in less than a month from 90 to 268.

A sampling of positive infections has found 664 cases in Minnesota involving the delta variant this summer.

The number of Minnesota counties with high transmission rates jumped to 16, according to CDC data, while another 42 were in the substantial range that triggers the federal mask-wearing guideline.

With some level of mask-wearing likely in K-12 schools, parents can help by finding masks that fit and are comfortable for their children, and by making sure they go to school with backups, said Dr. Nipunie Rajapakse, a Mayo pediatric infectious disease specialist. "Use [a style] they have worn before, that you know fits well, that they will be able to keep on."

Mask-wearing could be used temporarily to buy time for more vaccinations, but Dr. Gregory Poland, director of Mayo's vaccine research group, said the level of immunization that is needed is higher now because of variant strains. That means masks could be part of the longer-term strategy to confront the pandemic.

"If it was 70-80 percent back a year ago, it is probably 90-95 percent — something in that order," he said. "And by definition, we cannot get there based on the number of people who are rejecting vaccines."