A CDC flip-flop over whether tiny aerosols can spread the virus that causes COVID-19 through the air is not impacting Minnesota's response to the pandemic.

Many public health experts were surprised after the CDC on Friday stated that aerosols are a primary means of COVID-19 transmission — with some researchers claiming this risk has been underappreciated for months — but then they were equally surprised Monday when the federal agency withdrew the statement.

Minnesota health officials from the start have said aerosol transmission is a possibility, but is not as much a risk for COVID-19 as it is for pathogens such as measles. The state did work on airborne transmission of measles with a study showing how it spread far across the Metrodome at a 1991 Special Olympics event.

The state's COVID-19 guidance indicates a moderate risk when spending 15 minutes within 6 feet of an infected person, who could spread larger virus-carrying droplets while talking, breathing or coughing at others in that proximity. However, state infectious disease director Kris Ehresmann said other modes of transmission haven't been disproved.

"With COVID, we are absolutely learning new things all the time, so we continue to adapt our guidance," she said. "We will continue to look at the role that aerosolization can play in transmission."

The pandemic in Minnesota has reached 90,942 infections with the coronavirus that causes COVID-19, and 1,969 deaths — including 937 infections and four deaths reported Monday. At least one infection has been reported in 351 K-12 schools, but fewer than 100 have reported more than one so far.

State officials remain concerned that activity over Labor Day weekend and the restart of K-12 schools and colleges could result in infections of young adults who then spread the virus to older or sicker adults at greater risk of severe COVID-19.

One encouraging trend is a decline in the positivity rate of diagnostic testing from around 6% last month to 4.4%. On the other hand, the rate of infections from unknown community sources has risen to 36%, which hampers the ability of state contact tracers to slow the spread of the virus by identifying and quarantining people who have been exposed.

Concerns about the aerosol risk of spreading COVID-19 first emerged in Minnesota hospitals, some of which installed negative airflow systems in treatment rooms to prevent aerosols from seeping out into nursing stations and other areas.

Researchers have examined the community threat as well. University of Minnesota mechanical engineers conducted simulations this summer to assess how aerosols capable of carrying the COVID-19 virus could spread in classrooms, elevators and supermarkets. The researchers are soon to publish results of how musicians at Orchestra Hall could spread aerosols and what needs to be done to protect future audiences.

The CDC's policy switch was unexpected and unannounced when its website was revised Friday with guidance noting "growing evidence that droplets and airborne particles can remain suspended in the air and be breathed in by others, and travel distances beyond 6 feet."

After media reports, the agency withdrew the wording Monday and indicated that it was draft guidance that had been posted in error.

"Unfortunately an early draft of a revision went up without any technical review," said Jay Butler, a CDC deputy director for infectious disease.

Proponents of the aerosol risk of COVID-19 said they remain confident in their research findings and had hoped the CDC guidance would reinforce the importance of mask-wearing and encourage investments in ventilation and airflow systems to protect people while indoors.

"While my colleagues and I await the CDC's new guidance, it doesn't change the fact that transmission by aerosols is happening and that we know how to address it," said Linsey Marr, a civil and environmental engineering professor at Virginia Tech.

Some business owners are acting ahead of any government resolution on the aerosol question. Twin Cities restaurants that are part of the Jester Concepts group have been updated with needlepoint bipolar ionization systems that are designed to capture harmful particles and purify indoor air.

"When it comes down to it, the CDC, even the Minnesota Department of Health, they're still figuring everything out," said Brent Frederick, owner of Jester Concepts. "Cleaning the air to me was the most common-sense approach."

State Health Commissioner Jan Malcolm said her department would continue to review the latest science on COVID-19 and react as needed — even if contradicting CDC guidance. The state had ignored CDC guidance — which also was later reversed — discouraging testing of asymptomatic individuals.

"We do and have in the past continued to kind of look at the science ourselves and make decisions to, in some cases, stick with our guidance even when the CDC guidance might be changing," Malcolm said.

The state is rapidly expanding diagnostic testing as part of its strategy to identify more infections this fall and to alert people who may have been exposed to the virus.

The Health Department last week announced a "no barriers" strategy by which it would offer free testing clinics in four to six communities each week for the next month. On Monday, it unveiled its first testing site in Duluth using a new system that analyzes easier-to-obtain saliva rather than nasal or throat swabs.

"The sooner we can identify cases, the sooner we can take action," Ehresmann said.

Increased testing could on its own increase daily infection numbers, which Minnesota leaders use to assess their progress in controlling the pandemic and whether schools can remain safely open. Even if that happens, Malcolm said infection numbers and other statistics are only a starting point for discussion about policy or public health decisions in response to the pandemic.

"We have said all along, we need to look at multiple factors and how they play together," Malcolm said. "So no one number, whether it's case growth or hospitalizations, is the sole or most important measure."

The Washington Post contributed to this report.

Jeremy Olson • 612-673-7744