The University of Minnesota is recruiting 150 young adults to study how effectively COVID-19 vaccine limits spread of the corona­virus — a key question when health officials fear that a delta variant of the virus is spreading more rapidly.

While clinical trials last year established that three COVID-19 vaccines reduced the likelihood of severe illness and death, they didn't establish whether vaccinated people were less likely to carry and transmit the coronavirus to others, said Dr. Susan Kline, an infectious disease specialist leading the U arm of the national trial.

"I felt this was an unanswered question; it was a gap in our knowledge," she said.

The Prevent COVID U study being conducted by academic medical centers in 25 states was launched before the emergence of the delta variant in the U.S. that has caused widespread outbreaks in other states and is believed responsible for more than 80% of infections in Minnesota right now.

The Minnesota Department of Health on Wednesday reported 499 infections involving the delta variant that were identified through genomic sequencing analysis of a sampling of positive specimens. That total includes 190 specimens collected since July 1, making delta the most common variant identified over the past month.

Pandemic activity increased in July in lockstep with the rise in delta variant infections. The number of COVID-19 hospitalizations in Minnesota has increased from 90 to 231 over the past two weeks, and the positivity rate of diagnostic testing increased in July from 1.2% to 3.1%.

Kline said the study is not specific to the delta variant but will ultimately assess its infectiousness now that it is the dominant strain in the U.S. The Centers for Disease Control and Prevention last week recommended mask-wearing in K-12 schools and counties with substantial or high viral transmission rates, in part because of concerns that the delta variant was spreading more easily even among vaccinated individuals.

"In rare occasions, some vaccinated people infected with the delta variant after vaccination may be contagious and spread the virus to others. This new science is worrisome," CDC Director Dr. Rochelle Walensky said, in making the recommendation last week.

The study needs people 18 to 29 who haven't been vaccinated yet. Information is available at the study website at, or by e-mailing local study coordinators at

Participants will be vaccinated or placed in a comparison group and will provide daily nasal swabs for four months to monitor for coronavirus infections. Participants also will identify close contacts to whom they would likely spread the virus if they became infected, and those contacts will be asked to take part in a more limited role as well.

Some participants will provide blood samples for the study, which Kline said will assess the amount of virus in infected people and whether that varies by whether they are vaccinated.

The study initially was focused on college students, but Kline said it expanded to a broader universe of young adults — in part because college vaccine mandates this year have made it harder to recruit unvaccinated students. The study uses only the two-dose Moderna vaccine.

Other studies nationally have assessed whether vaccine prevents viral transmission in addition to severe COVID-19 illness and death. St. Luke's Regional Health Care System in Duluth is part of a federal group that assessed vaccination in health care providers and paramedics and found that it reduced the spread of asymptomatic infections among them.

Breakthrough coronavirus infections remain rare, despite concerns that vaccinated people infected with the delta variant can spread the virus.

The state on Monday reported 3,886 identified breakthrough infections out of more than 3 million fully vaccinated Minnesotans — a rate of 0.1% that suggests to health officials that the vaccine is highly protective.

The first-dose vaccination rate of eligible Minnesotans 12 and older is 67%, but Gov. Tim Walz and state health officials said they would like to see it closer to 80% to reduce the threat of viral spread at schools this fall and events such as the Minnesota State Fair.

Jeremy Olson • 612-673-7744