Breakthrough infections might be a significant part of the latest pandemic wave — with 15,819 coronavirus infections found in fully vaccinated Minnesotans — but state health officials and Mayo Clinic researchers say immunization remains a critical way to reduce severe COVID-19 risks.

The new breakthrough total, reported Tuesday by the Minnesota Department of Health, included 3,260 coronavirus infections identified in the past week out of more than 3 million fully vaccinated Minnesotans. The total includes 957 people with breakthrough infections who were hospitalized and 93 COVID-19 deaths.

Nearly 30% of positive COVID-19 tests in August involved Minnesotans who had been fully vaccinated — a reflection of the state's relatively high vaccination rate but also of the infectiousness of the fast-spreading delta variant of the coronavirus. The additional infections raised Minnesota's rate of breakthrough infections to 0.52% of fully vaccinated individuals.

Mayo researchers urged more vaccinations on Tuesday despite publishing a study showing that the risk of symptomatic breakthrough infections increased 150 days after being fully immunized with the Pfizer COVID-19 vaccine.

The risk of symptomatic COVID-19 remained far lower for vaccinated people at that point when compared with unvaccinated people, said Dr. John O'Horo, a Mayo infectious disease expert and co-author of the research, which was published on the preprint server medRxiv but has yet to be vetted by a peer-reviewed medical journal.

"What that really means is your odds have gone from being very, very low to very low for having an infection … You are still far better off vaccinated than not, but we do see this early signal of some waning immunity."

The study complements preprint Mayo research last month showing declining effectiveness of the two-dose Pfizer and Moderna COVID-19 vaccines in preventing any infections amid the delta variant wave, but strong protection against severe COVID-19 illnesses, hospitalizations and deaths. None of the research addressed the single-dose Johnson & Johnson vaccine, because it was approved later and makes up 4.5% of the doses administered in Minnesota.

A new federal profile report for Minnesota, released Tuesday, estimated that more than 97% of infections in the state are linked to the delta variant. The report listed Minnesota with the 14th lowest rate of new infections among states and the third lowest rate of new COVID-19 deaths in the latest pandemic wave — which continues to hit southern U.S. states the hardest.

Minnesota on Tuesday reported 12 COVID-19 deaths and 2,088 coronavirus infections, raising its pandemic totals to 657,492 known coronavirus infections and 7,856 COVID-19 deaths.

The update included the COVID-19 death of a Ramsey County resident in the 40 to 44 age range. That is the 120th COVID-19 death involving someone younger than 45 in the state, where 87% of the deaths have involved seniors.

The direction of the pandemic wave is unclear in Minnesota, where infection growth in recent weeks has been lower than predicted by Mayo's 14-day forecasting models. The positivity rate of COVID-19 diagnostic testing has hovered at 6.6% for the past week, while the number of COVID-19 hospitalizations declined from 629 on Sept. 1 to 615 on Sept. 3.

COVID-19 figures from the extended Labor Day weekend won't be released until Wednesday, though, and state officials remain concerned about the looming impact of large events such as the Minnesota State Fair and the restart of K-12 classes in kick-starting viral spread this fall.

Minnesota on Tuesday reported a first-dose vaccination rate of 70.9% among eligible residents 12 and older, and a rate of 59.8% for the entire state population.

The actual rates are higher, though, because the state count doesn't include shots administered by federal agencies such as the U.S. Department of Veterans Affairs and the Indian Health Service. The Centers for Disease Control and Prevention lists Minnesota's first-dose rate among eligible people at more than 73%.

Gov. Tim Walz on Tuesday urged continued vaccinations in Minnesota and commended the 3,118 people who received free shots at the Minnesota State Fair.

"We need every eligible Minnesotan to roll up their sleeves and get their shot so we can have a safe, healthy fall and make our schools safer for students and teachers," Walz said. "Our children who cannot yet get vaccinated are relying on you."

Mayo's study used a negative case-control approach, comparing people with positive tests for symptomatic illness between February and August with people who had negative tests in the same general time frames and locations. The study focused on recipients of the Pfizer vaccine because it was the first to be approved and was the most used early on in Minnesota.

The researchers said the findings back the universal use of COVID-19 vaccine but point to the need for more studies about waning immunity and the need for booster doses.

Mayo's findings about reduced severe illnesses in vaccine recipients matches with Minnesota's breakthrough infection data. The state rate of breakthrough hospitalizations in fully vaccinated Minnesotans is 0.031% and the rate of breakthrough COVID-19 deaths is 0.003%. Some of the hospitalizations involve people admitted for other reasons whose mild infections were discovered through routing testing.

Jeremy Olson • 612-673-7744