Minnesota is heading into the school year with modest but persistent COVID-19 levels in a state population that, for the most part, has already been hit by the pandemic.

An estimated 82% of Minnesota children have been infected with the coronavirus that causes COVID-19, according to an update this week by the Centers for Disease Control and Prevention. The estimate is based on a double-check of 675 blood samples submitted in May and June for various medical tests, and likely indicates a high rate of prior COVID-19 cases among Minnesota adults as well.

"I think the number is likely correct; this virus has saturated our population," said Michael Osterholm, executive director of the University of Minnesota's Center for Infectious Disease Research and Policy.

Exactly how previous COVID-19 exposure is affecting current viral spread is unclear, but Thursday's weekly state situation update showed little change in levels of pandemic activity or severity. Reported infection numbers climbed after the July 4th weekend in Minnesota but have remained about 1,400 per day since mid-July. The state has averaged about five COVID-19 deaths per day this summer.

COVID-19 hospitalizations in Minnesota rose above 500 earlier this month for the first time since early March. However, the 519 COVID-19 hospitalizations on Monday included only 38 requiring intensive care — keeping the pressure off hospitals' limited critical care capacities.

The number of COVID-19 patients in Minnesota ICUs was last above 50 on March 9 and dropped to a low of 25 in mid-April. At peak demand, hospitals reported 399 COVID-19 patients in their ICUs on Dec. 1, 2020.

The BA.5 variant of the coronavirus is causing around three-fourths of COVID-19 cases in Minnesota right now, and has broken through infection- and vaccine-based immunity at a much higher rate than previous variants. Vaccinated people made up 76% of identified infections among Minnesota adults in the last week of July, according to the latest state breakthrough data.

High immunity levels appear to be reducing severe illness, though. The rate of COVID-19 deaths over the past two months was nearly five times higher among unvaccinated seniors than Minnesotans 65 and older who received initial vaccine doses plus recommended boosters.

Vaccine activity has tapered in Minnesota. While 66% of eligible Minnesotans 5 and older have received at least one booster dose to maintain immunity after their initial shots, only 34% are considered up to date. Minnesotans 50 and older have hesitated to seek recommended second booster doses when eligible. Only 44% of children ages 5 to 11 and 10% of preschool children have received any COVID-19 vaccine at all.

Some health officials believe vaccine and booster rates will increase this fall with the arrival of reformulated COVID-19 vaccines that are designed to work better against the latest variants. Broad availability of the new booster options is expected as soon as next month.

Osterholm discouraged waiting for the new versions, because the existing versions provide short-term immunity from infection but longer-lasting protection against severe illness.

"Never postpone getting your dose today if you can, if you're eligible for it," he said.

The CDC has discontinued analysis of adult blood samples from commercial medical tests, but it will continue through 2022 to report so-called seroprevalence estimates of children with prior coronavirus infections. Earlier rounds of seroprevalence testing showed that infection rates were highest in children and declined with age.

Risk of severe COVID-19 remains greatest among the elderly and people with underlying health conditions. Of the 40 COVID-19 deaths identified so far in Minnesota in the first two weeks of August, 35 were seniors. The rest were Minnesotans ages 50 to 64.

Antiviral drugs have likely helped to tamp down the rate of infections in Minnesota resulting in severe illness and death. The state has received more than 106,000 courses of the most effective oral antiviral, Paxlovid, this year and has at least 27,000 courses left on hand.

A University of Minnesota clinical trial offered mixed results on the promise of adding existing medications to the COVID-19 arsenal. In results published Wednesday in the New England Journal of Medicine, the U researchers reported no benefit from taking low-dose fluvoxamine, an antidepressant, or moderate- to high-dose ivermectin, an anti-parasitic drug that some have championed as an off-label treatment.

The U's COVID-OUT trial failed to find any drug that lowered aggregate health scores for patients — as measured by whether they died, needed hospital care or suffered dramatic declines in blood oxygen as a result of coronavirus infections. However, participants who took the diabetes drug metformin showed lower rates of COVID-19 hospitalizations and deaths.

"The virus is continuing to change, so we want to continue to be prepared with new tools," said Dr. Carolyn Bramante, the U researcher who led the trial. "We don't know exactly what this virus will be doing next."