Revved-up riders are rumbling by the hundreds of thousands back to the Sturgis Motorcycle Rally in South Dakota, which starts Friday, raising concerns for health officials that they could spread the coronavirus that causes COVID-19 and bike it back home.

Epidemiologists last year found transmission chains of riders who brought the virus back to Minnesota, and they said the ingredients exist again this year despite the availability of COVID-19 vaccine.

"The risk increases with larger groups and longer durations of exposure, especially in settings where there may be many unvaccinated people and social distancing and wearing masks aren't routinely practiced," the Minnesota Department of Health said in a statement. "Any event or setting that is conducive to spreading the virus will continue to allow more variants to develop, undermining the gains we have already made with this virus."

Conditions in some ways look favorable in South Dakota, which ranks 27th among states for its COVID-19 vaccination rate of people 12 and older and has the second-lowest rate of new coronavirus infections in the nation, according to a federal state-by-state COVID-19 profile report. South Dakota Gov. Kristi Noem has endorsed the event and planned an appearance in Sturgis, where organizers expect as many as 700,000 attendees compared with 460,000 last year.

"The town is already packed," Sturgis city spokeswoman Christina Steele said. The event has no COVID-19 vaccine or mask requirements but offers sanitizing stations and free COVID-19 self-testing kits.

The city also has allowed attendees to drink on public property outdoors, where airflow levels dramatically reduce viral spread.

Concerns this year were heightened by outbreaks at other large events, particularly festivals in Provincetown, Mass., where a cluster of 469 coronavirus infections included 346 people who had been fully vaccinated. That outbreak weighed into the decision last week by the CDC to encourage masks in K-12 schools and in counties with substantial or high levels of viral transmission.

Beyond that, Wisconsin officials linked nearly 500 infections to crowds that attended Milwaukee Bucks games or gathered outside the team's arena — estimated as high as 100,000 one night — during the team's push to the NBA championship.

Minnesota businesses and organizations responded to the CDC with indoor mask-wearing requirements, at least for their workers. State leaders also have kept a watchful eye on large U.S. events as they consider the much-anticipated Minnesota State Fair later this month that Gov. Tim Walz has repeatedly offered as an incentive for getting vaccinated.

A statement on the fair website indicated that no capacity restrictions are planned but discouraged anyone with recent flu-like or cold symptoms from attending.

"If you've been vaccinated, you've already done your part," State Fair general manager Jerry Hammer said on the website. "If not, there's still plenty of time before the fair to be fully vaccinated."

State tracking hasn't identified as many smaller group outbreaks this summer. No bar or restaurant outbreaks have been identified in Minnesota since mid-June, and only five have been found in June or July at weddings or funerals.

Group outbreaks increased only after state infection numbers increased last year, though, said Kirk Smith, a supervisor of the state Health Department's foodborne diseases unit. "We have just been getting into the upper hundreds of cases per day in the last week. … In a couple of weeks, I would not be surprised if that leads to more detected outbreaks."

Minnesota is hoping a $100 incentive for new vaccine recipients will increase immunizations and the chance of unrestricted events and in-person K-12 classes this fall. More than 11,000 people signed up for the incentives on the first day they were available, and more appeared around the corner.

Laura Johnson, 70, of St. Paul, couldn't get her four grandchildren — ages 18 to 21 — to get their shots, but that changed Thursday.

"Now they're calling me to say, 'Grandma, where do I go?' " she said. "Before, it was me trying to convince them to go."

Nearly 3.2 million eligible people 12 and older have received at least first doses in Minnesota, or 67.7%.

A concern about Sturgis is that its free-spirit history could attract more people opposed to vaccination. T-shirts sold last year stated: "Screw COVID. I went to Sturgis."

Minneapolis attorney Earl Gray skipped the event for the first time in 15 years in 2020 but said his COVID-19 vaccination left him comfortable this year. He planned his usual rides in the day and meals, casino games and blues music at night.

"What are you going to do, live in your home for the next 10 years because people are concerned?" he said. "You just get vaccinated."

Zoltán Vári had already arrived from Hungary to his campsite in Sturgis on Tuesday. He got vaccinated against COVID-19, assuming it was a condition for making the trip into the U.S. "It's great to see a party of hundreds of thousands of people," he said.

Usually a sleepy town of under 7,000, the city of Sturgis tried to tamp things down last year, canceling most city-sponsored events and promotion. Bikers showed up anyway.

"The rally is a behemoth and you cannot stop it," said Carol Fellner, a Sturgis local who worried that this year's event would cause a fresh outbreak of cases. "I feel absolutely powerless."

Minnesota research found 51 infections among people who participated in the rally last year, and 35 secondary infections among close contacts back home. One-third of Minnesota counties reported infections likely traced back to the rally. The total cases included four hospitalizations and one COVID-19 death.

In total, Minnesota has reported 616,784 diagnosed coronavirus infections and 7,688 COVID-19 deaths — including 878 infections and five deaths reported Thursday.

COVID-19 levels bottomed out earlier this summer but are increasing because of the fast-spreading delta variant. COVID-19 hospitalizations in Minnesota have tripled from 90 three weeks ago to 270, and the positivity rate of testing for the infection has increased from 1.1% to 4.2%.

Despite concerns that the delta variant could cause more breakthrough infections in fully vaccinated people, health officials said the vaccines remain effective — especially at preventing severe illnesses, hospitalizations or deaths.

Minnesota has identified 4,477 breakthrough infections out of more than 2.9 million fully vaccinated individuals — a 0.15% infection rate.

The Associated Press contributed to this report.

Jeremy Olson • 612-673-7744