A sporadic burst of measles in Minnesota this summer is vexing public health officials, because they haven't yet nailed down the sources of this highly contagious disease.

A public health investigation is ongoing to find the origins of three infections last month of children 10 and younger from Anoka, Hennepin and Ramsey counties who had no apparent connections with one another. Those three infections led to seven more infections that were reported this week among children who were siblings, cousins, friends or neighbors, said Jayne Griffith, a senior epidemiologist with the Minnesota Department of Health.

The state total stands at 22 measles cases so far this year, all among unvaccinated people, and a couple more are expected to be added.

The latest increase "hasn't taken us by surprise," Griffith said. "We have been prepared for the close contacts to become cases."

Follow-up talks with the three infected children and their parents have unearthed details about the days before they contracted measles.

Griffith said there might be a tentative connection that could ultimately trace the origins of their infections back to children who contracted the measles in May as a result of overseas travels. That would be a relief, because the alternative is something state health officials want to avoid: community spread of measles within the state itself.

"The more you keep talking to people, it will sometimes trigger a memory" about events that could have led to measles exposure, said Griffith, adding that doctors have been placed on high alert to monitor for measles and test for the disease.

The one surefire common denominator is that all of the infected children were unvaccinated, prompting an urgent plea from M Health Fairview on Friday for parents to get their children scheduled for shots before the school year.

The share of kindergarteners in Minnesota who are up to date with the measles, mumps and rubella shots has fallen from nearly 93% in 2020 to 87%. In broad terms, that amounts to at least one more unvaccinated child per typical classroom.

Fairview's Roselawn Clinic is responding with a new approach, inviting parents to bring unvaccinated children to receive free shots in low-stress, comfortable environments. The Minneapolis-based health care provider also supports the state-funded No Shots No School program that allows parents to make clinical appointments for children just to receive their shots.

Vaccine hesitancy increased during the COVID-19 pandemic, when many people raised concerns about the shots that were created and distributed at first on an emergency basis to fight the pandemic.

"The biggest thing is these are preventable diseases," said Dr. Jessica Whelan, a family practitioner in Maplewood who is helping organize Fairview's new vaccine campaigns. "Rather than dealing with the complications, let's address things before they become issues."

About 55% of Minnesota's cases so far this year have needed hospital admissions for treatment of symptoms, which often begin with cold and fever symptoms and a characteristic rash that spreads from the head to the rest of the body.

Measles is one of the most contagious diseases on the planet. The ease of airborne transmission of the virus was famously documented by a 1991 outbreak during a Special Olympics event at the Metrodome in Minneapolis, where measles spread from an athlete on the field to fans in the upper stands.

The case count for 2024 already is the third-highest annual total in Minnesota since at least 2000. In 2017, an outbreak in Minnesota started among unvaccinated Somali children in child care sites and infected 75 people.

Griffith declined to identify the races or ethnicities of the children in the current outbreak. She said the decline in vaccine uptake has been uniform and increased risks across all racial groups in the state.