Three unrelated measles cases in the Twin Cities this month have state health officials concerned that the viral infection is circulating locally and presenting a threat to anyone who isn't vaccinated.

Investigators with the Minnesota Department of Health are continuing to search for connections, but so far have found none among the infected children in Anoka, Hennepin and Ramsey counties. All three cases were identified this week, and two involved children who were hospitalized. None had been vaccinated.

Close contacts of the infected children are being notified of their risks, but the state on Thursday cautioned that "anyone who is not vaccinated against measles could be at risk and should watch for symptoms of measles."

When close contacts are identified, the state in some cases may recommend post-exposure therapies to boost immune systems. Contacts who do not receive recommended prophylactic therapies may be asked to avoid high-risk settings where they could infect others for 21 days.

"Measles spreads easily, and it finds those who are vulnerable," said Jessica Hancock-Allen, infectious disease division director at the state Health Department.

Fifteen cases have been diagnosed so far this year in Minnesota, but prior clusters had identifiable relationships. Infections of three siblings were reported in May after their family had been traveling in a country in Europe where measles is common.

Measles is one of the most contagious diseases on the planet, causing initial cold and fever symptoms followed by a characteristic rash that often spreads from the head to the rest of the body. 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 spread is limited in Minnesota because more than 92% of children are vaccinated for measles by the age of 2, but the infection still finds susceptible pockets across the state. In 2017, an outbreak in Minnesota started among unvaccinated Somali children in child care sites and infected 75 people.