State health officials reported five more cases of measles Thursday, including one in Stearns County that marked the first time the current outbreak has spread beyond Hennepin County.

A total of 29 children have now been sickened since the end of March, making it the largest measles outbreak in Minnesota since 1990.

Like others reported so far, the Stearns County case involves a Somali-American child. Public health investigators are trying to determine how the child became infected, and if the family made trips to the Twin Cities area and came in contact with the highly infectious disease.

“We are going to have to do more sleuthing to understand what the connection is with this child,” said Kris Ehresmann, infectious disease director at the Minnesota Health Department.

Ehresmann said the department had considered it “highly possible that we would see cases spreading” beyond the metro area, particularly to areas such as Stearns County and Olmsted County, that have relatively large Somali-American populations. Low measles vaccination rates within the Somali community make them more vulnerable to catching the virus.

As recently as 2004, vaccination rates for young Somali-Minnesotan children matched those of the general population, but they plummeted starting a few years later, when an apparent rash of autism cases among Somali children triggered a scare over the vaccine. By 2016, measles vaccination rates for Somali 2-year-olds in Minnesota had fallen to just 42 percent.

In the Stearns County case, however, the child had been vaccinated for measles — but had received only one of the two recommended shots. One dose is 93 percent effective against the disease, according to the U.S. Centers for Disease Control and Prevention, while two shots provide 97 percent protection.

It is possible that even with just one shot the child will be less likely to infect others, said Ehresmann.

But the Stearns County case will require health officials to open up a whole new investigative arena, as investigators will need to catalog who came into contact with the child in Stearns County, including people at child care centers, homes and medical facilities.

Finding unvaccinated people is important, because even a brief exposure can cause sickness. The measles virus can linger in the air after an infected person has a left a room and still infect someone who does not have measles immunity protection.

That’s one reason why it is difficult to predict how many cases this outbreak will produce, though officials have said they expect the case count to continue rising in the near future. It has already outpaced a 2011 outbreak that sickened 26.

11 hospitalized

So far, the outbreak has been generally been confined to Somali-American children aged 5 and younger. Public health investigators have determined that 25 were unvaccinated. They are seeking immunization records for another three cases and are trying to pin down the race and ethnicity for four of the latest cases.

In Hennepin County, investigators have identified at least five child care centers where exposures might have occurred. Families of children attending those centers have been contacted, and if anyone is found to be unvaccinated, they are being asked to stay at home so they do not infect others.

Of the 29 current cases, 11 have required hospitalization.

The original source of the outbreak remains a mystery, but state health officials suspect it was imported by a traveler from a foreign country. Measles was declared eradicated in the United States in 2000 and no longer occurs naturally here.

Measles often starts with coldlike symptoms of cough, fever, runny nose and watery eyes, and eventually a rash spreads over the entire body. But it can produce severe symptoms in young children, and in extreme cases can produce lasting lung and brain damage, and even death.