Measles is popping up in more Wisconsin counties. Health experts say it may get worse.

Wisconsin health officials have identified yet another cluster of measles cases this year.

The state Department of Health Services on Tuesday reported five suspected cases in Rock County.

Officials are still investigating how the new infections started. But Wisconsin has already seen similar clusters of cases this year.

Chippewa County now has 11 confirmed cases after first identifying one infection related to out-of-state travel in late September. In Marathon County, six cases of measles have been confirmed since a single infection from out-of-state travel was found one month ago.

Of the 267 cases reported in Wisconsin this year, the vast majority have been linked to the ongoing outbreak in the southwestern region.

But Dr. Jim Conway, medical director for UW Health Immunization program, said it’s not surprising that unrelated cases are popping up given the level of measles activity around the country.

Since measles was declared eliminated in the U.S. in 2000, Conway said the cases that have been identified have been isolated to local outbreaks.

“Because we had relatively high immunization rates, they were manageable,” he said. “What we’ve now seen over this last 18 to 24 months is this evolution, where it’s starting to act more like what we call an endemic disease, which means it’s circulating in multiple places at multiple times, and then you’re going to get these sort of spinoffs.”

Conway credits state and local health officials with keeping these cases from turning into larger outbreaks by quickly and thoroughly identifying those who have been exposed to the highly-contagious virus.

Measles is the most contagious infection, according to pediatric infectious disease specialist Dr. Thomas Boyce from Marshfield Clinic Health System. He said it’s five times more contagious than COVID-19, and even a small number of cases in a community can be devastating.

Children under a year old, who are not eligible for the vaccine, have the highest risk of mortality from measles, he said.

“Even in an area where there’s good measles vaccine uptake, if an infant or a pregnant woman or an immunocompromised person, somebody who’s had a heart transplant or is immunosuppressed in some other way, acquires measles, it can easily be fatal,” Boyce said. “Occasionally it’s fatal even in previously healthy children.”

It typically takes 11 to 12 days after someone is exposed for the first symptoms to appear — what’s referred to as the incubation period. Boyce said that means an outbreak can look like more of a prolonged smolder than an out-of-control wildfire, like the recent outbreaks in Texas and Pennsylvania.

Boyce said most infectious disease doctors are worried about what will happen this winter, when the spread of all respiratory viruses picks up while people are stuck indoors.

“It’s certainly ripe for bigger outbreaks than what we’re seeing right now,” he said.

The virus’ long incubation period also means that public health officials have to stay alert for new infections for more than a month to ensure cases aren’t continuing to spread under the radar, according to Conway.

To keep a cluster of cases from turning into a full-blown outbreak, Conway said people need to make sure they’ve received two doses of the measles, mumps and rubella, or MMR, vaccine, especially anyone who grew up before the recommendation of two doses was adopted in 1989.

“These vaccines are extraordinarily safe and are really effective,” Conway said. “These vaccines have been around since the 1970s. I mean, we know more about these vaccines than almost any other vaccine.”

Individual choice over what vaccines a person or their children receive has become increasingly important to some communities since the COVID-19 pandemic. 

But Boyce emphasized that the choice not to vaccinate is not inconsequential. He compares it to a parent’s choice whether to use a carseat.

“You can either put your child in the car seat, or you can let them ride on your lap. But I think everyone would agree that there is only one best choice to make in that situation,” he said. “I would strongly argue that with vaccines and children, there is only one best choice, and that is to vaccinate your child with the routine vaccines that are recommended by your child’s physician.”