As the number of mumps cases climbed past 1,500 on Thursday, public health officials are seeing low turnout at free immunization clinics for young adults.

The state reported 1,552 confirmed, probable and suspect cases of mumps as of Wednesday, with disease activity in 74 counties. That was up from 1,487 cases as of Monday — one of the smallest reported increases since the outbreak began earlier this year.

"It's still too early for us to say that we're done with this yet," Iowa Department of Public Health spokesman Kevin Teale said.

Last week, the state divvied up 25,000 doses of the measles-mumps-rubella vaccine in 35 counties where colleges and universities are centered, but many in the targeted age group of 18 to 22 years old didn't show up for the shots.

"You have a population that may, very honestly, not be taking this mumps situation very seriously," Teale said. "We need to do our best to convince them how serious this is and get them out to get the shots."

Another series of clinics is planned this week for even more young adults — those ages 18 to 25 years old in all 99 counties. But higher attendance may still be difficult to garner as many students study for final exams as the spring semester comes to a close.

"We are looking at a population that it is probably one of the most stressful times of their life — finals week, school ending," Teale said.

One of the agency's main goals is to catch students who aren't immunized with the recommended two doses of MMR before they go home for summer break, and possibly spread the mumps further. Teale said there will be a third phase of the immunization effort, "but we'll need to decide based on what happens here with (phases) one and two before we decide what to do next."

In a copyright story in Thursday's The Des Moines Register, the newspaper reported that a survey it conducted showed that fewer than 20 percent of the 25,000 doses available were administered last week. Teale said public health officials throughout Iowa continue to submit immunization information to the state to help officially record how many doses have been used.

In Johnson County, home to the University of Iowa, public health officials told The Register that just 262 doses of the 1,300 it received were used during the first clinic.

Lisa James, associate director of the university's student health service, said students have been required to show proof of the two recommended MMR vaccinations for only three years. She said 1,200 students were recently sent e-mails saying they needed to update their immunization records. A flurry of faxes followed from those who had already had their two shots, James said.

Others didn't have time as they prepared for finals and graduation.

"Some of them just couldn't get it higher on their property list for the time that they had, and what they needed to do," she said.

In Polk County, where more than a handful of colleges and universities are located, 517 of 3,600 doses available were used during last week's clinic. So far this week, 23 additional doses have been administered, said Rick Cozin, a spokesman for the county health department.

"We feel good about the turnout," he said. "Our numbers indicate that we reached a significant percentage of the targeted population."

Mumps is a virus spread by coughing and sneezing. The most common symptoms are fever, headache and swollen salivary glands under the jaw. It can lead to more severe problems, such as hearing loss, meningitis and swollen testicles, which can lead to infertility. It does not respond to antibiotics.

Even with the recommended immunizations, about 10 percent of the population remains susceptible to the illness.