Indiana is battling its second measles outbreak in two years, even though its vaccination rate exceeds the national average. Health officials say the cases, traced to a Super Bowl event, illustrate just how vulnerable the public is to exposure from sources at home and abroad.

The 13 cases confirmed this month by state health officials have been confined to two counties, Boone and Hamilton. But all cases are linked to two infected people who visited the Super Bowl Village together on Feb. 3, prompting Indiana officials to reach out health departments in New York and Massachusetts for fear that the outbreak could spread across state lines.

Concerns about a widespread outbreak are well-founded, said University of Minnesota professor Kristen Ehresmann, who was part of a research team that studied the disease's spread across a large sporting event.

In 1991, a track and field runner from Argentina participating in the Special Olympics in Minneapolis unknowingly started an outbreak of measles, infecting spectators, athletes and event organizers.

"This was kind of Murphy's Law of disease transmission, with a highly infectious disease in a very, very crowded place so as to spread the disease as much as possible," Ehresmann said.

The Super Bowl setting has the same potential to spread the notoriously infectious disease, even though health officials in 2000 declared endemic measles — cases of the disease that originated in the United States — to be eliminated, due largely to increasing vaccination rates.

About 90 percent of all Americans are vaccinated against measles, said Dr. Greg Wallace of the Centers for Disease Control and Prevention. And in Indiana, more than 92 percent of children ages 19 months to 35 months received the vaccine in 2010, the latest year for which figures are available.

But concerns over the vaccine's safety have kept some parents from immunizing their children. In 2005, an outbreak of 34 measles cases in Indiana were traced back to a group of parents who didn't vaccinate their children because they believed it caused adverse health effects, according to a study published in the New England Journal of Medicine.

Ehresmann said claims that the vaccine causes conditions such as autism were debunked in several major studies.

In other countries, vaccination rates are dropping, which raises the risk of exposure. The number of cases in Western Europe has particularly spiked, with France reporting the third-highest number of measles cases in the world. More than 750 people have contracted measles in an ongoing outbreak in Québec, Canada, despite the country's high immunization rate.

The U.S. typically sees about 50 cases of measles each year. But there were 223 cases in 2011 — a 15-year high. Officials said the increase was due to Americans picking up the disease while abroad. Three-quarters of U.S. measles cases can be traced to importation, with the rest originating from contact with a visitor from another country, Wallace said.

Indiana's outbreak has prompted state health officials to issue near-daily updates on the number of cases and places visited by those infected in hopes of stopping the disease's spread. In Noblesville, where a suspected measles case has been reported, school officials have been cleaning classrooms and canceling evening activities. Spokeswoman Sharon Trisler said 98 percent of Noblesville students are immunized against the disease, and the rest are being asked to get the vaccine.

Wallace said health departments are typically able to control outbreaks as long as they catch them early, and doesn't predict a large-scale resurgence of measles in Indiana so long as immunization rates remain steady.

Ehresmann hopes news reports of cases at the Super Bowl remind people that measles are dangerous and contagious.

"You never know when it's possible to have an exposure," she said. "Who would think the Super Bowl would have any link or tie to the measles?"