US researchers said the outbreak of measles, which began among children in the Disney theme park, spread due to low vaccine rate.
As few as 50 to 86% of kids were vaccinated against measles in states like California, Arizona and Illinois where cases clustered, far below the 96 to 99% needed to create what is known by researchers as "herd immunity".
The study in the journal JAMA Pediatrics is the "first to positively link measles vaccination rates and the ongoing outbreak," the authors from Boston Children's Hospital said.
A total of 142 people from seven states have come down with measles in the outbreak that began December 28, 2014, the US Centers for Disease Control and Prevention said.
"The fundamental reason why we're seeing the number of cases we are is inadequate vaccine coverage among the exposed," said John Brownstein of the Boston Children's Informatics Program.
Brownstein is a digital epidemiologist and co-founder of HealthMap and VaccineFinder, an online service that helps connect people with locations offering vaccinations, such as the MMR vaccine that protects against measles.
"Our data tell us a very straightforward story that the way to stop this and future measles outbreaks is through vaccination," he added. "We hope these data encourage families to ensure they and their loved ones are vaccinated."
Measles is a highly contagious disease that causes a rash and fever, and can lead to complications ranging from brain damage to hearing loss and blindness, or, in rare cases, even death.