Of the 34 people who became sick during the 2005 outbreak: 33 were members of the same northwest Indiana church, 32 had not been vaccinated against measles, and 28 were school-aged. Of the 28 school-aged children, 20 were homeschooled.
The parents of most of the children who got sick said they had refused measles vaccination for their children because of fears about vaccine safety.
The experience shows that outbreaks of vaccine-preventable diseases can easily occur among relatively small numbers of unvaccinated people, says Amy A. Parker, MSN, of the Centers for Disease Control.
It also illustrates the importance of maintaining high vaccination levels. The outbreak did not spread beyond the church to the community at-large, where measles immunization coverage approached 98 percent in school-aged children.
"The outbreak stayed fairly well contained within the unvaccinated population," Parker tells WebMD. "Vaccination coverage rates are very high in Indiana and throughout the U.S., primarily because of school policies," she said, referring to the fact that most schools require students to have proof of vaccination.
Parker and CDC colleagues published their detailed investigation of the Indiana church outbreak in the Aug. 3 issue of The New England Journal of Medicine.
Measles was officially declared eliminated from the U.S. in 2000.
But it remains a major illness in most of the rest of the world, with 30 million measles infections and 454,000 deaths from the disease each year, according to the World Health Organization.
In mid-May 2005, an unvaccinated 17-year-old Indiana girl returned from a church mission trip, where she worked in a Romanian orphanage. She unknowingly became infected with measles during the trip, and had symptoms of the disease when she attended a large church gathering the day after she arrived home.
Around 500 people attended the church function, and roughly 50 did not have evidence of measles immunity.
During the six weeks in which the measles outbreak raged within the church, 33 church members - mostly children and teens - became ill. In all but one of those cases, the victims had not been vaccinated against the disease.
The single measles case that did not involve a member of the church occurred in a 34-year-old health care worker at a hospital where two church cases were treated.
The woman had been vaccinated as a young child, but may have received only one dose of vaccine instead of the recommended two.
CDC researchers interviewed church members who had declined vaccinations for their children.
Many cited press reports suggesting a link between measles vaccine and autism as a reason for their refusal.
"Most families with these concerns continued to decline vaccination, even in the midst of an outbreak involving hospitalizations among their own community members," the CDC researchers reported.
The fact that 20 of the 33 church-related illnesses occurred among homeschooled children illustrates the importance of promoting immunization among that group, Parker says.
Proof of measles vaccination is required for children entering school, but West Virginia is the only state in the U.S. with a similar requirement for homeschooled kids.
In an editorial accompanying the CDC report, infectious disease specialist E. Kim Mulholland, M.D., noted that the Indiana outbreak shows the importance of vaccinating everyone who travels to regions where measles is still an endemic disease.
Mulholland is a professor in the infectious disease epidemiology unit of the London School of Hygiene and Tropical Medicine.
He tells WebMD the outbreak also shows how vulnerable communities can be when vaccination rates are lower than they should be.
"I am not convinced that parents who choose not to vaccinate their children really understand all the risks and implications that go along with that decision," he says.
SOURCES: Parker, A.A. New England Journal of Medicine, Aug. 3, 2006; vol 335: pp 447-455. Amy A. Parker, MSN, MPH, epidemic intelligence officer, CDC. E. Kim Mulholland, MD, professor, infectious disease epidemiology unit, London School of Hygiene & Tropical Medicine.
By Salynn Boyles. Reviewed by Louise Chang, M.D. © 2006, WebMD Inc. All rights reserved