Children who did not get vaccinated against whooping cough contributed to the 2010 outbreak of the illness, when more cases were reported than in any year since 1947, researchers say.
Researchers who looked at the geography of the cases suggest that clusters of "nonmedical exemptions" to immunizations were one of several factors in the California outbreak. They reported their findings Monday in the journal Pediatrics.
In California in 2010, there were 9,120 cases of the illness that's also called pertussis — one-third of all the U.S. cases. Los Angeles had 1,000 of those cases. Whooping cough is a respiratory ailment marked by bouts of coughing that are accompanied by a noise that can frighten parents — hence the name.
An earlier study published in the New England Journal of Medicine suggests that the DTaP (diphtheria, tetanus and pertussis) vaccine loses some effectiveness after the fifth of the five recommended doses. That, too, was part of the reason for the outbreak, the Pediatrics scientists say. They also list the cyclical nature of pertussis and improved diagnosis as reasons for the high numbers.
The researchers from several institutions, including Johns Hopkins Bloomberg School of Public Health and the California Department of Public Health, found 39 clusters with high rates of non-immunization and two clusters of pertussis among children entering kindergarten from 2005 through 2010. More cases occurred within the non-immunized clusters than outside of them, the scientists said.
From 2000 to 2010, California's "nonmedical exemption" rates more than tripled, to 2.33%, with some schools reporting rates as high as 84%, the researchers said. Both those clusters and the high pertussis clusters "were associated with factors characteristic of high socioeconomic status such as lower population density, lower average family size, lower percentage of racial or ethnic minorities," higher incomes and other factors, the researchers wrote.
It is estimated, they wrote, that more than 95% of the population must be immunized to prevent outbreaks.
Before the introduction of vaccines, pertussis was a leading cause of childhood death in the United States.
The Los Angeles Unified School District has offered free vaccines to students, and in 2011 there were no deaths from whooping cough in California, a first in the state. And the number of cases in California fell to 2,795.