October 1, 2013
Nonmedical Vaccine Exemptions Likely A Factor In 2010 Pertussis Outbreak
redOrbit Staff & Wire Reports - Your Universe Online
Parents’ refusal to have their children vaccinated against whooping cough may have contributed to the 2010 pertussis outbreak in California, according to research published in the October issue of the journal Pediatrics.
According to Dr. Saad Omer of the Emory University School of Public Health and his colleagues, vaccine refusal was one of the factors that led to the state’s largest whooping cough outbreak since 1947. Over 9,000 cases (one-third the national total) and 10 pertussis-related fatalities were reported in 2010, USA Today’s Michelle Healy noted.
Using data from the California Department of Public Health, the study authors looked at non-medical vaccine exemptions for children entering kindergarten classes from 2005 through 2010 and compared them to the whooping cough cases from three years ago. Healey explained that the state allows parents to obtain exemptions if immunizations violate their religious or philosophical beliefs.
Their research turned up 39 significant geographical clusters which had high-rates of non-medical vaccine exemptions, as well as two statistically significant areas of pertussis cases. Census tracts within an exemption cluster were found to be 2.5 times more likely to also be a whooping cough cluster, and the association remained significant after adjusting for various demographic factors, Omer and his associates revealed.
“Several other possible causes of the outbreak have been put forward, including the notion that the acellular vaccine now used is less effective than the older whole-cell medicine,” Michael Smith, North American Correspondent for MedPage Today, explained in an article Monday. ”Researchers have also suggested that even if it has the same initial efficacy as the whole-cell vaccine, its effectiveness may wane over time, leaving children vulnerable.”
“But Omer and colleagues have been concerned for some time about the risks of clusters of children who don't get vaccinated against Bordetella pertussis,” he added. “In California, the rate of nonmedical exemptions… has been rising, Omer and colleagues reported. From 2000 to 2010, the overall rate in the state rose from 0.77 percent of children entering kindergarten to 2.33 percent. While those rates are still low, they noted, some schools reported as many as 84 percent of new students were exempted in 2010.”
The study authors were able to obtain the number of children starting kindergarten, the number of non-medical exemptions and the locations for 7,091 schools. They also had data pertaining to 8,521 of the 9,143 reported cases of whooping cough, according to Smith. Their analysis revealed that a cluster of exemptions were more likely to be in a pertussis cluster, and that more cases of the disease occurred inside exemption clusters than outside.
“Without understanding the reasons parents are seeking exemptions, [Omer] said, it's not easy to decide how to approach the problem,” the MedPage Today editor said, adding that the researchers “pointed out some other gaps in the data. California records the fact of an exemption, but not the specific vaccines, so that it's possible some children with an exemption might actually have been fully vaccinated against pertussis.”