Increases in whooping cough incidence correlate with areas that have high rates of religious exemptions for childhood vaccinations, findings from a 12-year New York state retrospective study reveal.
From 2000 to 2011, rates of religious exemption nearly doubled, with the overall annual state mean prevalence of religious exemptions for one or more vaccines increasing from 0.23% in 2000 to 0.45% in 2011 (P=0.001), Jana Shaw, MD, of SUNY Upstate Medical University in Syracuse, N.Y., and colleagues reported in Pediatrics.
New York counties that had a higher proportion of people who claimed religious exemption status for their children had greater incidence of pertussis than other areas, the researchers found.
Average pertussis incidence among exempted children was 14 times greater than among vaccinated children, but disease risk also rose significantly among vaccinated children who lived in counties with high exemption rates.
New York state requires children to be vaccinated against certain disease before they can begin school, but parents are able to request religious exemptions. The state health department then tracks these requests by county.
To understand how the rate of religious exemptions influenced pertussis rates, Shaw and colleagues compared the rate of such exemptions with pertussis incidence in all counties from 2000 to 2011, with the exception of New York City (because data on pertussis specifically was not available).
The researchers defined religious exemption to be “high” in a given county if at least 1% of all enrolled children had claimed the exemption. In 2011, 13 counties had high exemption rates compared with just four in 2000.
Among counties with high exemption rates, reported pertussis incidence was 33.1 cases per 100,000, compared with 20.1 per 100,000 in low-rate counties. The incidence ratio was 1.71 (P<0.001).
On average, pertussis incidence among exempted children in all counties was 302 per 100,000, significantly greater (P=0.02) than the 22 per 100,000 among vaccinated children.
In counties with low exemption rates, whooping cough among exempted children had no significant effect on the risk of the disease among the vaccinated majority, the researchers found. But high exemption rates in the community increased pertussis risk for both vaccinated and exempted children (P=0.008).
“More studies are needed to characterize differences in the process of obtaining exemptions among [New York state] schools, and education is needed regarding the risks to the community of individuals opting out from recommended vaccinations,” the researchers wrote.