Published: Jun 3, 2013 | Updated: Jun 3, 2013

By Michael Smith , North American Correspondent, MedPage Today

Action Points

• As rates of religious exemption from childhood vaccination rise, so does the incidence of whooping cough, even among vaccinated children.
• Point out that the findings imply that parents who refuse vaccination for their own children also put others at risk, especially if there are a large number in a single community.

As rates of religious exemption from childhood vaccination rise, so does the incidence of whooping cough, even among vaccinated children, researchers reported.

In a 12-year retrospective study in New York state, rates of religious exemption nearly doubled with the overall annual state mean prevalence of religious exemptions for one or more vaccines coming in at 0.4% from 2000–2011 and increasingly significantly from 0.23% in 2000 to 0.45% in 2011 (P=0.001), according to Jana Shaw, MD, of SUNY Upstate Medical University in Syracuse, N.Y., and colleagues.

And in counties with a high proportion of people claiming the exemption for their children, the incidence of pertussis was greater than in other areas, Shaw and colleagues reported online in Pediatrics.

Overall, the average incidence of pertussis among exempted children was 14 times greater than among vaccinated children. But if vaccinated children lived in a county with a high exemption rate, their risk of the disease rose significantly.

The findings have important implications, commented Gregory Poland, MD, of the Mayo Clinic in Rochester, Minn.

“This is really a community effect,” he told MedPage Today because parents who refuse vaccination for their own children also put others at risk, especially if there are a great many in a single community.

New York state requires certain vaccinations before children can start school. Parents can request a religious exemption but the state health department keeps track of the numbers on a county-by-county basis.

Shaw and colleagues compared pertussis incidence with changes in the rate of such exemptions from 2000 through 2011 in all counties except those in New York City, where a pertussis breakdown wasn’t available.

The rate of religious exemption varied markedly, but more than doubled in 34 of the 62 countries over the 12-year study period. For the analysis, Shaw and colleagues defined a county as having a high rate of exemption if it was at least 1% of all enrolled children.

All told, 13 counties had high religious exemption rates in 2011, compared with only four in 2000, they reported.

Counties with a high rate of exemption had an incidence of reported pertussis of 33.1 cases per 100,000, compared with 20.1 per 100,000 on low-rate counties. The incidence ratio was 1.71 (P<0.001).

On average, the incidence of pertussis 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.

But high exemption rates in the community increased the risk for both vaccinated and exempted children, and the increase in risk was significant among vaccinated children living in counties with high exemption rates(P=0.008).

The researchers cautioned that the study was retrospective and, among other things, could not account for unreported cases of mild pertussis, missing or unexplained vaccine status, or home-schooled children.

Poland noted that parents who use religious exemptions may not have deep spiritual feelings, but instead are using the process as a “mechanism” to avoid vaccination, which they oppose for other reasons.

“I don’t think that a new sect had moved in or that suddenly the state of New York became religious and decided this was no longer a good thing,” he said.

In general, he said, such parents are white, well educated, and well off. “This is a sociological phenomenon that impacts an entire community, an entire state, an entire country, and for which children are suffering,” he said.