State laws that set strict standards for children to be exempted from vaccines on religious or philosophical grounds could reduce the number of whooping cough cases, but not measles, mumps, haemophilus influenza type B (Hib) or Hepatitis B, according to a new study published today in the American Journal of Public Health.

The study examined how non-medical exemption laws for vaccines required for school or daycare entry impact the incidence rates for the five diseases targeted by the vaccines. The researchers reviewed all relevant laws and regulations for each year between 2001 and 2008 and rated them on their restrictiveness in granting exemptions from very low to very high.

"Our research shows that during the study period, if all states increased the restrictiveness of their non-medical exemption laws by one level, the number of U.S. whopping cough (pertussis) cases would have decline by 1.14 percent, resulting in 171 fewer cases per year," according to author Y. Tony Yang, ScD, MPH, associate professor, College of Health and Human Services at George Mason University.

The results also showed that increased levels of vaccinations could reduce whooping cough cases, but did not have a statistically significant impact on the average incidence for measles, mumps, Hib and Hepatitis B.

The researchers suggest that the differences in the findings for pertussis and the other diseases may be because of a "threshold effect," which suggests that laws may not have a significant impact until a disease's incidence rate reaches a certain level. Pertussis was much more prevalent than the four other diseases studied -- the mean incidence rate for pertussis was 18 per 100,000 individuals from 2001-2008. For Hib, Hepatitis B, measles, and mumps, the mean incidence rates were less than 1 per 100,000. 

In the US, states can mandate that children be vaccinated before entering daycare and elementary school. The laws and regulations governing exemptions for mandatory vaccination can vary considerably by state. Medical exemptions are permitted in all states for individuals with certain illnesses, allergies or immune system disorders. All but two states, West Virginia and Mississippi, permit individuals to exempt their children on religious or philosophical grounds. As of 2012, one third of states permit citizens to claim a philosophical exemption.

The study finds that the least restrictive states, or those where it was easiest to obtain a non-medical exemption, were California, North Dakota, Ohio, Oklahoma, Vermont and Washington, though California recently passed a law making their exemptions much more restrictive. The most restrictive states were Mississippi and West Virginia.

A number of past studies have found significant associations between immunization laws and the uptake, or vaccination rate, of children. This study aimed to clarify whether or not the restrictiveness of those non-medical exemption laws and vaccination rate then impacted disease incidence.

States with laws that permit individuals to easily claim religious or philosophical exemptions, such as those that allow for a parent signature without educational counseling or health department approval, tended to be rated as having "very low" restrictiveness. Conversely, states without non-medical exemption laws permitting religious or philosophical exemptions were rated as "very high." 

The study, "A Longitudinal Analysis of the Effect of Non-Medical Exemption Law and Vaccine Uptake on Vaccine-Targeted Disease Rates," was funded by a grant from the Public Health Law Research program of the Robert Wood Johnson Foundation.