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.
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.
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.
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.