Vaccine exemption is associated with an increased risk of measles and pertussis, as evidenced by a recent US study.

To assess if vaccine refusal, delay or exemption was linked to outbreaks of measles and pertussis in the US, researchers looked for studies published between January 2000 (post-measles elimination era) and November 2015 for the measles cohort and from January 1977 to November 2015 for the pertussis cohort. [JAMA 2016;315:1149-1158]

There were 18 published studies on measles comprising 1,416 cases in individuals ranging from age 2 weeks to 84 years (178 cases in individuals <12 months). Of these, 56.8 percent (n=804) occurred in individuals with no history of measles vaccination, while 14.1 percent (n=199) occurred in vaccinated individuals. Of 970 cases with detailed vaccination information, 574 cases were in individuals who were age-eligible for the measles vaccine yet did not receive one, with 70.6 percent (n=405) of them unvaccinated due to nonmedical reasons.

Among 32 pertussis outbreaks comprising 10,609 cases in individuals aged between 10 days and 87 years, 24-45 percent of the five largest outbreaks (Arizona in 1988, California in 2010 and 2014, Washington and Oregon in 2012) involved unvaccinated individuals. In eight of 12 reports with data on reasons for non- or undervaccination (fewer than recommended doses), 59 to 93 percent were unvaccinated due to vaccine refusal.

There were reports of pertussis epidemics in communities with high vaccination rates, an occurrence study authors attributed to a decline in immunity. However, there was still an association between vaccine refusal and pertussis risk in some communities, they said.

The authors believe that enforcement of vaccine mandates, be it at a school- or state-level, as well as increasing the difficulty to acquire exemption are crucial to raising vaccination rates in the community. However, they also highlighted the importance of acknowledging reasons for vaccine hesitancy among parents, such as perception on risk of vaccine-preventable diseases and safety of routine vaccination.

This study builds up on previous data that showed an association between vaccine refusal and risk of varicella, pneumococcal disease, measles, and pertussis.