Whooping cough rates in the US have been increasing since the 1980’s and reached a 50-year high in 2012. Reasons for the resurgence of pertussis infection in recent years may include: genetic changes in the bacteria that causes pertussis, decreasing vaccine-induced immunity over time, greater awareness of the disease, and general availability of better laboratory tests. Researchers suspect that vaccine refusal, which has risen in recent years in several states, may have also played a role.
Unvaccinated children are at higher risk for getting pertussis than their vaccinated peers. This vulnerable population brings the illness into a community in spite of high immunization rates. A study published in the journal Pediatrics (September 24, 2013) looked at the impact of non-medical vaccine exemptions on the rates of pertussis in children from California during 2010. The researchers identified groups of children living in the same community (called geographic clusters) whose parents filed non-medical vaccination exemption forms prior to kindergarten. This group was compared with geographic areas where there was an increase in pertussis cases. The researchers found that areas with many unvaccinated children were 2.5 times more likely to have a cluster of pertussis cases than areas with few unvaccinated children.
The study can’t prove that clusters of unvaccinated children contributed to pertussis outbreaks, but the findings are consistent with other studies that have found that clusters of unvaccinated children impact outbreaks of pertussis and other illnesses.