Vaccine Refusal Linked to Whooping Cough Outbreak

Areas with a high rate of nonmedical vaccine exemption were significantly more likely to overlap with clusters of pertussis cases during a 2010 outbreak in California, according to the results of a new study.

Clusters of individuals who refused to receive vaccinations for nonmedical reasons may share the blame for a 2010 pertussis outbreak in California when more cases were reported in the state than in any year since 1947, according to the findings of a study published in the October 2013 issue of Pediatrics.

During the 2010 resurgence of pertussis (also known as whooping cough) in California, approximately 9120 cases were reported in the state, accounting for one-third of all cases reported in the country and leading to 10 deaths. Waning vaccine effectiveness, improved diagnosis, the cyclical nature of the disease, and large groups of susceptible infants, have all been suspected as contributors to the outbreak. However, since the number of nonmedical exemptions to vaccines has reportedly been on the rise throughout the country and particularly in California, the researchers hypothesized that groups of unvaccinated individuals living nearby each other may also have played a role.

To test their hypothesis, the researchers compared data on the geographic location of kindergarten students whose parents had refused vaccinations for nonmedical reasons from 2005 through 2010 with data on the location of pertussis cases reported during 2010. The analysis included 8360 schools and excluded elementary schools with fewer than 10 students, which were assumed to be homeschools. The addresses of included schools were geocoded to their exact latitude and longitude and then translated into Census tracts.

The average nonmedical exemption rate within Census tracts increased from 1.6% in the 2005-2006 school year to 2.4% in the 2009-2010 school year. The analysis identified 39 statistically significant clusters of high nonmedical exemption rates, found in areas ranging from single Census tracts to large geographic expanses, and 2 areas with statistically significant clusters of pertussis cases. Census tracts with high nonmedical exemption rates were 2.5 times more likely to be in a pertussis cluster than were other areas. In addition, the incidence of pertussis within nonmedical exemption clusters was 20% higher than outside them, a difference that remained statistically significant after adjusting for demographic factors.

More than 95% of the population must be vaccinated against highly infectious diseases such as pertussis in order to prevent outbreaks and reduce the risk of spreading the disease to children too young to be vaccinated. Although vaccination rates for California children entering kindergarten are high overall, the researchers note that coverage rates are much lower in certain communities, which increases their risk of experiencing an outbreak.

The authors suggest that clusters of intentionally unvaccinated children may have been among the factors that contributed to the 2010 California pertussis outbreak.

“Our findings suggest that geographic areas with high rates of [nonmedical exemptions] are associated with high rates of pertussis,” they write. “The contribution of [nonmedical exemptions] to the changing epidemiology of pertussis should be acknowledged and explored in future studies along with other contributing factors, such as waning immunity of acellular vaccines.”