Pertussis Cases Concentrated Near Vaccine Refusal Clusters

Ricki Lewis, PhD

September 30, 2013

Clusters of nonmedical exemptions (NMEs) to pertussis vaccination overlap geographical areas of disease outbreaks in California, according to a report published online September 30 in Pediatrics.

In 2010, California reported 9120 cases of pertussis, the most in any year since 1947 and accounting for one third of all cases in the United States that year. Ten patients died.

Pertussis outbreaks have been attributed to several factors: waning immunity from the acellular version of the vaccine, better diagnosis, mutations in the pathogen, and the cyclical pattern of outbreaks every 2 to 5 years. In the new study, Jessica E. Atwell, MPH, from the Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland, and colleagues investigated a possible role for vaccine refusal in increasing incidence of pertussis.

Parents may obtain a nonmedical exemption for their child if a vaccine counters their religious, philosophical, or other beliefs. In California, NMEs for the vaccine have more than tripled in the past decade, and pertussis incidence has increased.

Using public health records, the investigators identified 39 statistically significant clusters of high rates of NMEs among children entering kindergarten between 2005 and 2010 and 2 statistically significant clusters of pertussis cases for the 5-year period.

Individuals in a NME cluster were 2.5 times more likely to also be within a pertussis cluster (odds ratio, 2.47; 95% confidence interval, 2.22 - 2.75) compared with children who received the vaccine. More cases occurred within the NME clusters than outside them (incident rate ratios, 1.20; 95% confidence interval, 1.10 - 1.30).

Vaccination rates below 95% of a population for highly infectious diseases such as measles or pertussis can threaten herd immunity, the researchers write. This places vulnerable individuals, such as infants and those who cannot mount an immune response to a vaccine, at elevated risk for infection.

The researchers conclude that clusters of NMEs may have contributed to the resurgence of pertussis seen in California in 2010.

Limitations of the study include the fact that some records do not indicate which vaccines a child refused, so some children counted as NME for pertussis may actually have had the vaccine. Also, the authors used kindergartners as a surrogate for the population and their protocol did not assess vaccination status of infected individuals. It is also possible that the number of cases in 2010 was underreported because pertussis was not diagnosed.

The researchers have disclosed no relevant financial relationships.

Pediatrics. 2013;132:624-630. Full text

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