Vaccine refusal linked to California pertussis outbreak

Michelle Healy | USA TODAY

The 2010 California outbreak of the extremely contagious respiratory disease pertussis (also known as whooping cough) was the largest in more than 60 years. It resulted in 9,120 cases and 10 deaths -- a third of all the cases seen in the USA that year. Several causes of the outbreak have previously been documented, including waning immunity of the pertussis vaccine. A new analysis offers evidence that geographic areas with higher percentages of people who intentionally refused the vaccine is also associated with increased cases of the disease.

Using data from the California Department of Public Health, the study in October's Pediatrics, online today, analyzed non-medical exemptions for children entering kindergarten from 2005 through 2010, and pertussis cases that were diagnosed in 2010 in California.

Parents in California can obtain exemptions if all or some immunizations are contrary to their beliefs, whether those beliefs are religious, philosophical, or related to other unspecified nonmedical reasons, says the study.

The analysis identified 39 statistically significant geographical clusters with high rates of non-medical exemptions and two statistically significant clusters of pertussis cases. Census districts within an exemption cluster were 2.5 times more likely to also be in a pertussis cluster.

Both clusters were associated with "factors characteristic of high socioeconomic status," such as lower population density, lower average family size, lower percentage of racial or ethnic minorities and higher median household income.

The study authors conclude that communities with large numbers of intentionally unvaccinated or undervaccinated persons can lead to pertussis outbreaks, putting vulnerable populations like young infants at increased risk.

They note that the cyclical nature of pertussis (cases tend to peak every two to five years) and improved diagnosis, have also been cited as likely contributors to the California outbreak.

According to the Centers for Disease Control and Prevention, infants too young for pertussis vaccination are at greatest risk for life-threatening cases of the disease, but it can cause serious illness in older children and adults as well. In addition to vaccinating children, CDC recommends pertussis shots for teens, new parents and anyone else who comes in contact with infants.