Vaccine refusal played a role in the large whooping cough outbreak of 2010 in California, found a study released today online from Pediatrics. That year, 9,120 cases of whooping cough were reported in California, the most since 1947.
In March, I wrote a blog about whooping cough, also known as pertussis. The impetus for that blog was new data that had emerged regarding that California outbreak. Evidence available then suggested that waning immunity associated with the new acellular pertussis vaccine may have contributed to the outbreak.
Unlike many states, California requires only a parent’s signature on a form to permit a child to enter school without vaccinations. The state keeps a record of these non-medical vaccination exemptions (NMEs). In California, pertussis is a disease that must be reported to the state health department. The authors of this study examined geographic clusters of NMEs and geographic clusters of pertussis cases to see if the clusters overlapped. The authors found that:
- The average rate of NMEs per census tract in California averaged 2 percent between 2005 and 2010
- 39 census tracts within the state had NME rates that were significantly higher than the rates seen in the remainder of the state
- The odds of finding a cluster of pertussis cases within one of these census tracks with high rates of NMEs were 2.5 times higher than the odds of finding a cluster of cases in the remaining census tracts; this difference remained after the results were adjusted for a number of demographic factors.
So what does this mean? For an outbreak or an epidemic of an infectious disease to occur, a large number of individuals need to be susceptible to that disease and that disease needs to spread easily from person to person. While vaccination reduces the risk of infection for individual recipients, it is estimated that vaccination of more than 95 percent of a susceptible population is required to prevent epidemics. Small pox was eradicated through mass immunization; polio has almost achieved eradication by the same approach.