Outbreaks of pertussis in California were more likely to occur in areas where many children went unvaccinated, according to results from a study published today in Pediatrics.
A resurgence of pertussis in California and many other parts of the country in recent years is likely the result of many factors. Previous studies have suggested that the protection offered by pertussis vaccines may wane over time, that a newer and safer version of the vaccine may offer less protection than an older version, and that the cyclical nature of the pertussis outbreaks may play a role for some of the uptick in cases.
But many have suspected that vaccine refusal, which has risen in recent years in California, may have also played a role. Most California children are vaccinated (90.7%), but the rate of children who have not been vaccinated before attending kindergarten because they received nonmedical exemptions to the required immunizations has grown from 0.77% in 2000 to 2.33% in 2010, according to the authors. This new study suggests this trend may have contributed to the increase in the state’s pertussis cases.
The researchers identified geographic clusters of children who received nonmedical vaccination exemptions to attend kindergarten and compared them 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 played a role in the pertussis outbreaks. In addition, the researchers did not look at specific vaccination records for the children, so some of the children who received a waiver may have been vaccinated for pertussis but skipped other shots. But the findings are consistent with other studies that have found that clusters of unvaccinated children contribute to outbreaks of pertussis and other illnesses and that children who miss doses of the pertussis vaccine or receive them late may not be fully protected.