Pertussis infection rates have dropped nationally by 50% since their peak in 2004 (see graph), but recent reports of an outbreak in the heavily Latino central valley of California has sparked a conversation about unvaccinated children and the concept of herd immunity. A study published in Pediatrics last year found that of 156 cases of whooping cough in Colorado children, 18 were among children whose parents refused vaccination. The researchers in a second analysis of infants 2 months-20 months old determined that 11% of the pertussis cases were attributed to parental vaccine refusal. While the converse of these numbers mean that 134/156 children vaccinated from pertussis still contracted it, and 89% of pertussis cases happened even though the children were vaccinated, the findings were enough to warrant a New York Times article condemning herd immunity.
The California epidemic, as the state health department has declared it, is on track to be the largest since 1950. There have been 910 cases with 600 under review, including 5 deaths of infants under 3 months old. Another New York Times article reports "Epidemics occur every three to five years in the United States, with the most recent in 2005, when there were more than 25,000 reported cases nationwide, and nearly 3,200 in California, where 7 people died." The 1500 new pertussis cases in California would currently add .45 cases per 100,000, as the CDC measures it, to the national total. To equal the 2004 peak, over 26,000 more cases (4 per 100,000) would need to be confirmed nationwide. However, only 5,000-7,000 cases are reported in a normal year, according to the same NY Times article.
While the California Department of Public Health did not release the vaccination status of the 1500 cases, they did mention that 80% of them were Latino. While no scientific proof exists to link race as a known factor in whooping cough infection, the State has urged Latinos especially to get vaccinated. In a separate statement, Ken August, a spokesman for the department, estimates that 2% of children who attend school in California have a personal belief exemption, meaning that for personal beliefs their parents have refused vaccination. In light of the silence on vaccination status of the infected children and the Colorado study, it seems a bit curious to blame 2% of a population for an outbreak if the vaccination system worked as it should.
McKinley, J (2010). "Whooping Cough Kills 5 in California; State Declares an Epidemic." New York Times Online Published: June 23, 2010
Jason M. Glanz, David L. McClure, David J. Magid, Matthew F. Daley, Eric K. France, Daniel A. Salmon, and Simon J. Hambidge (2009). "Parental Refusal of Pertussis Vaccination Is Associated With an Increased Risk of Pertussis Infection in Children." Pediatrics 123 (6) pp. 1446-1451 (doi:10.1542/peds.2008-2150).
