Editorial Commentary: Accounting for Pertussis

The resurgence of measles and pertussis in the United States [1, 2] is a reminder of the importance of maximum immunization coverage, even in a developed country with relatively high rates of childhood immunization. The exact parameters of herd immunity may not be established for some vaccine-preventable diseases, but maximal effective use of available vaccines providespopulation-level immunity toprevent or reduce transmission. Evidencebased strategies published by the Advisory Committee on Immunization Practices (ACIP) for improved control of pertussis promote effective use [3, 4]. Observation of vaccine effectiveness in routine clinical use provides insight that may not be available from clinical efficacy trials and may be used to inform recommendations. The article by Witt et al [5] in this issue of Clinical Infectious Diseases presents a retrospective study of acellular pertussis vaccine effectiveness made possible by the unique circumstances of a community-wide outbreak of Bordetella pertussis infection affecting a large population under the care of an integrated healthcare system.This allowedestimation of vaccine effectiveness by age group. The relative duration of protection achieved with diphtheria toxoid–tetanus toxoid–acellular pertussis (DTaP) vaccines (introduced in theUnited States in the 1990s) remains a question [6], one made more relevant by the possibilities arising from the availability of pertussis antigen containing vaccines (tetanus toxoid– reduced diphtheria toxoid–acellular pertussis vaccine for adolescents and adults [Tdap]) that allow for booster dosing of individuals >7 years of age. The authors were able to demonstrate less than expected effectiveness of DTaP in 8to 12year-olds, while also demonstrating substantial overall effectiveness of pertussis vaccines. On the basis of their observations, they suggest evaluation of earlier and more frequent doses of pertussis containing vaccine (Tdap) in outbreak settings, and even on a routine basis. More studies of strategic use of Tdap as a booster dose vaccine would be worthwhile, especially after the broad experience with the vaccine and its safety profile. The ACIP already recommends Tdap use in undervaccinated children as young as 7 years of age [3]. A question might be raised as to the use of polymerase chain reaction (PCR) as the exclusive diagnostic test for pertussis in the Witt et al study, especially as the minimal cough duration in the practice guidance was only 1 week. PCR is a widely used, sensitive test that is faster and easier to perform than culture and can identify evidence of B. pertussis infection in situations where culture results are negative. However, high sensitivity and low specificity, with pseudo-outbreaks described due to false positives and environmental contamination with B. pertussis genome [7, 8], suggest that PCR only be used with care and for patients with cough duration of≥2 weeks and a clinical picture consistent with pertussis. However, the observations by Witt et al [5] occurred in the presence of a widespread, welldocumented stateand community-wide outbreak of pertussis (high prevalence of diseaseimprovingpositivepredictivevalue), and testing was done in a central laboratory, using a closed system test platform, and simultaneous testing for Bordetella parapertussis. Furthermore, rates of testing were high across the spectrum of ages, but rates of positive PCR results are very different by age, so false-positive test results do not seem to be a factor contributing to the results unless there was a systematic bias in selection for testing by age. Pertussis remains a threat to infants, in whom protection awaits the initial doses of the DTaP vaccine series. Public health recommendations for the use of Tdap vaccines have been a work in progress since licensure in the United States, and approaches have been designed to protect older children, adolescents, and adults, in large part to protect infants they may expose to pertussis (as symptomatic and asymptomatic cases to the extent that overall reduction in incidence may reduce asymptomatic carriage). Initially, Tdap Received 27 February 2011; accepted 1 March 2012; electronically published 15 March 2012. Correspondence: Altred De Maria Jr, Bureau of Infectious Disease, Massachusetts Department of Public Health, William A. Hinton State Laboratory Institute, 305 South St, Jamaica Plain, MA 02130 (Alfred.DeMaria@state.ma.us). Clinical Infectious Diseases 2012;54(12):1736–8 © The Author 2012. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals.permissions@ oup.com. DOI: 10.1093/cid/cis313