Pertussis Vaccine Effectiveness Evaluation

The Vermont Department of Health and the Centers for Disease Control and Prevention (CDC) are conducting an evaluation of pertussis vaccine effectiveness.

The reported incidence rate of pertussis in Vermont increased from 2.9 to 103 cases per 100,000 population from 2010 to 2012. Part of this increased incidence may be the result of vaccine-associated waning immunity. There also have been genetic changes in the circulating strains of pertussis; however, the contribution of these changes to vaccine effectiveness is unknown. One such change is the recent appearance of pertussis bacteria that no longer produce a protein called pertactin, a component of both pertussis vaccines. In Vermont, greater than 90 percent of the isolates submitted by the Health Department Laboratory for testing to the CDC were classified as pertactin-deficient.

Assessment of Vaccine Effectiveness

To inform future vaccination efforts, an assessment of pertussis vaccine effectiveness against currently circulating pertussis strains is underway in Vermont. CDC identified Vermont as an ideal place to perform this type of evaluation, due to the Health Department Laboratory’s large and representative collection of pertussis specimens from across the state, which confirmed that the pertactin-deficient strain caused the majority of cases here in 2012.

Between March 17 and April 11, 2014 teams totaling more than 30 CDC epidemiologists and over 30 Health Department epidemiologists and nurses collected data to evaluate vaccine effectiveness in Vermont. Pertussis cases were identified through Vermont’s routinely collected surveillance data from 2011 through 2013. Three controls (patients without pertussis) were randomly identified from the same clinic as the case. Demographic and clinical information, and vaccine history was collected from medical records for cases and controls. For participants with unknown vaccination status (information not available in medical records), parents will be contacted to obtain vaccination history. By comparing vaccination histories of patients with pertussis to those without pertussis, we can see how well the vaccine works and how long it protects. The results will be shared with the Advisory Committee on Immunization Practices (ACIP) and on the Health Department website.

The Health Department and CDC staff are working with 96 Vermont practices that have agreed to participate, and have abstracted data from 3,271 charts – 98.6% of the target. CDC epidemiologists have finished quality assurance checks on the data that was collected. As expected, some records contain incomplete pertussis vaccination history. To ensure that our analysis of pertussis vaccine effectiveness is as accurate as possible, we need to verify that these patients did not receive pertussis vaccine. The parents of those children who still have missing or unknown vaccination history will be contacted to verify vaccine history.

A summary report of this evaluation will be available in the fall of 2014.

Return to Top