Should There be a Suspect Pertussis Case Definition? Comparison of Maine Pertussis Cases and Not-Cases, 2010-2012

BACKGROUND:  In 2012 Maine experienced epidemic level rates of pertussis. It began as a localized outbreak in one county in the last half of 2011. As of November 23, 2012, Maine had the sixth highest pertussis incidence rate in the country. With increasing numbers of confirmed and probable pertussis cases, there was also an increase in investigations of PCR-positive Bordetella pertussis classified as neither confirmed nor probable (not-cases). Since resources during outbreaks can become scarce and these investigations used similar resources as investigations resulting in cases, Maine Center for Disease Control and Prevention (Maine CDC) identified a need to further characterize pertussis not-cases.

METHODS:  All pertussis investigations completed by Maine CDC between January 1, 2010 and December 12, 2012 were analyzed (analysis of the remainder of 2012 data will occur once finalized). Cases were classified using the 2010 CSTE pertussis case definition. Multiple linear regression was used to determine predictors of being a pertussis case (confirmed and probable). Not-cases were classified as “suspect cases” if the individual was PCR-positive for B. pertussis and had no clinically compatible symptoms (paroxysms, whoop, post-tussive vomiting) or had at least one compatible symptom but had less than a 14-day cough.

RESULTS:  There were 73 pertussis investigations in 2010 (53 cases, 20 not-cases), 244 investigations in 2011 (205 cases, 39 not-cases), and 811 investigations in 2012 (678 cases, 133 not-cases). Overall, 17% of investigations resulted in not-case classifications. After adjusting for age group, sex, and year of disease, PCR-positive individuals had 3.16 times the odds of being a case compared to those PCR-negative, indeterminate, or did not have PCR testing done (p-value <.0001). Of 192 not-cases, 116 (60%) did not have at least one compatible symptom recorded, and 57 (30%) had a symptom with less than 14-day cough recorded. Of these 173 not-cases, 105 (61%) were PCR-positive. Of 105 “suspect” cases, 78 (74%) had at least one dose pertussis-containing vaccine.

CONCLUSIONS:  Pertussis case definitions may not reflect true case burden. Vaccinated individuals may not present with classic symptoms and have reduced cough duration. Timing of specimen collection, symptom onset, and treatment may also affect results. Despite these factors, PCR-positive individuals that do not meet clinical case definition are classified as not-cases while clinically compatible individuals who are PCR-negative do meet case criteria. This analysis supports the need to consider a suspect pertussis case classification for PCR-positive individuals who do not present with classic pertussis symptoms or duration.