Trends of Pertussis in Kent County, Michigan: A 20-year Retrospective Study

Background: Pertussis incidence is low in Kent County, Michigan in comparison with surrounding counties, and we suspect that the rates are increasing. We hypothesized that pertussis incidence has increased for all demographic characteristics, especially since the year 2005. Methods: A retrospective design was used in which data between the years 1996-2015 was used as gathered from the Michigan Disease Surveillance System (MDSS). Demographic characteristics were examined and incidence was compared to previous years in order to examine trends. Additionally, incidence rates pre- and post-2005 were compared. Data were analyzed using Poisson regression models in SPSS. Results: This sample included 189 confirmed cases. The sample was 49.2% male, 65.1% White and 2.7% non-White. The greatest proportion of cases were seen in the less than one year age group (n=80; 42.3%), with the second largest percentage seen in the 1-19 years age group (n=74; 39.2%). Grand Rapids and East Grand Rapids had the highest number of cases (n=51; 27%) and the greatest number of cases occurred in 2015 (n=20; 10.6%). Pertussis rates increased by 4% each year during the study period (IRR=1.04, p=0.003), but we did not observe a significant difference in pre- vs. post-2005 rates (p=0.213). Conclusion: We identified an increase of overall incidence during the time period. From this information, there are many implications for multiple groups within the population. We should promote increased promotion and prevention resources within Kent County related to the increasing trend of pertussis incidence within the community. Future work aims to target susceptible subgroups.