Modelling Bordetella Pertussis Incidence Rates, Illinois 1990 to 2013

Introduction
Pertussis is an infection of the respiratory system caused by Bordetella pertussis and its incidence has increased in recent years in Illinois and in the United States.

Method
We used Joinpoint regression to analyze incidence trends from 1990 through 2013. Joinpoint regression uses permutation tests to identify points (joinpoints) where linear trends change significantly in direction or magnitude (e.g., zero joinpoints indicate a straight line). The rate of change for each trend is tested to determine whether the change is significantly different from zero, and each trend in the final model is described by an annual percentage change (APC) with a 95% confidence interval (CI).

Results
There were three joinpoints during the study period 2001, 2004, and 2007. A steady decrease in pertussis incidence rates was observed from 1990 to 2001 APC -9.0 (95% CI, = -22 to 6.3). From 2001 to 2004 there was an increase in the incidence rates APC 125 (95% CI, -66.1 to 1397.5) followed by a decrease in trend from 2004 to 2007, APC -31(95% CI, -78.1 to 118.7). From 2007 to 2013, there was a statistically significant increase in incidence rates APC 27.3 (95% CI, 6.0 to 52.8).

Discussion
Modelling demonstrated increased incidence in disease trends. Incidence rate modelling could be used for pertussis disease control and planning by public health agencies by analyzing short term incidence rate projections obtained from extrapolating the trend in the incidence data.