The Impact of the 2011-2012 Pertussis Epidemic on Infant Vaccination in Washington State

Abstract

Background Washington State experienced a pertussis epidemic from October 2011 to December 2012. There was considerable variation in incidence by county; rates were highest amongst infants. Objectives To determine the effect of the pertussis epidemic on infant vaccination in Washington State and whether pertussis incidence within counties modified this effect. Methods We conducted an ecologic before-after study to compare the proportion of infants up to date (UTD) with a pertussis-containing vaccine at time points before (September 30, 2011) and during (September 30, 2012) the epidemic. Children aged 3-8 months enrolled in the Washington State Immunization Information System with documented county of residence were included. UTD status was determined by greater than or equal to 1, 2, or 3 doses of a pertussis-containing vaccine at ages 3, 5, and 7 months, respectively. Generalized linear models with extension to the binomial family and clustered robust standard errors were used to examine differences in the proportion of UTD infants between pre-epidemic and epidemic points. The potential modifying effect of pertussis incidence in county was examined. Results We found no significant difference in statewide UTD status with a pertussis-containing vaccine between pre-epidemic and epidemic time points (absolute difference: 2.1%; 95% CI: -1.6, 5.9). There was no significant modification by pertussis incidence in county of residence; nonetheless, there was considerable variability in the absolute difference in UTD status across counties. Conclusions A statewide pertussis epidemic does not appear to have significantly changed the proportion of infants who were UTD with a pertussis-containing vaccine.