Description and Effects of Vaccine Schedule on Pertussis Illness in 2 Month-6 Year Olds in Minnesota from 2009-2012 *

BACKGROUND:  In recent years parents have turned to alternative schedules to reduce the number of simultaneously administered vaccinations. Current pertussis vaccine recommendations are based on maximizing vaccine efficacy. Alternative schedules may lead to periods of pertussis susceptibility due to lack of appropriate boosting.  We describe pertussis cases and trends in Minnesota for 0-6 year olds from 2009-2012, and assess the effect of vaccine timing on duration of protection for vaccinated cases.

METHODS:  Minnesota Department of Health pertussis surveillance data from 2009-2012 were used for the analysis. Pertussis cases ages 2 months-6 years were included in the descriptive analysis. Adherence to the recommended vaccine schedule was evaluated for pertussis cases aged 19-84 months with at least 4 doses of vaccine. Cases were divided into 3 groups based on their adherence to the Advisory Committee on Immunization Practices (ACIP) vaccine recommendations using immunization data from the Minnesota Immunization Information Connection (MIIC) and provider records. These cases were analyzed by months from last pertussis vaccination until time of onset of illness. Estimated risk ratios were used to assess the relationship between time from last vaccination until onset of illness and adherence to the vaccine schedule.

RESULTS:  A total of 1,442 cases of pertussis were reviewed. Incidence rates were highest in children under 1 year of age (112 cases per 100,000) with peaks at 4 and 6 years of age (79 and 74 cases per 100,000, respectively). For cases ages 19-84 months of age who received 4 doses of pertussis containing vaccine (477 total cases), the group that received all 4 shots according to the recommended schedule had an estimated 5.5 (CI 3.82, 7.20) months of additional protection compared to those that received 2 or fewer doses on schedule. In addition, those that received 3 of the 4 doses according to schedule had an estimated 4.7 (CI 3.02, 6.39) additional months of protection compared to those with 2 or fewer doses on time.

CONCLUSIONS:  This evaluation reinforces the importance of adherence to the pertussis vaccine schedule. Abiding by the recommended vaccine schedule not only protects those at highest risk of severe illness, but it also provides several months of prolonged protection against waning immunity. This prolonged protection is important, as lacking the pertussis containing vaccine at the appropriate interval could leave a child susceptible to pertussis illness.