Background: Despite high pertussis-containing vaccine coverage in the United States, children who are unvaccinated or not fully vaccinated remain susceptible to pertussis. Over multiple birth cohorts of incomplete vaccination, the number of children not immune to pertussis will accumulate because of factors such as age-specific vaccination status and dose-specific vaccine effectiveness. The total number of pertussis-susceptible children 0-23 months of age in the United States is unknown.
Methods: Using data on age-specific pertussis-containing vaccine receipt among children evaluated in the 2013 National Immunization Survey (born between February 2011 and June 2012) and accounting for vaccine effectiveness and maternal transfer of antipertussis antibodies, we estimated the cumulative number of pertussis-susceptible children 0-23 months of age.
Results: Of an estimated 7,905,672 children 0-23 months of age in the United States, we estimated that approximately 22% (1,716,429) are susceptible to pertussis. Age was a large factor in susceptibility, with 89% of children less than 2 months of age not immune to pertussis compared with 7% of children 21-23 months of age. In sensitivity analysis, increasing maternal pertussis vaccination coverage from 10% to 42% decreased susceptibility in children less than 2 months of age to 68%. When considering waning immunity after the fourth dose of vaccine, the herd protection threshold was no longer reached.
Conclusions: These estimates underscore the need to monitor age-specific pertussis vaccine coverage, to increase childhood and maternal pertussis vaccine coverage, to maintain population-level immunity and to prevent the spread of pertussis among young children.