BACKGROUND: Pertussis, a cyclic disease, causes greatest morbidity and mortality among infants, particularly those too young to be vaccinated. In the context of a recent resurgence of pertussis and a 2012 recommendation to vaccinate for pertussis during every pregnancy to prevent infant disease, we describe US infant pertussis trends between 2000 and 2015.
METHODS: We analyzed infant pertussis cases (age groups: <2, 2–<4, 4–<6, and 6–<12 months) that were reported through the National Notifiable Diseases Surveillance System, using the Council of State and Territorial Epidemiologists case definition, with cough onset between 2000 and 2015. Incidence (per 100,000) was calculated using National Center for Health Statistics population estimates as denominators. Proportion of affected infants hospitalized and case-fatality ratios (CFRs) were calculated using cases with known outcomes; relative percent changes were calculated. Linear trends were assessed using negative binomial regression; p<.05 was significant.
RESULTS: From 2000–2015, 48,909 infant pertussis cases were reported, including 255 deaths; infants aged <2 months accounted for 38.7% of cases. Overall, annual incidence was highest among infants aged <2 months (range: 112.7–282.1 per 100,000) and was lowest among infants aged 6–<12 months (range: 10.0–62.1 per 100,000). The proportion of infants hospitalized decreased 47.6% overall, from 64.5% in 2000 to 33.8% in 2015 (p<.0001); cases among 4–<6 and 6–<12 months old had the largest annual relative decreases (-5.1% and -5.9%, respectively), while <2 month olds had the smallest relative decline (-1.5%) (p<.0001 for all). Although the CFR was highest among <2 month olds (1.6%), CFRs decreased significantly over time among <2 and 2–<4 month olds (p<.05).
CONCLUSIONS: Pertussis incidence remains highest among infants aged <2 months. While declines in hospitalization and case fatality may represent a decrease in disease severity, these changes may also reflect increased reporting of non-severe cases. Ongoing monitoring of infant pertussis is needed to better understand the impact of new prevention strategies.