BACKGROUND: Pertussis has been increasing in the U.S. since the mid-2000s. Reporting peaked most recently in 2012 with 48,277 cases, and annual counts continue to exceed those reported during the 1980s and 1990s. A number of factors may explain the pertussis resurgence, including surveillance bias driven by increased provider awareness; waning of immunity from acellular pertussis (aP) vaccine leading to increasing susceptibility in cohorts of children and adolescents as the time since their last pertussis vaccination increases; differences in immune response generated by acellular vaccines as compared to whole cell vaccines; and finally, molecular changes in B. pertussis, possibly resulting from selective pressure of aP vaccination leading to bacteria with increased virulence or the ability to evade vaccine-driven immunity.
METHODS: Pertussis cases reported to CDC through the National Notifiable Diseases Surveillance System (NNDSS) from 1/1/2010 to 12/15/2015 were included in this analysis. Supplemental surveillance data and B. Pertussis isolates collected by seven states participating in Enhanced Pertussis Surveillance (EPS) (CO, CT, GA, MN, NM, NY, OR) were also analyzed. Cases were assessed by state, age, clinical symptoms, diagnostic method, and vaccination status. B. pertussis isolates were tested for pertactin genetic variants, and results were linked to surveillance data to describe molecular trends.
RESULTS: During the study period 173,574 pertussis cases were reported to CDC. Cases occurred in all 50 states and the District of Columbia, annually. Infants aged < 1 year experienced the highest incidence of pertussis (range: 70.9 to 126.7 per 100,000 pop). The age distribution of reported cases, however, varied; during 2010 to 2011, children 1-6 years and adults ≥ 20 years each contributed ≥ 20% of cases. From 2012 forward, adolescents 11 to 19 years contributed the largest proportion of cases annually (range: 28%-34%). Polymerase chain reaction (PCR) was the most common diagnostic test; 88% of cases with known laboratory testing included a PCR result. Symptom severity was greatest among infant case-patients, who experienced 67% of hospitalizations, 31% of pneumonia diagnoses, and 82% of deaths. Of EPS cases with known vaccination status, 72% reported receipt of ≥ 1 pertussis-containing vaccine. Pertactin gene expression in pertussis isolates collected nationally decreased from 82% in 2010 to 0% in 2015.
CONCLUSIONS: The resurgence of pertussis in the United States continues and the cause is likely multifactorial. Studies must continue to bridge clinical and epidemiological findings with novel molecular findings to expand the evidence-base for the development of more effective vaccines against pertussis.