Updated Assessment of the Impact of the Modified Council of State and Territorial Epidemiologists Case Definition for Pertussis on Reported Pertussis

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BACKGROUND: Using Enhanced Pertussis Surveillance (EPS) data, we evaluated how the 2020 Council of State and Territorial Epidemiologists (CSTE) pertussis case definition change impacted the number of cases reported to the National Notifiable Diseases Surveillance System (NNDSS) as reported pertussis increased in 2024 to above pre-COVID-19 pandemic levels in the United States. METHODS: We compared how EPS cases with cough onset during 1/1/2021-12/31/2024 would have been classified under both the 2014 and 2020 case definitions. To assess the impact of the case definition change, the proportion of EPS cases newly reportable under the 2020 case definition was applied to 2021-2024 NNDSS data to estimate how many additional cases were captured. RESULTS: The proportion of newly reportable cases was 8.6% (10/116) in 2021, 14.1% (28/199) in 2022, 23.6% (79/335) in 2023, and 38.0% (1,903/5,006) in 2024. Using preliminary 2024 NNDSS data, we estimated 16,462/43,321 (38.0%) cases would not have met the 2014 case definition. Between 2019 and preliminary 2024 data, the observed increase in reported NNDSS pertussis cases was 133%. If the case definition had not changed in 2020, the projected increase would have been 44%. CONCLUSIONS: A large increase in reported pertussis cases was observed in 2024; however, it is important to understand the impact of case definition changes to accurately interpret trends in disease incidence over time. The magnitude of the increase was influenced by the CSTE case definition change, which resulted in an estimated 38.0% increase in reported cases.

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