Comparing Respiratory Illness Surveillance Case Definitions to Detect Bordetella pertussis in Children Aged <5 Years With Respiratory Illness in South Africa, 2017-2023.

Pertussis is vaccine preventable, and surveillance can guide interventions. Assessing the performance of syndromic surveillance and the World Health Organization (WHO) pertussis case definitions can identify improvements to enhance detection and monitoring of Bordetella pertussis. We analyzed respiratory illness sentinel surveillance data among children aged <5 years from January 2017 through December 2023. Participants were enrolled for surveillance as outpatients with influenza-like illness (ILI) or in-patients with severe respiratory illness (SRI). Nasopharyngeal swabs were tested for B pertussis via polymerase chain reaction (PCR). Sensitivity and specificity, and performance indicators of case definitions were evaluated against PCR results. Of 23 642 participants with PCR results, B pertussis was detected in 0.7% from ILI and 1.6% from SRI. When compared with the WHO pertussis case definition, a modified definition (including apnea, omitting cough duration) improved sensitivity (ILI, 30.0% vs 43.3%; SRI, 55.7% vs 60.2%) but reduced specificity (ILI, 90.5% vs 75.8%; SRI, 88.3% vs 80.9%). WHO and modified pertussis case definitions missed a large proportion of true pertussis cases (ILI, 70.0% vs 56.7%; SRI, 44.3% vs 39.8%). Current pertussis case definitions likely underestimate disease burden. Revising the WHO pertussis case definition and integrating pertussis into syndromic surveillance could improve detection while leveraging existing resources.