In Brief | Paediatrics & Infectious Diseases
Paediatrics: Do Clinical Case Definitions Contribute to Missed Pertussis Cases? Evidence from South African Children
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Study Snapshot
| Design | Retrospective analysis of prospective NICD sentinel surveillance for influenza-like illness (ILI, outpatient) and severe respiratory illness (SRI, inpatient), January 2017 to December 2023, across five provinces |
| Population | 23,642 children aged <5 years with a valid nasopharyngeal B. pertussis PCR result (4,125 ILI; 19,517 SRI) |
| Comparison | NICD ILI and SRI surveillance definitions, the WHO pertussis definition, a modified WHO pertussis definition and the WHO SARI definition, each compared against PCR as the reference standard |
| Primary Outcome | Sensitivity and specificity of each definition. WHO pertussis: sensitivity 30.0% (ILI) and 55.7% (SRI), specificity 90.5% and 88.3%. Modified WHO: sensitivity 43.3% and 60.2%, specificity 75.8% and 80.9% |
Summary
Reliable pertussis surveillance depends on case definitions that actually detect disease, yet these definitions have rarely been tested in routine syndromic surveillance. Bishop and colleagues evaluated several definitions in children under five enrolled in NICD respiratory illness surveillance at four clinics and eight hospitals across five provinces, using multiplex real-time PCR of nasopharyngeal swabs as the reference standard. B. pertussis was detected in 344 children (1.5%): 0.7% of outpatients and 1.6% of inpatients. Detection peaked in December each year, reaching 10.4% in December 2022, and only two cases were found between April 2020 and April 2022, when COVID-19 restrictions were strictest.
Hospitalised cases were very young (median age 1.7 months; 81.2% under three months) and often lacked fever (47.1% vs 65.2% in pertussis-negative inpatients), while classic signs such as paroxysmal cough, whoop, post-tussive vomiting and, in infants, apnoea were more common. The broad NICD inpatient definition captured 97.5% of cases but had a specificity of only 6.0%.
Both pertussis-specific definitions performed best in infants under three months and lost sensitivity with age: in hospitalised children aged one to four years, the WHO definition detected only 9.5% of cases. Positive predictive values were low throughout, with more than 90% of children meeting any definition being PCR-negative.
Among inpatients old enough to be protected, only 8.3% of pertussis cases had received three DTP doses, compared with 52.9% of negatives (crude OR 0.07). Because nasopharyngeal swabs and a strict PCR cut-off were used, true prevalence is likely underestimated.
The authors recommend age-specific revisions to the WHO definition and integrating pertussis PCR into syndromic surveillance, particularly to support evaluation of the maternal pertussis vaccination added to South Africa’s schedule in 2024.
References
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