To identify associations between nasopharyngeal Bordetella pertussis DNA load and clinical and epidemiological characteristics and evaluate DNA load prognostic value in pertussis severity.

Prospective observational multi-centre study including nasopharyngeal samples positive to pertussis DNA by real-time PCR collected from children and adult patients in more than 200 health centres of Catalonia (Spain) during 2012–2013.

B. pertussis load was inversely correlated with age (rho = −0.32, p < 0.001), time to diagnosis (rho = −0.33, p < 0.001) and number of symptoms (rho = 0.13, p = 0.002). Median bacterial load was significantly higher in inpatients versus outpatients (4.91 vs. 2.55 log₁₀ CFU/mL, p < 0.001), patients with complications versus those without (6.05 vs. 2.82 log₁₀ CFU/mL, p < 0.001), disease incidence in summer and autumn versus spring and winter (3.50 vs. 2.21 log₁₀ CFU/mL, p = 0.002), and unvaccinated-partially vaccinated patients versus vaccinated (4.20 vs. 2.76 log₁₀ CFU/mL, p = 0.004). A logistic regression model including bacterial load and other candidate prognostic factors showed good prediction for hospital care (AUC = 0.94) although only age and unvaccinated status were found to be prognostic factors.

We observed strong positive associations of nasopharyngeal bacterial load with severity outcomes of hospitalisation and occurrence of complications. Bacterial load and other independent variables contributed to an accurate prognostic model for hospitalisation.