Pneumococcal colonization and severity of pneumonia in hospitalized Cambodian children following introduction of the 13-valent pneumococcal conjugate vaccine

Objectives: This study sought to characterize pneumococcal colonization and clinical/radiological features in Cambodian children admitted to hospital with an illness compatible with pneumonia following national intro-duction of the 13-valent pneumococcal conjugate vaccine (PCV13).Methods: Children aged 0-59 months admitted to Angkor Hospital for Children who met the World Health Orga-nization (WHO) case definition for clinical pneumonia were enrolled over a 3-year period. Clinical, radiological and vaccination data were collected at enrolment. A nasopharyngeal swab was collected for detection of pneu-mococcal colonization using the WHO standard culture method.Results: Between 1 September 2015 and 31August 2018, 2209 analysable illness episodes were enrolled. Pneumo-cocci were detected in 943/2209 (42.7%) children. PCV13 serotypes were detected less frequently in children who had been vaccinated appropriately for their age compared with undervaccinated children: 309/567 (53.6%) vs 216/342 (63.2%) ( P = 0.006). Age-appropriate PCV13 vaccination was negatively associated with hypoxic presen-tation [adjusted odds ratio (aOR) 0.72, 95% confidence interval (CI) 0.60-0.87; P = 0.0006] and primary endpoint pneumonia on chest x ray (aOR 0.69, 95% CI 0.54-0.90; P = 0.006).Conclusions: The introduction of PCV13 in Cambodia was associated with a decline in vaccine serotype nasopha-ryngeal colonization, and clinical and radiological severity in children hospitalized with clinical pneumonia.