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Efficacy of PCV vaccine is primarily mediated by controlling pneumococcal colonisation density

Widespread use of Pneumococcal Conjugate Vaccines (PCV) has resulted in a reduction in nasopharyngeal colonisation and invasive pneumococcal disease caused by vaccine-types. In a double-blind, randomised controlled trial using the Experimental Human Pneumococcal Challenge (EHPC) model, PCV-13 (Prevenar-13) conferred 78% protection against colonisation acquisition and a reduction in bacterial intensity (AUC) in experimentally colonised volunteers as measured by classical culture. In this study, we used a multiplex quantitative PCR assay targeting lytA and pneumococcal serotype 6A/B cpsA genes to re-assess the experimental colonisation status of the same trial volunteers. Increase in detection of low-density colonised volunteers by this molecular method led to a decrease of PCV efficacy against colonisation acquisition (29%), as compared to classical culture (83%). For subjects who were colonised following pneumococcal challenge, PCV had a pronounced effect on decreasing colonisation density. These results have implications for vaccine efficacy and surveillance studies as they indicate that the success of PCV vaccination could primarily be mediated by the control of vaccine-type colonisation density which results in decreased transmission and the reported herd effect of PCVs. Studies assessing the impact of PCV should account for density measurements in their design. Clinical trial registration with ISRCTN: 45340436