The rise and fall of pneumococcal serotypes carried in the PCV era

Streptococcus pneumoniae is a major cause of meningitis, sepsis and pneumonia worldwide. Vaccination using pneumococcal conjugate vaccines (PCV) has therefore been part of the UK's childhood immunisation programme since 2006. Here we describe pneumococcal carriage rates in children under five years of age attending the paediatric department of a large UK hospital in response to vaccine implementation over seven winter seasons from 2006 to 2013. S. pneumoniae (n=696) were isolated from nasopharyngeal swabs (n=2267) collected during seven consecutive winters, October to March, 2006/7 to 2012/13. This includes the period immediately following the introduction of the seven-valent pneumococcal conjugate vaccine (PCV7) in 2006 in addition to pre- and post-PCV13 introduction in 2010. We show a decrease in PCV13 vaccine serotypes (VT) in the three years following PCV13 vaccine implementation (2010/11 to 2012/13). Serotype 6A represented the only observed VT following PCV13 implementation with all others (including PCV7 serotypes) absent from carriage. Overall pneumococcal carriage, attributable to non-VT (NVT), was consistent across all sampling years with a mean of 31·1%. The ten most frequently isolated NVTs were 6C, 11A, 15B, 23B, 15A, 21, 22F, 35F, 23A and 15C. Fluctuations in the prevalence of each were however noted. Comparing prevalence at 2006/07 with 2012/13 only 15A was shown to have increased significantly (p value of 0·003) during the course of PCV implementation. These data support the increasing evidence that the primary effect of PCVs is due to population immunity by reducing or eliminating the carriage of invasive VT serotypes. With IPD being increasingly attributed to non-vaccine serotypes, surveillance of carriage data continues to act as an early warning system for vaccine design and public health policy that require continual data of both carried pneumococcal serotypes and IPD attributed serotype data.