Herd protection or herding cats?

Recent advances in the epidemiology and prevention of Streptococcus pneumoniae infections

The introduction of pneumococcal conjugate vaccines (PCVs) 7 and 13 into national childhood immunization programs in the US in 2000 and 2010, respectively, proved to be remarkably successful in reducing infant mortality due to invasive pneumococcal disease (IPD), resulting in widespread uptake of these vaccines. Secondary herd protection of non-vaccinated adults against IPD has proven to be an additional public health benefit of childhood immunization with PCVs, particularly in the case of the vulnerable elderly who are at increased risk due to immunosenescence and underlying comorbidity. Despite these advances in pneumococcal immunization, the global burden of pneumococcal disease, albeit of unequal geographic distribution, remains high. Reasons for this include restricted access of children living in many developing countries to PCVs, the emergence of infection due to non-vaccine serotypes of the pneumococcus, and non-encapsulated strains of the pathogen. Emerging concerns affecting the elderly include the realization that herd protection conferred by the current generation of PCVs (PCV7, PCV10, and PCV13) has reached a ceiling in many countries at a time of global population aging, compounded by uncertainty surrounding those immunization strategies that induce optimum immunogenicity and protection against IPD in the elderly. All of the aforementioned issues, together with a consideration of pipeline and pending strategies to improve access to, and serotype coverage of, PCVs, are the focus areas of this review.