Direct and Indirect Effects of Pneumococcal Protein Conjugate Vaccine

Streptococcus pneumoniae is a major etiology of serious bacterial infection among children worldwide. Among the 91 serotypes, the majority of invasive infections are caused by 10 common serotypes, 14, 16, 18, 19, 23, 4, 9, 7, 1, and 3. However, the ranking and serotype prevalence differ by age group and country. The heptavalent pneumococcal conjugate vaccine (PCV7) was licensed for use among infants and young children in many countries including Korea. The routine use of PCV7 has resulted in a decreased incidence of invasive pneumococcal disease by the vaccine serotypes among the vaccines (direct effect). However, it is notable that substantial declines in invasive diseases among older children and adults ensued through indirect effects on transmission (i.e., herd immunity). In addition, there are increasing evidences to suggest that routine immunization with PCV7 is changing the epidemiology of pneumococcal diseases such as serotype distribution of invasive disease, nasopharyngeal colonization, and antibiotic resistance patterns. In contrast, there is a small increase in the number of invasive diseases caused by nonvaccine serotypes, so called 'replacement phenomenon', though it is still minor compared with overall declines of vaccine-serotype diseases. Of those, the increase in the 19A-related disease has become most prominent. In Korea, a remarkable increase of 19A was noted before the introduction of PCV7. The emergence of resistance and replacement of disease by nonvaccine strains should be closely monitored.