Twelve elderly residents living in an assisted care facility suffered from sinusitis, otitis media, and mild pneumonias during midwinter. Despite the fact that they had all received the 13-valent pneumococcal conjugate vaccine recently licensed for adults, S. pneumoniae was isolated from 10 of the patients. Which of the following is the best explanation for the pneumococcal infections?

There are more than 90 capsular immunotypes of Streptococcus pneumoniae. Immunity to S. pneumoniae is conveyed by antibodies against the specific capsular type. Vaccines have been formulated to contain the most commonly isolated capsular types. Originally, the S. pneumoniae conjugated vaccine contained seven serotypes. Unfortunately, the serotype replacement phenomenon occurred in which a nonvaccine strain began to cause disease among the vaccinated population. Over time this lead to the licensing of a 13-valent pneumococcal conjugated vaccine (PCV-13), which was recommended for children. A 23-valent nonconjugated vaccine (PV-23) was developed for children over age 2 and adults who are at risk for the disease. Recent research showed that older adults vaccinated with PCV-13 elaborated higher protective antibody titers than those immunized with PV-23. PCV-13 has now been licensed for adults 50 and older. The failure of the vaccine to protect these individuals is most likely due to the replacement phenomenon-the capsular type responsible was not present in the vaccine (c). Elderly patient do mount good immune responses to vaccines so (a) is not the likely cause. Some patients will not respond to the vaccine (b), but the effectiveness of the vaccine is higher than 17% (10/12, the number of patients who became infected). In fact, recent analyses of strains causing illness in those over 65 suggest that over 70% of the cases of invasive pneumococcal disease are covered by PCV-13. It is unlikely that the vaccine was defective (d), although this can occur.