including those 65 years or older. Generally the vaccine should be a single lifetime dose, as there is a high risk of side effects if repeated. The standard 23-valent vaccines are ineffective for children under two years old. The current guidelines of the American College of Physicians call for the administration of the immunization between ages two and 65 when indicated, or at age 65. If someone received the immunization before age 60, the guidelines call for one-time revaccination. A majority of pharmacists indicate that they recommend all adults 65 and older receive a pneumococcal vaccination. Revaccination at periodic intervals is also indicated for those with other conditions such as asplenia or nephrotic syndrome. Mechanism Polysaccharide vaccine The pneumococcal polysaccharide vaccine most commonly used today consists of purified polysaccharides from 23 serotypes (1, 2, 3, 4, 5, 6b, 7F, 8, 9N, 9V, 10A, 11A, 12F, 14, 15B, 17F, 18C, 19A, 19F, 20, 22F, 23F, and 33F). Immunity is induced primarily through stimulation of B-cells which release IgM without the assistance of T cells. This immune response is less robust than the response provoked by conjugated vaccines, which has several consequences. The vaccine is ineffective in children less than 2 years old, presumably due to their less mature immune systems. Non-response is also common amongst older adults. Immunity is not lifelong, so individuals must be re-vaccinated at age 65 if their initial vaccination was given at age 60 or younger. Since no mucosal immunity is provoked, the vaccine does not affect carrier rates, promote herd immunity, or protect against upper or lower respiratory tract infections. Finally, provoking immune responses using unconjugated polysaccharides from the capsules of other bacteria, such as H. influenzae, has proven significantly more difficult. Conjugated vaccine The pneumococcal conjugate vaccine consists of capsular polysaccharides covalently bound to the diphtheria toxoid CRM197, which is highly immunogenic but non-toxic. This combination provokes a significantly more robust immune response by recruiting CRM197-specific type 2 helper T cells, which allow for immunoglobulin type switching (to produce non-IgM immunoglobulin) and production of memory B cells. Among other things, this results in mucosal immunity and the eventual establishment of lifelong immunity after several exposures. The main drawbacks to conjugated vaccines are that they only provide protection against a subset of the serotypes covered by the polysaccharide vaccines. depitte, J.; Gove, Sandy; Breiman, Robert F. Research Due to the geographic distribution of pneumococcal serotypes, additional