Vaccinating the elderly: recommendations and rationale.

Pneumococcal vaccination is given only once. It is not currently recommended that patients who have been given the 14-valent vaccine be revaccinated with the newer 23-valent vaccine. Vaccination is more difficult for influenza than for pneumococcal pneumonia. An understanding of the annual changes in influenza vaccine recommendations requires some knowledge of the virus' ability to undergo antigenic variation and produce different strains each year.