This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax firstname.lastname@example.org with numerous contributions by others.
According to a chart I got from Kaiser, one dose is good for life,
except for immune suppressed or immunodeficient patients, who should
get a booster two years later. _Travel Medicine Advisor_ also says
that no booster is required. _AHFS Drug Information_, however, says
that antibodies are elevated at least five years in healthy adults,
but decline to prevaccination levels after ten years in some.
The reason for the apparent conflict in recommendations is that
allergic reactions are more common after the booster shots, but, at
the same time, the booster shots are useful for maintaining
immunity. For this reason, there has been some debate about the
booster shots; the most recent recommendation is "revaccination with
pneumococcal vaccine after six years in people with high-risk chronic
conditions" (Journal Watch for Oct 18, 1994). (An example is a person
without a functioning spleen.) The 23-valent vaccine was introduced in
1993; prior to that the vaccine was only 14-valent.
Journal Watch for Oct 18, 1994 summarizes an article in the Archives
of Internal Medicine (1994 Oct 10; 154:2209-14) on pneumococcal
boosters. "Antibody levels wane significantly within six years after
vaccination, necessitating revaccination of high-risk patients. This
interesting study evaluated immunogenicity associated with
revaccination." Shots of pneumococcal vaccine were found to increase
antibody levels "at least 1.4-fold in about 55 percent" of both
previously unvaccinated adults and those who had been vaccinated 5.5
to 9 years previously.