In most cases, each shot creates only partial immunity to the disease, and subsequent shots build on that immunity. As a general rule for shots that come in a series, the first dose provides approximately 50 percent immunity, the second dose bumps it up to about 75 percent, and the third dose gives around 90 percent immunity. A few diseases require a fourth dose about a year after the third, and then another booster dose about four years later. These are necessary to keep immunity levels high enough to be effective.
Some shots, however, like those for measles, mumps, and rubella and chicken pox, don't require a whole series to create immunity. This is because they are live-virus vaccines, which work well with one initial dose and sometimes a booster years later.
-- Adapted from The Vaccine Book: Making the Right Decision for Your Child, by Robert Sears, M.D. (November; Little, Brown and Company)