[emamum]

http://www.metrokc.gov/health/immunization/system.htm

QUESTION:
I have heard that some people get diseases that they have been vaccinated against. How could this be true?
ANSWER:
Modern vaccines are extremely effective, but are not perfect. For example, a vaccine that is 90% effective means that one in every ten people who is vaccinated is not fully protected from the disease. Should disease affect a community, those that are unprotected are likely to be infected - which includes those who were not vaccinated and the 10% of people who were vaccinated but for whom the vaccine didn't work. The 10% for whom the vaccine did not work may still have partial immunity; if infected, these individuals may experience a milder form of the disease. Because most diseases that vaccines prevent are transmitted from person-to-person, the more people in a community who are immunized, the less likely that disease will be transmitted and "find" the few that are unprotected.

Most vaccines require more than one dose to reach maximum immunity. Some, like tetanus and diphtheria, require booster doses every 10 years throughout life to continue that immunity.