Why do some vaccines require several shots?

Short Answer

For some vaccines, a series of shots has a cumulative effect and improves the level of protection against a disease with each dose. For other vaccines, additional shots help account for changes or mutations in some of the disease-causing viruses and bacteria. There are still other vaccines that do not last forever and need periodic booster shots to remain effective.

Source:

http://www.chop.edu/service/vaccine-education-center/vaccine-safety/#Why_are_some_vaccines

Vaccines may be given multiple times for a few reasons:

Some vaccines are given more than once so that people can be fully protected against the disease. For example, when adults get the chickenpox vaccine, 78% will be protected after one dose, but that number rises to 99% after the second dose. Since the additional dose is not harmful and provides extra protection, everyone is recommended to get the second dose.

Some vaccines provide a low level of protection after a single dose, but additional doses provide a greater response, so that individuals are better protected. This is true of the vaccine for Haemophilus influenzae type B or Hib.

Another reason for multiple doses is that immune responses last longer after more than one dose of some vaccines. The pertussis vaccine is an example of this.

Herd immunity develops when a high percentage of people in a community are immune to a disease. With herd immunity, even people who cannot receive vaccines are protected from the infectious disease.