Vaccines are preparations of killed microorganisms, living weakened microorganisms, etc. introduced into the body to produce immunity to a specific disease by causing the formation of antibodies.
Vaccines are very delicate compounds, which if handled or administered incorrectly will be ineffective or neutralized. In general vaccines that need to be mixed should be used within one hour, they should be kept cool and out of sunlight.
No vaccine is 100 percent effective. Effectiveness depends on such things as age of the animal, passive immunity the animal possesses when vaccinated, the stress on the animal, diseases, and other factors we don't understand. Vaccines against some diseases are more effective than others.