An initial (may also be called a primary) vaccine, is the first vaccination of a pet. This vaccination must not be before 3 months of age and is only good for 1 year. 

A booster is any vaccination following the initial vaccination, as long as the initial vaccination is still current. The booster vaccination can be good for 1, 2 or 3 years. If any vaccination (initial or booster) is allowed to expire before re-vaccination can occur, then the next vaccination will automatically be an initial vaccination and only good for 1 year. 

After initial vaccination there is usually a 21-day waiting period. It is after that time that the pet would be immune to the disease. 

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