Rabies is required once on or before 16 weeks of age in puppies, boosted again 1 year later (1 year, 4 mos. of age) and then every three years going forward. Dogs are required by law to be vaccinated against the rabies virus because this deadly virus can spread to humans. Why? If an unvaccinated puppy breaks the skin of a person or another dog using those sharp puppy teeth, your local laws will require you to have the puppy quarantined for a minimum of 10 days (could be as long as 6 months.)