We have been administering Panzer's puppy vaccines ourselves, purchased at Tractor Supply. We've been using the 5-way vac for the 6 week, then the 7-way vacs for 9 and 12 weeks. I thought we were done (except rabies from the vet), but now am noticing the brochure also refers to a 15 and 18 week shot. Are the 15 and 18 week necessary? Are the doses possibly lower in the do-it-yourself varieties?