Heptavac p + covers 9 different things and so the passive immunity lasts for varying length's and the data says that by age 4wks the pasteurellosis cover is beginning to weaken but cover for some of the other diseases can remain strong up until 12wks of age , so you pick a time to give the lamb its first injection some where in between BUT if pasturella was a known flock problem then you would want to inject as early as possible . The immunity goes up and down until 2 WEEKS after the second injection when it is at its peak eg first inj 3 -4wks old second inj 4-6 wks later , the lamb has maximum immunity at 9-12 weeks of age . First injection 8-10 wks then second injection 4-6 wks later maximum immunity is at 14- 18 weeks of age