It is produced by yeast cells, into which the genetic code for HBsAg has been inserted.[1] A course of two to three (2–3) vaccine injections is given, the second injection at least one month after the first dose and the third injection being administered six months after the first dose. The first and second dose offer complete protection. The final injection (second or third depending on number of vaccines being administered) is to prolong protection against the HBV.[2] Afterward an immune system antibody to HBsAg is established in the bloodstream. The antibody is known as anti-HBs. This antibody and immune system memory then provide immunity to HBV infection.[3] The first vaccine became available in 1981.