Figure 1. Safety and efficacy of replicating and non-replicating vaccine approaches. There are a number of distinct technologies used to develop viral vaccines. Some of the most efficacious vaccines in use today, such as the measles, mumps, and rubella vaccines, are based on live-attenuated viruses. The smallpox vaccine, which was used to eradicate the disease, is also a live vaccine. It is frequently called a 'Jennerian' vaccine because Edward Jenner discovered that a related live animal virus (cowpox virus) could be used to vaccinate humans against smallpox. Although live viral vaccines are highly efficacious, there is also potentially a greater risk involved with these vaccines because they are replication-competent. Vaccines that are not based on replication-competent viruses are perceived to be safer and have been used to develop a number of important vaccines such as Hepatitis B virus (subunit), HPV (virus-like particle), and inactivated poliovirus vaccine (inactivated virus). Numerous AIDS vaccine candidates have been developed using the non-replicating strategies, notably Merck's Ad5 vaccine candidate, but so far none have been effective. This image was adapted from one provided by Wayne Koff and Chris Parks of IAVI.