At the molecular level, the compounds were almost identical. Yet in animals the two were very different. Human results matched the animal tests. Ganciclovir has a role in treating transplant and HIV-infected people. All this was defined by prudent animal testing.
Vaccines protect people from disease by stopping infections before they can wreak their havoc.
Let's take as an example the development of a vaccine against HIV to prevent AIDS, which is devastating the African and Asian continents with 16,000 new cases a day and continues to spread throughout the world unchecked. Because vaccines work by stimulating the body's immune system to fight back against the virus, there is no way to test in cells, or on a computer, how the vaccine will work in the whole animal. These animal model experiments define whether novel vaccines are safe enough to initiate clinical trials. More important, they show whether the vaccine is good enough to justify large-scale testing, which may involve tens of thousands of people and cost tens of millions of dollars. Non-human primates, especially macaque monkeys, are critical for the development of an HIV vaccine.