Merck's HIV vaccine, which has been 10 years in development, has failed in a human trial and has been canceled. The Phase II clinical trial has showed the vaccine was completely ineffective in protecting from a HIV infection. The Phase II trial, co-sponsored by Merck & Co., Inc., and the HIV Vaccine Trials Network, (which is funded by the National Institute of Allergy and Infectious Diseases) has involved 3,000 healthy volunteers at high risk for HIV infections.

The Phase II trial, dubbed STEP, found that from one placebo group of 762 volunteers 21 became infected with HIV, while from another group of 741 who received the experimental drug 24 became infected with HIV.

"Clinical testing of AIDS vaccines is a scientific process and, while this is a disappointment, it is in no way the end of the search for an AIDS vaccine. These data are certainly not the ones that we had hoped for. [...] we also note the success of the STEP Study trial design in providing a swift answer to the critical question of whether or not the vaccine provided any benefits," said Executive Director Mitchell Warren from The AIDS Vaccine Advocacy Coalition (AVAC).

The vaccine, an adenovirus-based vaccine (codenamed MRK-Ad5), was made from a common cold virus which had copies of three HIV genes. It was previously determined that the vaccine appeared to be able to turn on the immune system, but the clinical trial unfortunately showed that the vaccine failed to prevent infection, and also failed to reduce levels of the virus in the bloodstream of people who became infected.

Merck has however had some success with its new HIV drug Isentress (active substance: raltegravir) which belongs to the antiretroviral integrase inhibitor class. Integrase is an enzyme that is used by the HIV virus to integrate its genetic material into human cells, and raltegravir interferes with this process by inhibiting the enzyme. Raltegravir's chemical formula is C₂₀H₂₁FN₆O₅. Recently, an advisory panel for the U.S. Food and Drug Administration has recommended rapid government approval from the U.S. Food and Drug Administration because of its proven effectiveness in Phase III trials.