Merck & Co says it has started a Phase II clinical trial of one of its investigational HIV/AIDS drug candidates in patients considered to be at risk of contracting the disease.

The study is testing a Merck vaccine candidate that is based on adenovirus – a common cold virus that has been modified so that it cannot reproduce and cause a cold in humans. The adenovirus is used to transport three synthetically produced HIV genes into the cells, which stimulates the body to generate an immune response to HIV, producing an army of killer cells – called T cells – that are programmed to recognise and kill cells infected with HIV. Most vaccines work by eliciting an antibody response.

It is being carried out in partnership with HIV Vaccine Trials Network – an international collaboration of scientists and institutions aiming to speed up the search for an HIV vaccine, which is partly funded by the US National Institutes of Health – and aims to include around 1,500 men and women at risk of contracting HIV in North and South America, the Caribbean, and Australia.

Merck says that it is conducting this so-called “proof of concept” because it enables the researchers to test the concept that the vaccine candidate prevents HIV infection, or results in lower HIV levels in the blood of those who become infected with HIV. Future research will be conducted if the data generated by the study show that the vaccine candidate provides some protection against HIV, or delays or diminishes the course of HIV infection.

According to UNAIDS statistics, nearly 40 million people were living with HIV in 2004. More than 13,000 people are newly infected each day – some 95% of these occur in developing countries.