'Great challenge'

Mark Feinberg, vice president of medical affairs at Merk’s vaccine unit, told Reuters: "No one really knows when and if we will ever have an effective HIV vaccine because the virus is such a great challenge."

The vaccines contained a cold virus, along with three proteins found in the HIV virus.

"The concept was that if someone getting the vaccine is later exposed to HIV, the immune system would recognise those HIV proteins and go after the virus," said Keith Gottesdiener, another senior Merck research executive.

The company is unsure if another attempt will be made to develop other HIV vaccines in the wake of the failed trial.
 
"At this point, we can't give an answer. We're just starting to look at this data and all its implications," Gottesdiener said.

Further obstacles 

Merck, based in the US state of New Jersey, was developing the product in partnership with the HIV Vaccine Trials Network, the largest clinical trials programme for developing and testing HIV vaccines, and said that two other early-stage trials of the vaccine had also been stopped.

"Today is a very sad day for the industry because Merck's vaccine had ... [given] many people optimism it would work"

Sarah Alexander, spokeswoman, HIV vaccine trials network
The HIV vaccine trials network is also said to help co-ordinate vaccine efforts around the world.

Study investigators have been instructed to discontinue vaccinating volunteers and to monitor them, Merck has said 

The study aimed to determine whether the vaccine prevented HIV infection and whether it reduced the amount of virus in those who were infected.

Although a number of medicines have been introduced in recent years that control the virus and keep symptoms of the fatal disease at bay, doctors have said that a preventive vaccine is the best way to control the spread of the disease.

"Today is a very sad day for the industry because Merck's vaccine had shown an ability to turn on the immune system, which gave many people optimism it would work," Sarah Alexander, a spokeswoman for the trials network, told Reuters news agency

Major setback
 
Mitchell Warren, executive director of the AIDS vaccine advocacy coalition said that although more than two dozen other HIV vaccines are now being tested for safety in human trials, only one is far enough along in trials to be tested for actual effectiveness.

"The Merck vaccine was a step along the way that will give us clues how we can better trigger the immune system with other vaccines, Warren said.
At least 25 million people in sub-Saharan
Africa have contracted Aids [Gallo/Getty]
In 2003, a HIV vaccine deemed to hold promise was developed by VaxGen, a US biotech company. It was later declared a failure.

Millions of people have contracted Aids since it gained prevalence in the early 1980s, with the worst outbreaks in Africa.

The virus is believed to be spreading quickly in India, China and other parts of Asia.

As many as 1 million residents in the US are believed to be infected with the virus.
 
However, many have insurance that covers expensive drug cocktails needed to prevent uncontrolled growth of the virus. But the drugs, which can cause serious side effects, must be taken for life because they cannot totally eradicate the virus.

Meanwhile, Step, Merck's international trial, which had been stopped, had involved 3,000 HIV-negative volunteers from diverse backgrounds, between the ages of 18 and 45, at high risk of HIV infection.

But an interim efficacy analysis, conducted in about 1,500 volunteers expected to have the best response to the vaccine, had showed that the drug was ineffective.