The National Institutes of Health on Thursday canceled a US human trial of a human immunodeficiency virus vaccine doubting its effectiveness and saying it might increase infection.

In a news release, the NIH said it had taken the decision to drop the HIV trial known as PAVE 100 “after soliciting and considering broad input from the scientific and HIV advocacy communities.”

More exactly, the decision was taken after Dr. Anthoni Fauci, director of the National Institute of Allergy and Infectious Diseases (part of NIH) had met with scientists to try to understand why a vaccine developed by Merck failed in trials last year. Merck’s vaccine was similar with the vaccine supposed to be tested this year on about 2,400 men in the US and other 6,000 volunteers in South America, the Caribbean, and Africa. The trial had been viewed as a test of a promising vaccine that used virus strains from around the world to prompt immunity.

Merck’s vaccine failed to prevent HIV infection or reduce the amount of HIV in the blood of patients. Moreover, results showed that it might have increased the risk of infection for some patients.

However, the NIH decided to run a smaller trial designed to see whether the vaccine has a significant effect on the amount of virus in a person’s blood. Only if this trial proves efficient, then the NIH will run additional trials.

“Show me that the vaccine works by lowering the amount of HIV in the blood. Then we will move to a larger trial that will document the link with a particular immune response.” Until then, “doing a large trial is not justified,” he added, according to the New York Times.

The NIH has been trying to find a vaccine to prevent HIV infection for almost twenty years now. “An HIV vaccine continues to be our best hope for ending the HIV pandemic.”

For the first time since AIDS was identified in the mid-1980, the United Nations said last month the numbers of AIDS deaths and infections have started to decline. However, the number of people infected with HIV is still high. An estimated 32.2 million people worldwide were living with HIV in December 2007 compared with the figure of 39.5 million the two institutions released the year before. Also, the annual rate of new infections seemed to have declined over the last decade. There were 2.5 million new infections in 2007 down from 3.2 million infections in 1998.

Half of all HIV infection are adult women, 61 percent of them being located in Africa’s sub-Saharan nations. In fact, sub-Saharan Africa remains the epicenter of the global malady. Of the 32.2 million people living with HIV, about 22.5 are registered here, as well as 76 percent of those who die because of AIDS annually. Also 90 percent of all HIV positive children in the world are in sub-Saharan Africa.

The number of AIDS deaths in 2007 was estimated at 2.1 million down from 3.9 million in 2001, most probably because treatment against AIDS was made available in recent years, the United Nations said. However, the organizations needs to do much more in order to meet its goal, namely to reverse AIDS rates by 2015.