WASHINGTON (Reuters) - More than 3,000 people who volunteered to receive an experimental Merck and Co. AIDS vaccine are being told to come back and get extra tests because the jab may itself raise the risk of infection.

Researchers stress that they do not yet have enough information to say whether those who got the shot indeed are more susceptible to infection with HIV. But they said initial information from the trial, which was stopped suddenly last month, is worrisome.

"At present, there is a tremendous amount of data being analyzed from the ... trial to see if there is, in fact, any greater risk of infection in those volunteers who received the vaccine," Dr. Mark Feinberg, vice president of medical affairs and public health for Merck, said in an e-mail letter.

Two studies were stopped in September after the independent board monitoring one of the trials noticed some troubling data.

"Specifically, 24 cases of HIV infection were seen among the 741 volunteers who received at least one dose of the investigational vaccine, while 21 cases of HIV infection were seen in the 762 participants who received at least one dose of the placebo," the National Institute of Allergy and Infectious Diseases, which was co-sponsoring the trial with Merck, said in a statement.

This trial, which began in 2004, had enrolled volunteers around the world in the United States, Peru, Brazil, Dominican Republic, Haiti, Jamaica and Australia.

The second trial had begun in South Africa earlier this year, and had enrolled 800 volunteers.

Both Merck and the NIAID say there is no way the vaccine itself could cause infection.

DNA SNIPPETS

The investigational vaccine used in both ... studies cannot cause HIV infection because it contains only synthetically produced snippets of viral material. There is no way for these snippets to reconstitute an intact virus," the NIAID, part of the U.S. National Institutes of Health, said.

"Researchers, however, are analyzing available data to better understand if there may be an increased susceptibility to acquiring HIV infection among those volunteers who received the vaccine."

The vaccine uses three pieces of DNA from the human immunodeficiency virus that causes AIDS, carried by a common virus that normally causes upper respiratory infections, called an adenovirus.

"Frankly, I think everyone is still trying to figure out what the data means," said Mitchell Warren of the AIDS Vaccine Advocacy Coalition, who was not involved in either study.

Warren said he hoped the investigation would not frighten people away from taking part in AIDS vaccine trials.

"How people understand this information is going to be critical for this research to continue," Warren said in a telephone interview.

Close to 40 million people around the world are infected with HIV, which has no cure. The virus has killed 25 million people.

Experts agree that a vaccine would be the best way to fight AIDS but efforts to develop a vaccine have so far been almost completely ineffective. Dozens of potential vaccines are in trials now.