American researchers said today that they have made an important discovery about the African pattern of AIDS that may provide a clue to developing a vaccine for the fatal disease.

Harvard University scientists Max Essex and Phyllis J. Kanki, speaking at an international conference on the phenomenon of acquired immune deficiency syndrome in Africa, said their discovery had removed some of the obstacles blocking the development of an AIDS vaccine.

Much more work remains to be done, they emphasized, but the prospects for a vaccine are not as bleak now as they were believed to be only several months ago. Essex said their discovery about the AIDS virus in Africa provides the "best way to approach the vaccine question at this time."

Essex also presented new evidence supporting the theory that the African green monkey transmitted AIDS to humans on the continent, where the disease is now considered by many experts to be a major public health problem in several Central African countries. In contrast to the United States, where homosexual males are the most frequent victims of AIDS, the disease in Africa strikes men and women equally.

Reflecting the sensitivity of many African governments over the origin of AIDS, however, a group of African participants challenged the popular implication of the monkey theory -- that the disease originated in Africa and then spread to the United States and Europe.

The findings that gave Essex and Kanki hope about the development of an AIDS vaccine were the result of the examination of blood samples from hospital patients and prostitutes in Dakar, Senegal. The Harvard researchers, in collaboration with French scientists, looked at about 30 samples out of the larger group of 411 prostitutes and patients whose blood was examined for the AIDS virus, Essex said.

None of the 30 donors exhibited signs of AIDS or was unhealthy, Essex said, yet they all had antibodies that suggested that they had been exposed to an AIDS-like virus indistinguishable from the virus that infects wild African green monkeys. The monkeys are believed to be generally unaffected by the virus.

Essex said that at some time in the past, the monkeys may have spread the virus to humans, possibly by biting hunters, and that the harmless virus may have mutated into its present deadly form.

But the fact that there are humans in Africa with the harmless virus may provide clues revealing "what has changed about the virus that causes the disease," said Kanki. "That's very important."

"Conceptually, it's at least possible that it [the development of the virus] happened naturally," Essex said. "It's somewhat analogous to the situation with cowpox and smallpox, but we can't oversimplify it to say, as they did with cowpox, you just scrape it off" and inoculate people.

"You have to go through a lot of sophisticated procedures . . . to get something that would be usable," he said.

Research suggesting an African origin for AIDS produced controversy, as the African group issued a statement contesting the theory. "During the symposium papers presented did not show any conclusive evidence that AIDS originated in Africa," said the statement. "It is a global problem, and not an African problem alone."

Efforts to associate Africa with AIDS, the group's statement said, "do not contribute to future control programs." The group consisted of about 50 African doctors and government representatives.

In another indication of the sensitivity about AIDS in Africa, the scheduled presentation of several American and European researchers on AIDS in Zaire was canceled, and the scientists stayed away from the meeting. One of the scientists, who asked not to be identified, said last month that he feared the Zairian government would halt his research in the country if he spoke at the meeting.

Papers presented at the meeting also provided new evidence about the extent of the disease and related illnesses in Africa, but there was little in the way of hard numbers about how many cases of AIDS have been reported in specific countries.