The unplanned session was organized on the second day of the Amsterdam conference, which lasts until Friday and has drawn an estimated 10,000 AIDS researchers from around the world. The scheduled scientific meetings focused heavily on vaccine research and new animal models for testing vaccines. The most dramatic event, however, was prompted by a report in the current edition of Newsweek that quoted several physicians who had seen persons exhibiting the symptoms of AIDS - including pneumonia, Kaposi's sarcoma and rapidly deteriorating immune systems - who nonetheless repeatedly tested negative for HIV.

In other developments today at the conference, scientists reported encouraging progress toward breaking one of the biggest bottlenecks in AIDS research, the difficulty in finding a way to test whether any of dozens of AIDS vaccines under development actually work without trying them out on humans.

One way around that problem was presented by two teams of researchers working independently at the Dana Farber Cancer Institute in Boston and Kyoto University in Japan. Using genetic engineering, they created a hybrid of HIV and SIV called SHIV, which acts enough like SIV that it can infect monkeys, but is so physiologically similar to HIV that it is a potentially good model to test a human AIDS vaccine.