Despite the failure last fall of Merck & Co.'s AIDS vaccine, a fledgling Atlanta biotechnology company, GeoVax Inc., is charging ahead into advanced tests of a new drug that it says shows "significant promise." "We've had excellent results in our early stage human trials," said Emory University's Harriet Robinson, leader of a team of scientists working on the vaccine. Its vaccine is the only one of several being developed that is on the verge of being moved into Phase 2 tests by the HIV Vaccine Trials Network, which is supported by the National Institutes of Health. GeoVax's trials are expected to start this summer. ...