Australia on Friday canceled a roughly $750 million plan for a large order of a locally developed coronavirus vaccine after the inoculation produced false positive test results for H.I.V. in some volunteers participating in a trial study. Of the dozens of coronavirus vaccines being tested worldwide, the Australian one was the first to be abandoned. While its developers said the experimental vaccine had appeared to be safe and effective, the false positives risked undermining trust in the effort to vaccinate the public. The Australian setback showed the missteps that can inevitably occur when scientists, during a pandemic that has killed more than 1.5 million people, rush to condense the usual yearslong process to develop vaccines into a matter of months. The trouble that arose with the Australian vaccine, developed by the University of Queensland and the biotech company CSL, was related to its use of two fragments of a protein found in H.I.V. The protein formed part of a molecular "clamp" that researchers placed on the spikes that surround the coronavirus and allow it to enter healthy cells. The clamp stabilizes the spikes, allowing the immune system to respond more effectively to the vaccine. The use of the H.I.V. protein posed no risk of infecting the volunteers with that virus, the researchers said. But the clamp generated the production of antibodies recognized by H.I.V. tests at higher levels than the scientists had expected. The researchers decided to abandon development of the vaccine.