A vaccine trial against H5N1 bird flu has begun, targeting a strain that has ravaged bird populations globally but has not yet transmitted between humans. The trial tests a jab designed to protect against the virus before it potentially becomes a pandemic threat. H5N1 has infected millions of birds and spread to some mammals, including dairy cattle in the United States, but human-to-human transmission remains absent. Health authorities have monitored the strain closely for signs of mutation that could enable pandemic spread. The vaccine development represents a precautionary public health strategy, similar to pandemic preparedness efforts launched after COVID-19. Researchers aim to establish whether the jab generates adequate immune responses and can be rapidly deployed if H5N1 ever acquires the ability to spread widely among people. Results from the trial will inform vaccination stockpile decisions and guide pandemic planning. No timeline for completion was specified.