Scientists have developed a nasal spray vaccine for H5N1 bird flu that has proven successful in animal trials. When tested on mice and hamsters, the vaccine triggered a high level of protection and stopped the virus from taking hold. The spray remained effective even in animals that already had some flu immunity from previous sickness or shots, which is a problem that often makes new vaccines fail. Researchers say that applying the vaccine through the nose protects the lungs and nasal passages better than a standard needle, which could help lower the risk of severe illness and slow the spread of the virus.