Most immunologists agree that live vaccines – vaccines containing living but non-infective bacteria and viruses – are more effective at protecting target animals and humans than killed vaccines. A group of National Institutes of Health medical researchers have developed a live vaccine against the deadly H5N1 strain of the avian influenza virus.

During the past two years, the H5N1 strain has devastated poultry flocks in Asia. It has also infected approximately 120 humans, killing more than 60. Health experts fear that the virus could mutate into a strain that can spread from human to human creating a pandemic that could potentially kill millions of people.

The NIH vaccine was made with the live H5N1 strain of the virus and is applied as a mist sprayed in the nose. Immunizing people through their noses would be faster and possibly more effective protection, than immunizing them with injections.

The NIH researchers Dr. Brian Murphy and Dr. Kanta Subbarao are currently conducting animal studies. If the vaccine – dubbed FluMist – protects the animals, it will be tested on human volunteers.

“By no means are we confident we're picking the right strain” because influenza mutates so easily, Subbarao cautioned. She chose vaccine strains from those that U.S. scientists who are monitoring influenza in Asia cull from ducks, chickens, and geese, and ship them to the United States for research.

Subbarao must customize those strains for safe vaccination: First, using a new technique called reverse genetics, she selects genes for H and N antigens and removes genetic segments that make them dangerous. Then she adds the remaining gene segments to the regular weakened FluMist virus. Stocks of the custom virus are grown in fertilized chicken eggs. Each is then carefully opened to drain virus-laden liquid that in turn ispurified and put into a nasal spray.