JACKI LYDEN, host:

Every year, scientists fret over the flu vaccine. The viruses mutate from year to year, and health officials have to guess which types of viruses are coming months before the flu season starts. The vaccine has to change to anticipate these mutations.

Now, a group of American scientists has made a discovery that might mean no yearly changes would be needed. They found a section of the flu virus that doesn't mutate.

NPR's Joanne Silberner has more.

JOANNE SILBERNER: Dr. Wayne Marasco and 18 other scientists did not start out to study regular influenza.

Dr. WAYNE MARASCO (Associate Professor of Medicine, Dana-Farber Cancer Institute): The study started out, really, looking at our ability to make human antibody drugs for the prevention and treatment of bird flu, avian influenza.

SILBERNER: The bird flu virus has infected more than 400 people and killed more than half, mostly in Asia. Health officials fear a single mutation that makes it highly contagious could spark a worldwide pandemic.

Influenza viruses protect themselves with an ever-changing protein coat. Come up with a vaccine or antibody that attacks one strain and the virus coat evolves, and the virus goes unnoticed by the immune system.

The team, based at Harvard, the Centers for Disease Control and Prevention and an independent lab, wanted to find an accessible part of the bird flu virus coat that doesn't change. They created billions of antibodies in the lab, and found one that attaches itself to a little pocket just inside the coat.

Dr. MARASCO: We consider this an Achilles' heel of the virus.

SILBERNER: It's a weakness that doesn't change. It's in all four strains of the bird flu virus they tested. So they looked at a recreated version of the virus that caused the great influenza pandemic of 1918 - not because they expected it would work, Marasco says, but for that very most scientific of reasons.

Dr. MARASCO: We were curious.

SILBERNER: And the antibody did attach itself to that pocket.

Dr. MARASCO: When we learned from that study that the antibodies inactivated that virus as well, we knew we had something special.

SILBERNER: So they kept on going. They tried the antibody on regular flu viruses in the lab and it worked on many of them, they report in the current issue of Nature Structural and Molecular Biology.

And that's impressive for two reasons, says Dr. Anthony Fauci. He's head of the National Institute of Allergy and Infectious Diseases, which partly funded the study. The antibody the group found appears to attach to a whole lot of flu viruses and neutralize them.

Dr. ANTHONY FAUCI (Director, National Institute of Allergy and Infectious Diseases): If it doesn't change from species to species, but it doesn't neutralize, who gives a damn? It's not going to block the virus. But if it neutralizes but also remains constant from species to species, from strain to strain, then you've got something.

SILBERNER: What they've got, at least in theory, is two ways to stop all sorts of influenza viruses. Scientists could take the neutralizing antibody, and inject it into people who've already been infected. Or they could use the traditional vaccine approach and inject proteins from that sensitive pocket to create a vaccine.

Dr. FAUCI: Something that when you inject it into the human, it actually induces an immune response. And if it induces the response that you think it's going to induce, that will be a neutralizing antibody, and that will be very good news.

SILBERNER: Good enough to stop worrying about bird flu? No, not yet. More work needs to be done. The team is already working on using the neutralizing antibodies and developing a vaccine, but it's likely to take a year or two to see if things work in the human body, not just in test tubes.

Joanne Silberner, NPR News.

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