Briefing: The Bird Flu

The flu-vaccine shortage never materialized, but that doesn't mean we're free from the flu yet. Health officials from around the world gathered in Vietnam last week to discuss a virulent form of bird flu that has already caused dozens of deaths in Asia. U.S. officials are moving quickly in case it spreads. Here's what you need to know.

Where has bird flu been found so far? Cases of the most powerful form of avian flu, caused by the H5N1 virus, have been reported in Hong Kong, China, Cambodia, Thailand and Vietnam.

Why are health officials so worried? There is currently no vaccine against H5N1, so it could spread easily in populations with no immunity to the virus. So far, there have been no confirmed cases of human-to-human transmission (all victims so far have contracted it from chickens or ducks). But because the virus is so unstable, it may only be a matter of time before it becomes efficient at moving from person to person. Even if a vaccine were available, it would take months before the correct combination of viral-strain types could be incorporated into the shot. In the meantime, millions of people could get infected.

Why does it take so long to develop a vaccine? All flu shots contain a blend of several virus strains, which take about four months to grow into a single strain inside chicken eggs in the lab. Doses can't be saved from year to year because the same strain rarely shows up in two flu seasons in a row.

How are we preparing for a possible pandemic? The U.S. is testing a prototype vaccine and has already placed an order for a limited supply of this shot, which can be mass-produced if needed. Health officials also plan to fund a year-round supply of the chicken eggs needed to grow the vaccine virus strains. In addition, they are stockpiling the antiviral drug Tamiflu, which is effective in preventing and treating even new viral strains like H5N1. --By Alice Park