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Since the flu circulates seasonally among birds and humans, an industry is in place that prepares a vaccine each year. Ordinarily, the vaccine is prepared by assessing the three flu subtypes likely to be a threat in a given year. A number of monitoring stations, overseen by the World Health Organization, make the determination. Researchers grow samples of those viruses in chicken eggs, inactivate the viruses with chemicals, and then harvest their surface proteins, which are mixed into a vaccine -- the yearly flu shot. The vaccine produces immunity by fooling the human body into producing antibodies to those combinations of proteins, and thus to the virus subtypes that include them.
The problem with vaccine approach to preventing flu pandemics is that the influenza virus is a fast-moving target. By the time a vaccine is in production (the production process can take up to nine months), the prevailing flu viruses in circulation may have changed so much that any antibodies developed in response to the vaccine may be useless. The process is also labor-intensive and limited by the number of available eggs. Even in a high-production year, only some 300 million doses of vaccine are made -- not nearly enough in the case of a serious pandemic which might infect billions.
H5N1 presented vaccine researchers with an even bigger obstacle. Since its most virulent forms are 100 percent lethal in chickens, traditional methods of preparing a vaccine proved unusable. After some experimentation, a vaccine based on a modified strain of H5N1 that can be grown in chicken eggs has been developed and is currently in clinical trials; the early results are promising, but its effective dosage thus far appears to be extremely high, so it is unclear whether enough vaccine can be produced to protect a significant number of people.
Credit: Micah Fink
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Briefing
Read a briefing on the dangers posed by avian influeza and the challenges nations must overcome in order to fight against it.
Interactive Map
Explore how avian flu is spreading across Asia.
Filmmaker Notes
Go behind the scenes with producer Micah Fink.
Info-Graphic
Learn how chemical and biological weapons work.
Interactive Challenge
Test your knowledge of the HIV/AIDS pandemic.
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