
The most commonly used process for manufacturing an influenza vaccine was developed in the 1940s -- one of its co-inventors was Jonas Salk, who would go on to develop the polio vaccine -- and has remained basically unchanged since then. The process is coordinated by the World Health Organization and begins with the detection of a new virus (or rather one that differs significantly from those already going around); in this instance, the Pandemic H1N1/09 virus.
Once the pandemic strain has been identified and isolated, it is mixed with a standard laboratory virus through a technique called genetic reassortment, the purpose of which is to create a hybrid virus (also called the 'reference virus strain') with the pandemic strain's surface antigens and the lab strain's core components (which allows the virus to grow really well in chicken eggs). Then the hybrid is tested to make sure that it grows well, is safe, and produces the proper antigen response. This takes about six to nine weeks.
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Kottke.org: How the H1N1 vaccine is made