Selecting the Viruses in the Seasonal Influenza (Flu) Vaccine | Health Professionals | ... - 0 views
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It takes at least six months to produce large quantities of influenza vaccine. For vaccine to be delivered in time for vaccination to begin in October and November, manufacturers may begin to grow one or more of the virus strains in January based on their best guess as to what strains are most likely to be included in the vaccine.
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Currently, only viruses grown in eggs can be used as vaccine virus strains. If specimens have been grown in other cell lines, they cannot be used for vaccine strains. However, more and more laboratories do not use eggs to grow influenza viruses, making it difficult to obtain potential vaccine strains. In addition, some influenza viruses, like H3N2 viruses, grow poorly in eggs, making it even more difficult to obtain possible vaccine strains.