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When scientists choose viruses for the influenza vaccine based on biological and clinical data, what indicates that a certain strain will circulate and likely be dominant in a certain season?

Does a low HI titer (meaning antigenic novelty) for a virus antigen relative to an antiserum of a virus that is currently prevalent mean that the new virus will likely dominate in the next season? OR is a high HI titer a better choice for the vaccine since it means the virus in question is very similar to currently circulating viruses?

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Again, the full answer is too complex to answer here. The CDC has a broad explanation (Selecting Viruses for the Seasonal Influenza Vaccine) and the WHO has a presentation summarizing the process (The WHO Vaccine Strain Selection Process: Review of the Evidence).

The major factor for choosing a new vaccine virus is based on surveillance from the preceding season and from the opposite hemisphere's season. The surveillance data shows which virus groups are expanding or shrinking. Antigenic variation (high or low HI titers) is also a part of the decision, but every season has a number of viruses that have low HI titers to the current vaccine and the real question is whether those occasional outliers are just sporadic, or whether they're showing signs of becoming increasingly common.

Interpreting whether particular virus families are increasing or not requires a lot of familiarity with influenza behavior, genetics, and antigenicity. You can look at some of the data used in strain selection on the WHO site, e.g. Recommended composition of influenza virus vaccines for use in the 2018- 2019 northern hemisphere influenza season.

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  • $\begingroup$ Thank you @iayork. Can you tell me how one can gain familiarity with influenza behavior, genetics, and antigenicity in order to understand as much as possible? What book(s) should I read to understand this better? $\endgroup$
    – FIREREED
    Commented Feb 4, 2019 at 14:10
  • $\begingroup$ @FIREREED I don't have a simple answer. You might find some of the chapters in The Immunological Basis for Immunization: Influenza Vaccines interesting, and Trevor Bedford's web site and his NextStrain site may be useful. $\endgroup$
    – iayork
    Commented Feb 4, 2019 at 14:34

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