Reply #21: Flu comes in three major types.

[SheilaT]

Imaginatively called Type A, Type B, and Type C. Type A is the most virulent. The 1918 (Spanish Flu) and H1N1 are examples of Type A flu. Types B and C are progressively less virulent, although they are always around, and from year to year it's as if they take turns as to which one becomes most common.

An important reason people over the age of 60 or so tended not to get the flu in the 1918 pandemic was that they'd all been through a Type A flu epidemic about fifty years earlier, and so were mostly immune to this new Type A.

Every year an educated guess is made as to which subtypes of flu are likeliest to show up for the next flu season and the vaccine is configured based on that guess. All in all they (I'm honestly not sure who does the guess work, the CDC maybe) do a very good job of guessing what will show up. Each year the flu vaccine is configured to at least two different sub-types of expected flu virus. I have no idea if there's a practical limit to how many sub-types could be included in one particular vaccine, but I suppose there is.