The Flu vaccine no longer protects against just one strain, but a multitude of strains, that takes out a chunk of guess work in trying to work out Northern/Southern Hemisphere trends and projecting that onto the next Flu Season.
Multitude is a little misleading 3 or 4 strains (2 respiratory & 2 stomach typically) isn't how I personally would define a multitude.
Flu vaccines cause antibodies to develop in the body about two weeks after vaccination. These antibodies provide protection against infection with the viruses that are used to make the vaccine.
The seasonal flu vaccine protects against the influenza viruses that research indicates will be most common during the upcoming season. Most flu vaccines in the United States protect against four different flu viruses (“quadrivalent”); an influenza A (H1N1) virus, an influenza A (H3N2) virus, and two influenza B viruses. There are also some flu vaccines that protect against three different flu viruses (“trivalent”); an influenza A (H1N1) virus, an influenza A (H3N2) virus, and one influenza B virus. Two of the trivalent vaccines are designed specifically for people 65 and older to create a stronger immune response. - source
Edit, misspelled respiratoryLast edited by The_Yoda - on 21 April 2020