forgive my ignorance but is vaccine effectiveness and antibodies against flu virus X the same thing?
Yah. The vaccine induces your immune system to produce antibodies. The vaccine is effective so long as you have enough antibodies to protect you from the virus.
Edit to add - This is one of the places where "detectable" and "effective" matter. A study or two have claimed they detected antibodies even decades after exposure to some flu strains. But that doesn't speak to effective immunity.
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