Except for the nasal spray version, flu vaccines use inactivated virus pieces that have surface proteins to which the body’s immune system reacts, producing antibodies. Here is more detail:
https://www.publichealth.org/public-awareness/understanding-vaccines/vaccines-work/
Thank you both for your response.
Thanks for the link.
What I find amazing is that the body’s immune system, which is comprised of microscopic brainless cell matter, can ‘recognize’ and ‘remember’.