I had the flu a number when living in America. I would get a high fever, chills, weakness.
Since then about 30 years ago or so, I have gotten the flu vaccine every year. While living in America, and now in Bangkok. I have not gotten the flu since then.
Since living in Bangkok, I get my vaccine at a major international hospital. I was told by one of my doctors that the vaccine is for the flu strains in Southeast Asia only. Not sure about the accuracy of that. But it makes sense.