A new flu vaccine is formulated it released every year explain why one shot against influenza does not necessarily protect a person from year to year think about the genetic material a virus
The genetic material in viruses mutate to suit different hosts. Eventually the influenza cells will adapt to the new environment and develop it's own immunity.
The influenza strands mutate every year and together create a new strand that was resistant to the prior vaccine. This is why new flu shots are made every year. In order to combat the new stands.
