https://www.cdc.gov/flu/about/season/vaccine-selection.htm https://www.cdc.gov/flu/about/season/vaccine-selection.htm The information I gave was relevant and e

https://www.cdc.gov/flu/about/season/vaccine-selection.htm https://www.cdc.gov/flu/about/season/vaccine-selection.htm The information I gave was relevant and explained the correlation. But if you are not convinced, read above. Vaccine is one common way for an animal (including humans) to become immune to a targeted virus. That’s the purpose of receiving a vaccine, otherwise you would have to catch a real flu (of all classification) in order to develop immunity. As virus changes its structures, our immune system may not be able to recongize the new virus, thus we get sick, and therefore pharmaceutical companies have to develop a new vaccine. If flu virus doesn’t mutate, we would only have one type and one shot would be suffice for life time. I hope this helps.