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wildfire393 t1_jdn441v wrote

Influenza is a lot more common and contagious, which gives it much more opportunity to mutate. Often these mutations will allow it to bypass a previous vaccine that was formulated for an older version. So a new vaccine needs to be developed to deal with the new strain.

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LordRobin------RM t1_jdn9h1a wrote

But some immunity fades even for the same strain. That was the reason for the first COVID booster. They couldn’t guarantee if would last longer than six months.

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wildfire393 t1_jdnij8h wrote

There's some dropoff over time even against the same strain, but there's also better immune response after more exposures. The first COVID shot was two doses because testing showed a better immune response for two doses at a certain concentration several weeks apart, rather than a larger dose one time.

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