When is a good time to take the seasonal flu shot fir seniors?beginning or middle of october ? how long it usually stays in the system before it wears off?

As soon as it is available. The idea that protection fades in a few months is a myth. The problem is there are 8 or more flu strains circling the globe/aided by international air travel & only 4 strains can be put in the shot or it is too weak to protect against those 4. They pick the most active strains & the protection against those 4 lasts all season & into the next one.

Agree with Dr. Ferguson. Get the flu shot when it becomes available. Take Covid vaccine also, if you have not already done so. Wish you good health!