Flu Shot

I finally got the flu shot. I delayed but was worn down by the flu fear. I was thinking about invasion, immunity, protection. I was thinking of the fantastic book by Eula Biss on immunity an inoculation.  I thought about that scene in the John Adams series when, Abigail Adams exposes her kids to smallpox.  It is strange and mysterious to me the way our bodies work, the way we try and protect them. Its sort of like watching those inflight safety movies before your airplane takes off.  But I believe in shots, even though I cannot trace the line of cause and effect. There is a gray area where all the work happens that is hidden by my powers of observation and reason.   How do I know the flu shot works? Can I know ? How can I know that the probabilities are correct? How are they calculated? This thing “knowledge” starts to become complex and complicated.