A hypersensitive immune reaction to the vaccine is what causes most people's to experience a 'yucky' feeling after getting a flu shot.

Another proven contributor to these post vax yuckies is actually our own self again, but this time it's actually our brain doing causing the issues! And by this I mean, that studies have been able to reproduce results that suggest some people only feel sick after the vax because they expect to...aka they feel sick from the Placebo effect.

The data are not well established across the entire population, but some studies suggest that the number of people who present with clinical symptoms of post vax hypersensitivities but do not present any diagnostic test result that suggest hypersensitivity, and furthermore do not suggest any reason for the to be sick at all, is higher than the number of people who do experience real HS reactions from the vax!!

Meaning it's possible, and even likely, that most people who experience yuckie feelings after a vax are actually causing their own maladies through some type of physiological reaction induced entirely through placebo!.

Of course the those who do actually experience HS also tend to present symptoms of greater magnitude than their test results predict, which means they make their own symptoms worse through placebo too!

So basically placebo is just fucking us all and making everyone feel yuckie from the stupid shot, and honestly there's not a ton we can do about that :(

Note: A Hypersensitivity Reaction is what causes the majority of common our common allergy symptoms. The hypersentivity that results from the flu shot is the same exact mechanism that causes allergies (only true with novel virus vaccines. Second exposure can lead to more severe type hypersensitivity that are much more dangerous than an allergic type reaction)

The actual part of the vax that causes 95+% of allergic type responses are the adjuvant, which is the part of the vax that actually causes your body to signal to the adaptive immune system to go check out that site we just got poked at. Because the shot, unlike the mist, uses an inactivated virus, the adaptive immune response would never be able to respond punctually before our more primitive immune system, which perform most custodial roles required by our bodies, had cleaned up and disposed of the helpless pathogens.