The majority of side effects of the Covid vaccine are thought to be nocebo.
This article in Nature says 76%

People who haven't heard that there are side effects to vaccines, don't experience side-effects. İn my opinion, that means we should stop posting on social media about how we were in bed for 2 days after the vaccine.

https://www.nature.com/articles/d41586-022-00146-y#:~:text=%27Nocebo%27%20underlies%20vaccine%20side%20effects&text=Researchers%20reviewed%2012%20randomized%20clinical,fatigue%2C%20after%20the%20first%20dose.