That we all have individual "chemistries", this applies to all Rx and OTC products. The vaccines for Covid 19 have been administered to date several billion times. The side effects reported are at a very low rate and amongst the lowest of any vaccine or Rx drug. There are some indications that one might have that should be cleared with your MD before hand. As for the influenza "shot", at best it does not cover all variants of the influenza virus and therefore may not be more than 50% 0r 60% effective. Pneumonia, however, can arise from other circumstances other than influenza.