I am a physician. People are more likely to have a stronger immune reaction to their second shot, and with the second dose, a good percentage of individuals will really not be in good enough to work comfortably for the next 24-48 hours. Nothing very severe and by the end of the time period, everyone is back to normal. This applies to both Pfizer and Moderna vaccines.
People with a prior history of Covid-19 infection more often than not get a decent reaction to their first shot! They already have antibodies in place to react to the initial shot.
There is a percentage that will get a reaction to the first shot. In my opinion, these individuals who react to the first dose likely had a prior infection with Covid-19 that either was completely asymptomatic or so mild as to have gone unappreciated and undiagnosed.