My boss got the first shot of Moderna and he had pretty intense flu like symptoms for several days after. I told him to look on the bright side, at least you know its working. It’s pretty random with some having minimal reaction after shot 1 and others having full blown side effects. I do wonder if it has anything to do with prior exposure to Covid. Maybe some of the people who have a bad reaction to the first shot already had asymptomatic Covid and so the first shot is acting like the second shot for them. I have no scientific evidence to support that, but it makes some sense that if the second shot generally has a larger reaction due to your immune system already recognizing the virus and ramping up faster, if you were already naturally exposed your first shot could have a similar reaction to most people’s second shot. Interesting theory from a “non-medical background” guy so take it with a grain of salt as just a maybe.