Totally understandable! Most people don't understand how clinical trials work so this can seem very alarming. Additionally, reading clinical trials results can be confusing. First, research regarding mRNA vaccines has been occurring since around 2013 so the science behind this isn't new. Second, there are multiple committees that have to sign off on clinical trials and patient enrollment before the study can even begin. Everything is well regulated and has significant oversight. Third, there are a number of phases any clinical trial has to go through before it ever meets the public. These phases can be found on a number of websites and through a quick google search. Lastly, in phase 3, Pfizer alone had 21K people receive the vaccine (43K people were enrolled but some were given placebo which is standard; I'm working on a clinical trial that has less than 10 people enrolled nationwide. Because this is/was a nationwide pandemic, there were more people available for study enrollment). There was a 95% efficacy in infection prevention across demographics. This is important. It means that there isn't a small group of people seeing benefits.
Basically, I totally get the fear and concern. I highly recommend everyone to question everything in the science community. However, a large number of people received the covid vaccine prior to it becoming publicly available. Also, there were minimal side effects. Even adverse effects that occurred were similar in both the vaccine and placebo groups.