I very much want to get my bivalent vaccine. however, im questioning their safety as i understand they have only been tested on mice. is this true? will mixing vaccines cause unforeseen problems? myocarditis is also a concern for my age? 21?

It was not tested in mice but in volunteer people. However, the volunteers were only checked for antibodies with spike proteins of original covod-19 and omicron BA.1, not checking how effective the vaccine can protect you from the infection. So the testing is different from the original studies when the original vaccine was studied. Myocarditis is always a concern for young man with mRNA vaccine.

Repeat question. See previous reply. You're irrationally worried. Get the booster.