New job, 7w5d

Erin • Early MC Oct `14, Missed MC April `15 & Sept ‘18 💔
I started a new job yesterday, didn't know I was pregnant until after I accepted the offer.  
I'm wondering when to tell them and if there could be potential of being let go since I may not be able to receive some vaccines that are a condition of my employment (if my vaccines aren't up to date). 
They can't fire me right?