RNA uses uracil/uridine rather than thymine, but uridine is actually quite immunogenic. That's what has prevented people from using mRNA as a therapy until recently, when the founders of BioNTech figured out that they could use pseudouridine (abbreviated as Ψ) instead. See [1] for more information.

[1]https://www.wired.co.uk/article/mrna-coronavirus-vaccine-pfi...