BioNTech's mRNA vaccine technology is a pretty great example of a VC funded startup success story that had huge impact and profit.
If you want to constrain your question to Silicon Valley specifically, which isnt the center of biotech VC, there are still lots to look at[1].
You can also just hook at the helthcare portfolios of the SV VCs themselves and find several sucessful exits.
Venrock [2], e.g Anacor, regenxbio
https://techcrunch.com/2018/07/28/home-run-exits-happen-stea...
https://www.venrock.com/companies