Can someone explain how they can make a vaccine before the virus exists?

The mRNA covid-19 vaccine from Moderna was ready on 7.7.2020 and it’s still active,
https://patents.google.com/patent/US10702600B1/en

Can someone explain how they can make a vaccine before the virus exists?
The mRNA covid-19 vaccine from Moderna was ready on 7.7.2020 and it’s still active,
https://patents.google.com/patent/US10702600B1/en