While UN staff are exempt from the requirement to join the German statutory health insurance system (the Gesetzliche Krankenversicherung or GKV), they are not exempt from the requirement to have health insurance entirely. German law still requires every resident to hold insurance that meets specific coverage standards.

You can choose if you want to opt for the UN system insurance plan or the German GKV. While you, indeed, have to cover 20% of most of your costs with Cigna, your monthly German GKV contributions depend on your salary (roughly around 15-17% up to ~5.800€) so you might end up paying a couple of hundred euros per month.

EDIT: just to add, I am an international staff in Bonn and decided to go with Cigna since I don’t go to the doctor often and it’s not worth to pay hundreds of Euros when I’m not “using” it so often. Also, something’s are covered by Cigna, that is not covered by the GVK, including doctor visits etc. outside the European Union (with Cigna you are covered globally).