YF jab is not really recommended for those over 70. We did this trip in 2019, travelled to Iguazu and did the falls on both sides, staying in Iguazu. We did not have YF vaccinations. Read up on the rules as from what I can see you only need to be vaccinated if you have been in a Yellow Fever area six days before arriving and have stayed overnight or longer. We had no problems on return to Australia.