Doesn't developing our own immunity mean we have to catch the disease first? There are two problems with this.
Firstly a likely death rate of 1 per cent which means 250,000 Australians might well die.
Secondly nobody can say how long acquired immunity will last.
Keith
50-60% infection rate would then create herd immunity, ie there wouldn't be enough new people for the disease to keep spreading too. If you then kept those in the highest risk categories (over 60 and immunocompromised) under tighter social distancing measures, and then make sure those who get it in a bad way have access to quality care you might keep the casualties to 20,000. It's not a great outcome, but likely the least worse option at the moment.
Once you have had it, your body has immunity to that strain, if it mutates then we are back at the beginning, but that will likely be the same case for a vaccine as well.
There is no guarantee that there will be a vaccine, and if so when it would be available, but the testing phase takes about a year. I have heard testing a vaccine likened to growing corn, doesn't matter how much money you throw at it, how many crops you plant, it is still going to take the same time.