Here is one explanation: https://www.forbes.com/sites/roberthart/2021/07/13/how-well-does-the-pfizer-vaccine-protect-against-the-delta-variant-heres-what-we-know/?sh=7009da99ff33
Also, in highly vaccinated countries like the USA, the rates of vaccination are patchy. The areas that have poor vaccine uptake are now thought by some to be causing a "pandemic of the unvaccinated". https://www.npr.org/2021/07/16/1017002907/u-s-covid-deaths-are-rising-again-experts-call-it-a-pandemic-of-the-unvaccinated
There is a lot of information about this from reputable sources out there. You can easily check this out for yourself.
The bottom line is: get vaccinated using one of the vaccines passed by the regulation authorities in Australia.
