Any type of hormone induced birth control i.E. The pill, the shot (depo), the patch , the ring are 99% effective when taken as directed. With the shot just as long as she is taking it when she should ( every 3 months, every 6 months, or every 9 months), whatever is directed, you guys only have a 1% failure rate. It is up to you if you decide to have unprotected sex or not. I would not worry. The reason why women for the most part go on birth control is to not become pregnant without protected sex. I have been with my boyfriend for 2 years and have never used protection with him and am not pregnant to this day. Best of luck.
Natalie