Actually, the manufacturer's website claims that Mirena alone

is highly effective at preventing pregnancy, over 99%, and I am willing to believe it because I used it alone for many years when married without any incidents, but now that I have non-monogamous relationships, I also use condoms, which means that the effectiveness of what I use surpasses that of sterilization via tubal ligation (the old method) and has added benefits because Mirena removes menstruation and cramps and adds a bit of cancer prevention protection.