She started sucking on her index finger on the same hand though so she's not rid of her addiction, but I was surprised to see how fast it happened, almost from one day to the other. What happened was that she got a cut or a chap on her thumb due to a combination of a constantly wet thumb and very cold weather, and we put cream and a plaster on and told her to suck on the left thumb or a finger instead. At first she was just trying to remove the plaster but after a day or so she asked for more cream and a new plaster if it had come off (this seemed to coincide with her becoming more conscious - or perhaps this was the cause - of boo-boos and wanting us to blow on the "wound" and comfort her even though she hasn't really got hurt at all) as well as for sympathy for her wounded thumb. And after two days she started sucking on her right index finger and even though a couple of days later her thumb was fine and she didn't have to wear a plaster anymore, she didn't go back to her thumb, not even in her sleep when you'd think the reflex of putting her thumb in her mouth, something she's been doing since the age of five months, would be the strongest.
So now we have a little finger-sucking girl. Her dad sucked the same finger when he was little - though with the palm of his hand up while she does it the other way around - so maybe it's genetic ;)
* I noticed later after I had written my post that it really was "I'm addicted to..." but hope I'm excused for using Isolde's bad habits :)