Of course, there's always a few that may slip by the AV.
Then if you think about, if a virus or trojan is new then the AV probably won't recognize it as a threat since it probably doesn't have it in it's definition file. But that's why they have regular updates. We will never be 100% safe online because as long as there's an internet there will be threats, and the good guys will keep making tools to get rid of them, and the bad guys will keep changing them and making new ones to avoid detection.