Most anti viruses, if not all , have these days sophisticated mechanisms to deal with unknown malwares : behavior blockers, heuristics, HIPS, generic signatures....
To expect to get a sample first and add detection after is impossible these days ; I remember one of ESET officials saying " this sample was seen only 10 times by ESET , in the whole word, that's why we did not detect it"
I was shocked by such statement.
novice received kudos from BeanSlappers in AV-Comparatives Real-World Protection Test February-June 2018