''denn je'' vs. ''als je zuvor''
In my preparation book for ÖSD exam I got a title that says: ''Mehr Neuinfizierte als je zuvor''.
In my notebook entry, I wrote: ''Heutzutage brauchen Kinder Haustiere mehr als je zuvor'', and I was surprised when I got corrected: ''Heutzutage brauchen Kinder Haustiere mehr denn je''.
Please, could someone explain the difference between the two and how to use them properly?