SEP security update

I have SEP 12.1.5 installed in my environment. I understand that Symantec has released a security update SYM15-011 for 12.1.6 MP1. I would like to know if this update is applicable for 12.1.5 systems as well. Do I need to update this patch on my system? Is there any impact if I do not apply this update on my system? Please help