When I messaged USC, they said, “We have a large increase in applications this cycle, so timing might be stretched. Not all initial interview have been done yet. Faculty typically start contacting applicants early on and sometimes all the way into April. However, the majority of the formal interviews are done within February which is when our formal interview day is held. All applicants are considered, until they receive a deny letter.”
When I messaged my PI, he said, “I believe invitations for interviews started going out today. I don’t think everyone has received a reply yet. You should receive an email soon.” (this was on January 13th).