It's been a decade since I had to worry about such things. I remember reading that breast milk is - when available and plentiful - the preferred method. Formula is always second best. But this is a numbers game and I think the lab coats don't say formula child will suffer consequence A as a result. It's always there is a 5% higher chance of catching this or that (and I pulled that number out of thin air). But this is the margins I think I read about when it mattered.

Child #1 got supplemented with formula 60/40 at first; child #2 never had formula. Child #2 has spent more time in pediatricians' waiting rooms. It's a numbers game where you can do everything "right" and still not "win." Big air quotes on those terms.

If you are a new parent or are about to become one and you're reading this thread and you're freaking out: please take a deep breath. You'll figure this out.