Interesting discussion, and it's an area where I am not sure what I think and keep changing my mind
You see, I really don't think 'one feed' makes a measurable difference, and to talk of it making a 'huge' difference to immunity is (surely) to exaggerate - what do you think, mears? The 'medicinal' effect of one single feed is going to be overwhelmed by the effects of formula given thereafter, isn't it? Somehow jollying women into this one feed needs to be done honestly, IMO. This means being clear about the effects of not breastfeeding, and supporting openness to giving bf a try, explaining how bf meets so many of the baby's normal needs.
Obviously, the 5 ml or whatever of colostrum is a nice thing for the baby to have, and the close contact and connection ditto, and we should be making it easy for mothers to enjoy a single feed if that is all they are prepared to do (for whatever reason - and these reasons may feel pretty powerful to her, and that's her business!). These early moments with the baby can mean special memories (as long as the mother does not feel pressured or bounced into doing something she really recoils from). But don't lets pretend it is a mega-deal health-wise, as it isn't....surely.
If the 25 per cent of babies who currently get no breastmilk at all were all to get one single feed, and one single feed only, then we would see no difference in public health outcomes at all. However I suspect that some mothers, supported to give that one bf, will continue to give breastmilk/breastfeed for longer, to their surprise...and that will make a difference.
Those who don't want to bf, and those who have a rotten experience bf and who then use formula, need to know that many of the non-milk effects of bf can be replicated with bottles of formula, and that in fact, it's important that they should be, from the baby's point of view. I'm meaning that mothers who use formula need to know it's important for the mother to give the majority of bottles, to feed in response to the baby's cues for feeds, to be skin to skin when she can or at least snuggled up, to talk and interact with the baby and so on. Lots of mothers already do all or some of this, and they need to be aware of how good it is that they're doing it. I am pretty sure it is healing for the mother who feels sad at not bf to know that feeding, however it's done, can be loving, responsive, life-enhancing and relationship-strengthening.