Ask an Expert: Breast feeding vs formula and mother-to-child HIV transmission

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In our first Ask-an-Expert submission, Emily Blynn from Washington, DC, writes:

I have read in various places that for HIV positive mothers, if formula is not consistently available for some reason, be it financial or physical access or otherwise, consistent breastfeeding is less likely to transmit the virus to the infant than switching off and on between formula and breast milk. This obviously seems contradictory, and though I’ve seen this fact a few times, I’ve never seen an explanation for it. If the reason is that the infant builds up some sort of immunity to the virus over time with consistent breastfeeding, couldn’t this be a potential line of research into prevention strategies?

Answer from Center Scientific Advisory Committee member Sten Vermund, MD, PhD:
There are many references that explain the seeming paradox by Anna Coutsoudis, PhD , and others. A summary is that food and water are seen by the infant gut immune system as “foreign” and immunological cells are recruited to fight the foreign antigens.  This brings infectious breast milk in closer proximity to target CD4+ cells and macrophages.  Breast milk stimulates no such immunological mobilization, so that even with infectious breast milk, the exclusively breastfed baby is less likely to get infected, given the normal gut-associated lymphoid tissue (GALT).

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