The research into OA as a treatment against varroa done by LASI in 2013 and published in 2015 was, I think, intended solely to compare three methods of delivery (spraying, drizzling and vaporising) at three different doses (four in the case of vaporisation). In order to do the comparison, they had to remove the brood before starting treatments to get a fair comparison. I don't think they really expected beekeepers to open up their hives, lift out frames and remove all brood as a matter of routine.
The LASI research showed vaporisation with 2.25g of OA (not Apibioxal because it was not invented then) was safe and the most effective treatment. As Phil has pointed out above, beekeepers are ahead of the academics and realised that OA did not kill bees and that if you vape every 5 days for the length of the brood cycle, you catch all the varroa as they emerge with the bees from brood cells.
Why 5 days? Well, I think came about because 5 days is the minimum "phoretic" period before a mite returns to a brood cell - get 95% the mites before they return to the safety of a cell and you're nearly there. Do it three times, five days apart and you've pretty much covered the worker bee reproductive cycle or four times if you've got drone brood (or still have a big mite drop after 20 days).
We don't know for sure how OA kills mites - do the mites somehow ingest the treatment or do they absorb it? Does it burn their feet as they move about on bees and they let go and die under the hive? Does the acid poison them through absorption by their soft mouth parts? Beekeepers don't need to know the answer to these questions but others are curious.
A lot of stuff was written about varroa before we found out that the varroa are not phoretic (meaning in a purely transport phase) when on bees outside the cells - they are parasitizing the bees. They tuck themselves between the abdominal plates of adult bees primarily on the underside of the abdominal region on the left side of the bee and they eat the bees' Fat Body (an essential organ for many different reasons). They seem to be fairly safe there but we don't know how long they stay at one feeding site or indeed on one bee so I think they are vulnerable to OA when they leave a feeding site to find another, which is why the OA crystals need to have an active life - to be effective until the next treatment. Having a broodless period is not the panacea it was once thought to be. We now know that it means that the mites are all feeding on adult bees, reducing their lifespan, reducing their ability to regulate temperature and humidity, reducing their ability to deal with toxins, etc.
Bees do nothing invariably and reacting to OA treatments is no exception. I've had one or two colonies that for some inexplicable reason needed five OA treatments to get the mites under control. Vaping is a simple procedure that is made difficult by having to clean up after Apibioxal leaves sugary residues in equipment - ah well, can't have everything.
CVB