According to research, one of the best times to use OAV is when bees are brood-less. Simply because OAV does not penetrate capped brood, therefore not being able to reach mites which may in the brood, underneath the caps. December is recognized as a good time, a favorite time to treat with OAV in accordance in my area. Similar can be said of late summer because of dearth in my area, for the same reason. Dearth is usually a time of less brood and a good time to treat with OAV leaving mites little place to 'hide'.
There is much which can be leaned here at Beemaster for the beekeeper, the search bar is a great place, whether that beekeeper is a beginner, intermediate and or well seasoned in various areas of beekeeping, (not only treating of mites). However for the last few post, (and please accept my apology Bob), we are talking primarly Oxalic Acid Vapor treating.
There are 'several topics' discussing the use of of OAV here, along with 'sources' included and provided many times, posted for backup. Sources including Scientist, Universities, and Bee Researchers, as well as individuals here at Beemaster doing their own personal research.. Including types of treatment, treatment schedules, treatment dosages, treatments during time of brood, etc.
So far I have never heard of any case of Immune problems caused by oxalic acid vapor in any research paper or otherwise. I would like to read that source or sources of papers if you have them available. It is a big world and research is a steady progress, still reveling many things, while there are many good things yet to be learned.
I also realize 'opinions' may vary from beekeeper to beekeeper, some stemming from myths while others from facts... A great asset of Beemaster is; Together we 'discuss and learn' sharing the two and hoping to separate the two, for the good of beekeeping, the beekeeper, and bees alike.
Very glad to have you here BagziK. Keep your good contagious enthusiasm coming!
Phillip