There seems to have LONG been a 'controversy' about the brand "Pneumabort K", KV rhino vaccine,which is probably the one in widest use. Some have felt that there are, or have been, issues w/ the 'carrier'(adjuvant) of the vaccine, with the process, or even with the vaccine itself.
Early on, when I was still breeding, I gave it faithfully at 5, 7, and 9 months. However, I began to read of many concerns about it. I haven't studied my records to see if there seemed to be a connection with any of the abortions that occurred here(I had several over time--one at about three months, a couple much later, at @ 7 and 9 months, which linger in my mind; am sure there may have been others, too.)What I DID find was that the ONLY site abcess I EVER had occur was following the administration of that particular vaccine...and several instances of serious soreness, swelling and heat at the site(and I do not give shots in the neck). In my later years of breeding, I either gave NO such shots, or, gave it only once or twice, like at 7 and 9, or only at 9, months of gestation. I had no late term abortions during that time.
My personal feeling about this is that there are several factors in play, including whether or not some/all of your horses leave your premises for venues where many horses gather, and how often; whether 'outside' horses come to your premises, and how often; whether you have other horses living nearby--and so on. My 'guess' is that you *might* have a 'better chance' of being able to 'do without' this particular vaccination in a breeding situation if your horses mostly aren't hauled to shows, esp. those where MANY horses compete or have 'come and gone' in the facility,your location is not 'next to' other horses, esp. ones that might NOT be being kept on a good vaccination and husbandry schedule...you see where I am going. I don't think anyone could EVER know for sure what approach would 'reliably work' for them; but I do think it wise to weigh these kinds of factors in deciding what position YOU will take.
I am thankfully no longer breeding. I give a once yearly multiple booster that includes E, W, and VEE, tetanus toxoid, flu, and KV rhino, and a separate WN. IF I were to haul anybody to a show, or any venue where they'd stay in a facility where other horses also stayed, esp. over one or more nights, I'd be sure to booster at least two weeks preceeding w/ a rhino/flu (or, if one were to suffer an open injury and it's been more than three months since their booster, I'd give a tetanus toxoid booster)-otherwise, that's pretty much 'it'. My horses are all now fully adult-no young stock.
Margo