The new datasheet, which was published as part of its UK licence approval in April, simply states duration of immunity 12m. The earlier imported ones said duration of immunity 12m but you can boost at 6m if you like. Many vets are still using up imported stock so will still have the data sheet which suggests booster at 6m.
FHB spoke to the manufacturers about this and I also spoke to them at the RWAF conference. Reassuringly, they told us both the same thing ;-) In essence, immunity has always been established as at least 12m in their clinical trials. But meat farms were reporting continued outbreaks of RHD2 before the 12 months were up. Which is pretty obvious really because a meat farm will have a very high turnover of animals - if they are batch vaccinating on a single day, there will be animals in varying states of health/pregnancy/lactation/days old/not born yet/not old enough for the vaccine, and these animals are therefore still potentially able to get infected before the next batch vaccination is due. So because of this, the data sheet recommended that a further booster could be given to all the animals after 6m.
The situation with our pet rabbits is completely different - with each of them reviewed and health checked individually and confirmed that they are an appropriate age/health etc to have the vaccine, and a record made that they have definitely had it. So for the vast majority of cases, the duration of immunity should be 12m as per their clinical trials. Of course there is always a little risk that a vaccine hasn't 'taken' for some reason, hence it may be wise to err on the side of caution in circumstances where animals are under high stress for some reason or if there are cases very close by. It shouldn't be necessary in most cases, as the vaccine should still be good for a year, but I guess some may wish to play it extra safe. Personally I'm happy with every 12m now I understand how that advice arose in the first place.