I think footvax does scald too, but I dont have any handy to check.
According to NOAH, Footvax is for:
the active immunisation of sheep as an aid to the prevention of footrot and reduction of lesions of footrot caused by serotypes of D. nodosus.
And for those, like me, who can never remember which way round it is, from NADIS:
Scald and footrot share the same primary cause, the bacterium Fusobacterium necrophorum which causes damage to the superficial layer of the skin between the claws and enables the establishment of other bacteria, including Dichelobacter nododus, which causes footrot.
In other words, Footrot is a secondary infection following infection by the scald causative agent, F. necrophorum. Footvax protects against the secondary infection, D. nodosus.
So Footvax won't help reduce scald, but will protect scaldy lambs from progressing to full-blown footrot.
Ah, just seen Welshcob already picked up the NOAH bit. Never mind, we've got the bit about the two bacteria now as well.