Hola Stuart

Estar a salvo is a set expression in Spanish meaning "to be safe [out of danger]". It is always using the preposition "a". We also use "ponerse a salvo", meaning "to get to safety", for example:

"Los residentes de la zona se pusieron a salvo del incendio, huyendo en sus coches."

"The residents of the area got to safety from the fire, by escaping in their cars."

I hope it helps,

Inma