THE farmer vaccinates because he has or had a problem , so the sheep may bring orf onto your land . The vaccine only lasts for 12mths giving protection to the treated animal and is contagious for up to 7wks after being applied . If you buy in any new sheep ( unless you ask and are told no orf in this flock ) you run the risk of introducing orf , so since you know these sheep are from a flock with orf you are increasing the odds .