Since you got your silkies from a hatchery and they are not with the flock, that is sounding more promising. They are extremely unlikely to be the source of an outbreak which I was particularly concerned about. Yes, if you opted to have them vaccinated then Marek's is one of the 2 main vaccines that are given,. the other usually being coccidiosis. The vaccine is not a guarantee that they will not get the disease but certainly lessens the risks.