Revaccination of chicks for Marek's disease at twenty-one days old

Chicks vaccinated for Marek's disease (MD) at both 1 and 21 days old did not suffer significantly different losses from MD than did chicks vaccinated at 1 day old only. This was true even with vaccinations of 1-day-old chicks with increasingly higher titers. The failure of revaccination to reduce losses from MD could be attributed to the age at which natural exposure occurred.