Long term studies comparing the efficacy of cell-free versus cell-associated HVT vaccines against Marek's disease

In long-term field trials lasting approximately 12 months attempts were made to determine whether the cell-free HVT vaccine was as effective in reducing MD losses as the cell-associated HVT vaccine. Four trials were conducted in which flocks were vaccinated with either the cell-free or the cell-associated HVT vaccine. The cell-free HVT vaccine was as safe and effective in preventing MD as the cell-associated HVT vaccine. Although there were some losses due to MD in all flocks, there were also losses due to lymphoid leukosis, multiple hemangioma, liposarcoma or coccidiosis. Egg production was essentially the same in flocks vaccinated with the cell-free HVT vaccine as in those vaccinated with the cell-associated HVT vaccine.