Fig 1. Schematic overview of one ‘lot’ (2 lots per replicate, 1 for each shedder vaccination status) of the MD transmission experiment.

In each lot, shedder birds were all either HVT-vaccinated or PBS sham-vaccinated. All contacts were unvaccinated. The experiment comprised 16 replicates, each consisting of 1 lot in which 3 infected vaccinated shedders were placed in 48-hour contact with 15 naïve unvaccinated contacts at 2 time points, and 1 equivalent lot with sham-vaccinated shedders (4 additional sham-vaccinated lots were added because 2 of these had only 2 shedders because of early death). In total, there were 1,080 contacts and 106 shedder individuals distributed into 72 contact bird groups. Contact bird groups each had roughly equal numbers of males and females. All indicated time points (not to scale) are relative to the day of shedder inoculation with wild-type virus. Open and closed symbols represent uninfected and infected chickens, respectively. For all birds, necropsy was carried out to determine the presence and severity of disease symptoms (tumours and peripheral nerve enlargement) at 8 weeks post-infection (shedders) or post-contact (contact birds), or upon death/euthanasia, whichever was the sooner. HVT, herpesvirus of turkeys; MD, Marek disease; PBS, phosphate buffer solution.