Many livestock and human vaccines are leaky because they block symptoms but do not prevent infection or onward transmission. This leakiness is concerning because it increases vaccination coverage required to prevent disease spread and can promote evolution of increased pathogen virulence. Despite leakiness, vaccination may reduce pathogen load, affecting disease transmission dynamics. However, the impacts on post-transmission disease development and infectiousness in contact individuals are unknown. Here, we use transmission experiments involving Marek disease virus (MDV) in chickens to show that vaccination with a leaky vaccine substantially reduces viral load in both vaccinated individuals and unvaccinated contact individuals they infect. Consequently, contact birds are less likely to develop disease symptoms or die, show less severe symptoms, and shed less infectious virus themselves, when infected by vaccinated birds. These results highlight that even partial vaccination with a leaky vaccine can have unforeseen positive consequences in controlling the spread and symptoms of disease.

许多牲畜和人类疫苗存在泄漏，因为它们可以阻止症状，但不能防止感染或进一步传播。这种泄漏令人担忧，因为它增加了预防疾病传播所需的疫苗接种覆盖率，并可能促进病原体毒力的进化。尽管有泄漏，疫苗接种仍可减少病原体负荷，影响疾病传播动态。然而，对传播后疾病发展和接触者传染性的影响尚不清楚。在这里，我们使用马立克病病毒（MDV）在鸡中进行的传播实验来表明，接种泄漏疫苗可以显着降低已接种疫苗的个体和所感染的未接种疫苗的接触个体的病毒载量。因此，当被接种疫苗的禽类感染时，接触禽类出现疾病症状或死亡的可能性较小，表现出不太严重的症状，并且自身散发的传染性病毒也较少。这些结果强调，即使使用泄漏疫苗进行部分疫苗接种，也可能在控制疾病的传播和症状方面产生不可预见的积极后果。