Pathogen transmission from vaccinated hosts can cause dose-dependent reduction in virulence

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.诸多畜禽与人用疫苗均属于漏型疫苗（leaky vaccine）：此类疫苗可阻断疾病临床症状，但无法阻止感染发生以及病原体的后续传播。此类漏型特性令人担忧，因其会提升疾病传播防控所需的疫苗接种覆盖率，还可能推动病原体向毒力增强的方向演化。尽管存在这类漏型保护缺陷，疫苗接种仍可降低宿主体内的病原体载量，进而对疾病传播动力学产生影响。然而，此类疫苗接种对接触个体在感染后的疾病进展与传染性的影响，目前仍不明确。本研究以鸡感染马立克氏病病毒（Marek disease virus, MDV）的传播实验为模型，结果显示：接种漏型疫苗后，接种个体及其所感染的未接种接触个体体内的病毒载量均显著降低。因此，当接触鸡被接种疫苗的鸡感染后，其出现临床症状或死亡的概率更低、症状严重程度更轻，且自身排出的感染性病毒量也相应减少。本研究结果表明，即便仅针对部分群体实施漏型疫苗接种，仍可在控制疾病传播与临床症状方面带来未被预见的积极效果。