A little goes a long way: Weak vaccine transmission facilitates oral vaccination campaigns against zoonotic pathogens

PLoS Neglected Tropical Diseases
http://www.plosntds.org/
(Accessed 9 Mar 2019)

Research Article
A little goes a long way: Weak vaccine transmission facilitates oral vaccination campaigns against zoonotic pathogens
Andrew J. Basinski, Scott L. Nuismer, Christopher H. Remien
Research Article | published 08 Mar 2019 PLOS Neglected Tropical Diseases
https://doi.org/10.1371/journal.pntd.0007251
Abstract
Zoonotic pathogens such as Ebola and rabies pose a major health risk to humans. One proven approach to minimizing the impact of a pathogen relies on reducing its prevalence within animal reservoir populations using mass vaccination. However, two major challenges remain for vaccination programs that target free-ranging animal populations. First, limited or challenging access to wild hosts, and second, expenses associated with purchasing and distributing the vaccine. Together, these challenges constrain a campaign’s ability to maintain adequate levels of immunity in the host population for an extended period of time. Transmissible vaccines could lessen these constraints, improving our ability to both establish and maintain herd immunity in free-ranging animal populations. Because the extent to which vaccine transmission could augment current wildlife vaccination campaigns is unknown, we develop and parameterize a mathematical model that describes long-term mass vaccination campaigns in the US that target rabies in wildlife. The model is used to investigate the ability of a weakly transmissible vaccine to (1) increase vaccine coverage in campaigns that fail to immunize at levels required for herd immunity, and (2) decrease the expense of campaigns that achieve herd immunity. When parameterized to efforts that target rabies in raccoons using vaccine baits, our model indicates that, with current vaccination efforts, a vaccine that transmits to even one additional host per vaccinated individual could sufficiently augment US efforts to preempt the spread of the rabies virus. Higher levels of transmission are needed, however, when spatial heterogeneities associated with flight-line vaccination are incorporated into the model. In addition to augmenting deficient campaigns, our results show that weak vaccine transmission can reduce the costs of vaccination campaigns that are successful in attaining herd immunity.
Author summary
Zoonotic pathogens pose a significant health risk to humans. Mass vaccination programs have shown promise for controlling zoonoses in reservoir populations and, in turn, lessening the health burden posed to neighboring human populations. Despite some significant successes, major logistical challenges remain for programs that seek to establish and maintain herd immunity in free-ranging animal populations. Specifically, limited host access and costs associated with vaccine distribution may hinder efforts to vaccinate a host population and preempt spillover of a zoonotic pathogen. We use mathematical models, parameterized with data from campaigns in the US that target rabies in wildlife, to illustrate how transmissible vaccines can overcome these challenges. Specifically, we find levels of vaccine transmission necessary to boost vaccination efforts that seek to preempt the spread of rabies, and also predict the cost savings that could be realized with a transmissible vaccine.