Unraveling the Transmission Ecology of Polio.

Micaela Martinez-Bakker,Pejman Rohani,Aaron A King,Andrew Fraser Read

doi:10.1371/journal.pbio.1002172

Abstract

Sustained and coordinated vaccination efforts have brought polio eradication within reach. Anticipating the eradication of wild poliovirus (WPV) and the subsequent challenges in preventing its re-emergence, we look to the past to identify why polio rose to epidemic levels in the mid-20th century, and how WPV persisted over large geographic scales. We analyzed an extensive epidemiological dataset, spanning the 1930s to the 1950s and spatially replicated across each state in the United States, to glean insight into the drivers of polio’s historical expansion and the ecological mode of its persistence prior to vaccine introduction. We document a latitudinal gradient in polio’s seasonality. Additionally, we fitted and validated mechanistic transmission models to data from each US state independently. The fitted models revealed that: (1) polio persistence was the product of a dynamic mosaic of source and sink populations; (2) geographic heterogeneity of seasonal transmission conditions account for the latitudinal structure of polio epidemics; (3) contrary to the prevailing “disease of development” hypothesis, our analyses demonstrate that polio’s historical expansion was straightforwardly explained by demographic trends rather than improvements in sanitation and hygiene; and (4) the absence of clinical disease is not a reliable indicator of polio transmission, because widespread polio transmission was likely in the multiyear absence of clinical disease. As the world edges closer to global polio eradication and continues the strategic withdrawal of the Oral Polio Vaccine (OPV), the regular identification of, and rapid response to, these silent chains of transmission is of the utmost importance.

Highlights

Poliovirus, like other members of Picornaviridae, usually generates mildly symptomatic infection
Historical epidemics that predate the use of vaccines can be used to disentangle the epidemiology of disease from vaccine effects
The race for the polio vaccine during the post-World War II era led to the development of the Inactivated Polio Vaccine (IPV) and the Oral Polio Vaccine (OPV), which reduced the global incidence to less than 0.1% of prevaccine levels [6]

Summary

Introduction

Poliovirus, like other members of Picornaviridae, usually generates mildly symptomatic infection. Wild poliovirus (WPV) is transmitted fecal–orally and in the Northern Hemisphere exhibits seasonal epidemics in late summer and autumn [1,2,3]. Missing the 2014 goal of globally stopping WPV transmission has left eradication elusive, primarily because of political and social obstacles for effective vaccine distribution, including vaccine hesitancy and mistrust. In light of this—and the call for innovative solutions [7]—an understanding of polio’s ecology can help guide alternative strategies. Looking toward eradication and beyond, a polio-free world requires an understanding of the mode by which polio originally emerged and historically persisted. We contend that a retrospective study of the ecology of WPV in the absence of vaccine interventions can inform future planning and may pinpoint vulnerabilities in WPV’s epidemiology that could be leveraged for eradication

Methods

Results

Discussion

Conclusion