Polio

This chapter tells the story of polio, beginning with its American history when early in the twentieth century, small outbreaks of an unknown disease called infantile paralysis struck primarily young children in the summer months and then metamorphosed into progressively larger polio epidemics in NA and Europe. Early misunderstandings about the route of infection meant it was several decades before the variable clinical response to poliovirus infection was understood, with no symptoms at all in ~90% of infections; mild, transient symptoms in 9.5%; and the paralysis that led to extreme public anxiety in only 0.5% of all poliovirus infections. Poliovirus was found to be highly contagious and infectious, so even though paralytic disease was rare, polio left many young survivors in wheelchairs and leg braces. At its peak in the 1940s and 1950s, polio paralyzed or killed over half a million people – mostly young children – in the developed world every year. The major research efforts which developed in response are described, including identification of the causative virus with its RNA genome, extremely high mutation rate and its three major strains; the epidemiology of the clinical illness; and (ultimately), the pathophysiology of the disease. Within a half century of the first major US polio epidemic, American researchers developed two highly effective vaccines and the incidence of polio in developed countries plummeted. The exciting story of the development of these two vaccines – an injected, killed virus vaccine developed by Jonas Salk and an oral attenuated live virus vaccine developed by Albert Sabin – is described in the chapter. Major societal events which dramatically affected the course of the disease are also presented. When polio survivor Franklin Delano Roosevelt (FDR) became US president, he mobilized the American public to support research to prevent and treat the disease. With his leadership, nationwide grassroots fund-raising campaigns emerged, campaigns so successful that they completely funded all the research which led to development of the two vaccines. FDR also focused attention on physical rehabilitation, which emerged as an important medical subspecialty, an enduring polio legacy. Finally, disabled polio survivors galvanized the rise of social and civil disability rights. With these successes, two effective vaccines and dramatically decreased polio cases, this history of polio reads like a success story.

The second half of the chapter addresses the effort to eradicate the poliovirus after the world belatedly realized that polio was a global disease. From lameness surveys identifying flaccid paralysis, the global incidence of polio cases was estimated to have been at least 600,000 individuals annually. In 1988, the WHO established the Global Polio Eradication Initiative (GPEI), dedicated to complete eradication of the poliovirus. Using the oral polio vaccine, this largest public health program in history has vaccinated more than 2.5 billion children against polio, with a global decrease in the number of polio cases by more than 99% and a reduction in the number of polio-endemic countries from 125 in 1988 to three by 2012. However, despite sustained efforts, there have been new wild poliovirus cases every year with a tragic upsurge in 2019. At the same time, cases of vaccine-associated paralytic poliomyelitis have been recognized since 2000 due to spontaneous mutation of the attenuated live virus strains in the oral polio vaccine with consequent recurrence of neurovirulence and transmissibility. Called circulating vaccine-derived polio viruses (cVDPVs), these mutants cause local polio outbreaks, infecting and paralyzing unprotected children. Outbreaks of polio due to cVDPVs occurred in seven African countries in 2019. As long as the live polio vaccine is used, it will inevitably release potentially potent polioviruses into the environment, making eradication impossible – but replacing the oral vaccine with the injected, killed vaccine is a serious challenge. In May 2019, the WHO adopted the Polio Endgame Strategy 2019–2023 to address this dual challenge.