Polio: A Tale of Two Vaccines

Abstract

Medicine for Policymakers is a Journal column that provides decision makers with brief explanations of the meaning and implications for biosecurity of clinical issues. The articles describe, for a nonmedical audience, hospital practices, medical challenges, healthcare delivery issues, and other topics of current interest. Readers may submit ideas to the column's editor, Amesh A. Adalja, MD, through the Journal's editorial office at jfox@upmc-biosecurity.org.

Recently, polio has been in the headlines and on the minds of policymakers as the World Health Organization (WHO), with backing from the Gates Foundation, has redoubled its efforts to eradicate poliovirus. Eradication and control strategies for polio employ vaccines that protect individuals from infection with the poliovirus. However, because there are 2 very different polio vaccines, understanding the control of polio is more nuanced than is the case for other vaccine-preventable illnesses.

Paralysis Occurs in a Subset of Cases

Although the common conception of polio invokes images of children using walkers, people breathing via iron lungs, and people walking with a persistent limp, only a small minority of individuals infected with the poliovirus progress to paralysis. Polio is acquired by contact with the virus from the gastrointestinal or upper respiratory tract of another human, and over 95% of infections with poliovirus are not symptomatic. Only 1 in 1,000 (0.1%) infections leads to paralysis.¹ The fact that most cases of polio are not clinically apparent makes polio eradication very difficult.¹ By contrast, infection with smallpox—the only other human infectious disease to be eradicated—was always clinically obvious, and therefore vaccination could be precisely targeted to areas where the disease was seen.² For polio eradication to work, vaccination must be very widespread.

The Salk and Sabin Vaccines

The first vaccine available to combat polio was developed by Dr. Jonas Salk at the University of Pittsburgh in 1955. This vaccine, which is made from 3 inactivated (or “killed”) polio virus strains, is still used. Currently, it is administered in a series of 4 injections. The introduction of the Salk vaccine, or inactivated polio vaccine (IPV), led to a tremendous decrease in polio in the U.S. However, there were concerns that this vaccine would not provide sufficiently robust or long-lasting immunity. In fact, breakthrough cases of polio occurred in individuals who had been fully vaccinated.

In parallel to the development of IPV, an orally administered vaccine made from 3 attenuated (“weakened”) live virus strains was developed by Dr. Albert Sabin. The theoretical advantage of the oral polio vaccine (OPV) is that ingested live attenuated virus reproduces in the intestines of the vaccinee, more closely mimicking natural infection with poliovirus and therefore producing better immunity. Also, the live vaccine virus that is growing in the intestines can spread through fecal-oral transmission to other nonimmunized people, conferring immunity on them as well—potentially important for immunity in households and other crowded living conditions. In addition, the oral polio vaccine is much less costly than the inactivated version.¹

Vaccine-Derived Polio and the OPV

One of consequences of using OPV was that, in certain relatively rare circumstances (1 in 2.6 million doses), the genetically modified vaccine strain virus could further mutate and regain some of its potency, causing paralysis in some individuals. Because OPV is capable of person-to-person spread, these newly unattenuated vaccine-derived polio viruses (VDPVs) can themselves rarely cause limited outbreaks of vaccine-derived polio.¹

U.S. Polio Vaccination Policy

Since 1955, there have been several polio vaccination policy changes. In 1963, the U.S. moved from an IPV-only policy to an OPV-only policy to provide better immunity. Eventually, as the incidence of naturally occurring (or “wild-type”) polio diminished and the risk of vaccine-derived paralysis became relatively greater, the U.S. migrated to a combination OPV-IPV policy and, finally, to the current IPV-only 4-dose policy (see Figure 1). Polio caused by the “wild” virus has been eradicated from the United States since 1979.³

Figure 1.

Polio Vaccines Used in the U.S.

OPV in the Developing World

Because of its low cost, ease of oral administration, and ability to engender immunity in individuals who have not been vaccinated (by being shed from vaccinees), OPV is the vaccine used in the developing world. The global polio eradication efforts use this vaccine. However, although wild-type polio can theoretically be eradicated using OPV—and OPV is responsible for the eradication of 1 of the 3 strains of polio already—it will cause rare cases of vaccine-derived paralysis. IPV can prevent vaccine-derived paralysis, but it is not practical in most places where polio is still endemic because of its cost and the need for administration via injection. Thus, isolated cases and limited outbreaks of vaccine-derived paralysis will continue even if wild polio is eradicated as long as OPV is used.⁴

References

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