In the developed world, polio is often thought as a disease of the past. We have fought against this debilitating infection with scientific breakthroughs in the last century. We have beaten off and reconquered much of what was once under polio's seemingly unclenching fist. Yet, parts of the world are still plagued with polio, causing lower limb paralysis and disfiguration every day. There you can find the frontlines of the current polio eradication war. Why has it been so difficult to eliminate polio? What are its biological components and mechanisms? How far are we from eradicating it entirely from the lives of vulnerable children worldwide? This module aims to explore these pressing questions and more about the poliovirus.

Learning Objectives

1.       Recount the history of polio, its impact, and how the virus has been battled.

a.        Highlight major events from its discovery to vaccine development.

2.       Understand the basic biology of the poliovirus.

3.       Describe early symptoms common to polio.

4.       Describe symptoms of neural involvement in polio infection.

5.       Understand the transmission of the poliovirus.

6.       Discuss the mechanisms of the oral and inactivated polio vaccines.

7.       Describe in general terms the development of vaccine-derived poliovirus.

8.       Summarize the World Health Organization's initiatives for polio eradication.

9.       Explain the biological factors of the poliovirus that impact eradication efforts.    

a.      What factors aid eradication efforts?

b.      What factors hinder eradication efforts?

The History of Polio


RK: Side note to Dr. Lamorte : Looks like Softchalk 8 has created this Presentor activity to solve the issue of the slideshow activity only showing you the Photo OR the content, but not showing both at the same time. So I started making this as a Presentor activity, but it looks like the content is limited to only 300 characters. SO I gave up and made a Slideshow activity above.

Anyway, I thought you would like to sample the latest interactivity :)

The Basic Biology and Transmission of Poliomyelitis


Figure 8: A Poliovirus Bound to a Neuron Receptor (10)

Quick Facts:

Tab1: Wild polio enters the body through the mouth, typically by hands, food or water contaminated with fecal matter (12).


Figure 9 Poliovirus enters via the mouth (13)


Figure 10 Children can contract poliovirus by touching contaminated objects and putting their hands in their mouths (14)

Tab 2. Once inside the body, polio replicates itself in the pharynx and gastrointestinal tract (15).


Figure 11 The pharynx and gastrointestinal system (16)


Tab 3:

It then travels into local lymphatic tissue and the bloodstream, where it may infect Central Nervous System (CNS) cells. In the event that polio infects CNS cells, it can then replicate in motor neurons in the anteriror horn and the brain stem.

The damage caused by this can result in nervous cell and tissue destruction, leading to the symptoms of paralysis in polio victims (17).


Figure 12 Cross-section of spinal cord displaying ventral (anterior) horn (18)

Tab 4. As polio remains present in the throat for approximately one week (19) and sheds itself in the feces of an infected individual, even in asymptomatic cases, disease can rapidly spread in areas with poor hygiene and sanitation. It spreads particularly quickly amongst young children who are not toilet trained (20). Polio can also be spread through an infected person's saliva (19) and there is evidence of passive transmission of the disease by flies (20).



Figure 13 Polio spreads in areas of poor sanitation, such as the community pictured above (21)




Figure 14: The Life Cycle of Poliovirus on a Cellular Level (10)

Video on Polio Transmission (video length: approximately 1:45)

RK: What a nice video from Smithsonian museum on how the poliovirus finds a host and starts the disease process. Looks like they have embedded this as a Flash activity. And they haven't given us access to the html code. Looks like we can't embed this, but we can link out to it. I wish you had given a little description of it so that people would be more apt to click it and view it. It's ok, don't worry. You have lots of good stuff in this module.

For further information, view the CDC Pinkbook chapter on Poliomyelitis:

Can Polio Save Lives?

New Research into the Therapeutic Use of Polio in Cancer

ABC News Segment on Duke University's efforts to treat brain tumors using polio. (1:10)

See More at:

The Symptoms and Progression of Polio Infection


Figure 15: Some of the common muscles affected by polio (22).

The majority of polio-infected individuals are asymptomatic, while about 4-8% will demonstrate mild symptoms. Less than 1% of polio cases result in permanent limb paralysis. Of the individuals who are paralyzed, 5-10% die as a result of paralysis of the respiratory muscles (11).

For people who are symptomatic, but never progress to paralysis, polio symptoms generally last one to ten days. They include: fever, fatigue, vomiting, neck stiffness, headache, and limb pain (12). These symptoms can be mistaken for a cold or flu. Some people can also develop meningitis from a polio infection (23).

For people who develop paralysis, the initial signs and symptoms are the same. However, if the virus affects the spinal cord and/or brainstem, the symptoms specific to paralytic polio appear within a week. These symptoms include: loss of reflexes, severe muscle aches or spasms, and flaccid paralysis, often worse on one side of the body. The onset of paralytic symptoms may be sudden (23).

As the majority of people infected with polio have no symptoms and are unaware of their infection, the virus may spread through a large population before even being recognized. Based upon an epidemiological model representing a population of 200,000 within a developing country, the estimated basic reproductive number of wild polio infection is 12. A single case of polio paralysis is often the first sign of an epidemic (20). However, if enough children in the community have been fully immunized against polio, the virus will be unable to find a susceptible host and will die out through herd immunity (24).

Figure 16: A Group of Children in India Affected by Polio (25)

Figure 17: Historic Polio Therapy (26)


 Clip from "A Fight to the Finish: Stories of Polio," a documentary by Ken Mandel. The clip shows interviews with survivors and physicians who treated children with Polio. (clip length 3:20)


Did You Know?

Post-Polio Syndrome


Polio survivors can experience post-polio syndrome ten to forty years after recovery from polio infection. This syndrome can cause fatigue, progressive muscle weakness and deterioration in muscles originally affected by polio, joint pain and bone deformities. There is no treatment, and the cause is unknown (19)



"Faces of Polio" video from Rotary International.


This video contains interviews with Polio survivors and physicians. There is discussion of Post-Polio Syndrome, its symptoms and the difficulty with diagnosis. (video length: 25:54)

The Polio Vaccine

The polio vaccine provides lifelong immunity and is the only means of polio prevention. There are two types currently available: the oral polio vaccine (OPV) and the inactivated polio vaccine (IPV). The vaccine results in humoral (circulating antibody) and mucosal (secretory immunoglobulin A) immune responses (27).


Oral Polio Vaccine (OPV)


Figure 18 Oral Polio Vaccine being administered to a child (28)

Albert Sabin's OPV is a live vaccine. It is much less expensive than the injectable vaccine and is easy to administer by mouth. OPV provides short-term intestinal immunity to wild polioviruses and long-term humoral immunity against paralytic polio disease (29). OPV given at or soon after birth increases antibody levels and seroconversion rates of subsequent doses (27).

However, while rare, OPV can cause paralytic polio and those who are vaccinated using OPV, as well as their unimmunized contacts, are at risk of developing paralytic polio. The risk is estimated to be 4 cases per 1,000,000 births annually in countries administering OPV, a slim risk though possible (27).

Inactivated Polio Vaccine (IPV)


Figure 19 Children given polio vaccine at Maryknoll School, Los Angeles, California, May 19, 1955 (30)

The IPV, created by Dr. Jonas Salk, is administered by intramuscular or subcutaneous injection. IPV is one of the safest routine vaccines, but it is more expensive and more difficult to administer than OPV. IPV also triggers a lower level of immunity in the intestine than OPV (31).

As polio rates decrease or are eliminated, many countries have added or switched to sole use of IPV due to the risks associated with OPV (29). Once all wild poliovirus is eradicated globally, only IPV will be used (29). 

Vaccine-Derived Polio


Figure 20 Vaccine-derived polioviruses (VDPVs) detected worldwide, July 2009 - March 2011 (29)


Map Key:

cVDPV = circulating VDPV

iVDPV = immunodeficiency-associated VDPV

aVDPV = ambiguous VDPV

AFP = acute flaccid paralysis


Because receipt of the oral polio vaccine results in the shedding of the weakened vaccine-virus in feces, in areas of poor sanitation with populations that are under-immunized, if an excreted vaccine-virus is allowed to circulate for at least one year, it can mutate into circulating vaccine-derived poliovirus (cVDPV), which causes paralysis. 20 outbreaks of cVDPV have been recorded in 20 countries to date (32).

In rare cases, individuals with primary immunodeficiencies have been administered the oral polio vaccine prior to diagnosis of immunodeficiency, and contracted vaccine-derived poliovirus. Studies indicate that they have then shed polio in their stool for up to several years (15). Notably, HIV/AIDS does not seem to be a risk factor of those with immunodeficiencies experiencing this effect (27).

Looking Forward: The Future of Polio

In 1988, the 41st World Health Assembly (WHA) declared a goal to eradicate polio by 2000. Tremendous progress has been made, yet polio persists 13 years after the launch of the Global Polio Eradication Initiative's initial goal. In 2012, with the continuing urgency toward reaching eradication, the WHA declared polio eradication a global public health programmatic emergency. 

Type 2 poliovirus has been eradicated, yet Type 1 persists at high levels and Type 3 at low levels (33). Today, global health leaders continue to fight for progress toward eradicating the final 1%.


Figure 21 Rotary International's Campaign: 'The Fight to End Polio' (34)

Eradication: Why It Matters

As of 2013, the polio eradication effort has prevented paralysis in more than 10 million people. Vaccination campaigns have also promoted other health issues including the systematic administration of Vitamin A, which alone has stopped 1.5 million childhood deaths (35).

 Furthermore, beyond the humanitarian benefits of eradicating a debilitating and preventable disease, the economic benefits are immense. The net benefit of the eradication effort has been estimated to be between 40-50 billion dollars from 1988 through 2035. Developing countries account for approximately 85% of these net benefits, an enormous amount that can go a long way toward other public health goals (36).


Eradication: Where We Stand

The WHO issued 'The Polio Eradication & Endgame Strategic Plan 2013-2018.' This optimism illuminates a world free of polio in the near future (37).


The plan has four primary objectives:

1.       Detect and stop polio worldwide by 2014

2.       Eliminate the use of OPV and implement stronger immunization systems

3.       Implement a certification process stating regions have gone three years without an incident case

4.       Plan polio's legacy on sharing eradication knowledge and expertise


Biology & Technology: Working With Us

As polio can only live in humans, animal vectors and animal reservoirs are not sources for the disease transmission. Therefore, eradication efforts need only to focus on humans. Despite the high level of communicability of polio, the limited incubation period reduces the period for shedding, primarily in the feces, to occur.

Figure 22 Timeline of the Biological Presentation of Poliovirus Infection from an American Medical Textbook published in 1959 (38)

The polio vaccines have been one of the greatest contributors to the possibility of polio eradication. In particular, community health workers without advanced medical training can administer the OPV. This has allowed access to communities hesitant toward outsider intervention and areas difficult to reach due to conflict or other mitigating factors.  

Technology preserving vaccines has also been developed ensuring vaccine effectiveness since high temperatures can compromise performance. Vial monitors indicate if a vaccine's effectiveness has been compromised due to heat. If vaccines become ineffective, vial monitors will change color indicating ineffectiveness (39).


Figure 23 Vaccine vial monitors (39)

When outbreaks occur in regions previously declared polio free, quick response is of the utmost importance. A type 1 polio outbreak occurred in China in 2011. The virus was detected in a 16-month-old girl who had no history of travel outside of Xinjiang.  21 cases were confirmed between July 3 and October, 2011 and the Chinese Ministry of Health issued the highest public health emergency possible. Daily surveillance began and more than 5 million vaccines were delivered to Xinjiang for the first vaccine campaign launched by September 8. In total from August 2011 to April 2012, 43.7 million doses of OPV were administered via five supplementary immunization rounds. The outbreak was stopped 1.5 months after laboratory confirmation demonstrating how China's immediate and intense response was able to prevent a potentially devastating epidemic in the world's most populous country (40).

Biology & Technology: Working Against Us

One of the biggest obstacles is the lack of clinically visible symptoms of individuals infected. The disease can spread silently with at least 200 infected, asymptotic individuals for every one reported paralyzed polio case. This can lead to transmission unknowingly occurring within communities (41).  Professor D.A. Henderson, who headed the international effort to eradicate smallpox, cites this feature as one of the biggest challenges regarding polio eradication. He said during an interview, "There were no [smallpox] patients with subclinical infections. Thus, we could readily identify infected areas and contain the outbreaks" highlighting his skepticism toward polio eradication (42).

 Another obstacle is the highly communicable nature of the disease and cross-border transmission. Polio had not been seen in Somalia since 2007, yet between April 2013 through October 1, 2013, 175 cases were reported illustrating the rapid nature of disease transmission across borders. Fourteen cases were also discovered in Kenya since the last confirmed cases in July 2011 (43). The map below of poliovirus cases in the world illustrates how clusters of countries often emerge.

Figure 24 Districts with cases caused by wild poliovirus (44)


A case study of vaccine efficacy

 Issues with vaccine effectiveness in certain regions of the world have also occurred as demonstrated in Northern India.  The trivalent oral vaccine was unable to efficiently treat the disease despite immunization coverage in children in the area. High prevalence of diarrhea and other diseases required a more potent vaccine.  This outbreak demonstrated the need for proper surveillance for factors potentially impacting vaccination effectiveness. For more on this study and the crucial nature of proper surveillance, please see [link to pdf]


The Human Factor

Other contributing factors hampering polio eradication efforts have been faulty management and oversight practices for mass vaccination campaigns, community mistrust of the safety of vaccines, inaccessible populations due to migration or conflict, and insufficient financing for programs (41).

 Recently Syria has seen cases of polio for the first time in 14 years. Polio is often spread in war-torn communities as health infrastructures, including clean water and adequate sanitation, become compromised. Access to immunization is often threatened as well; immunization rates have dropped to less than 70% compared to 90% in 2010. Vaccination campaigns have been planned however the success of them is unknown at this time. As Siddharth Chatterjee notes discussing the Syrian outbreak, "Principles over politics must be the clarion call when it comes to children and their wellbeing, everywhere" (46).

 Polio has become a top public health priority over the last 20-years and advocates have been successful in garnering global support financially, politically, and socially which have lead to enormous strides in the fight against polio. CDC and WHO initiated the Stop Transmission of Polio (STOP) program in 1999. Since then, it has mobilized 1,563 volunteers in 69 countries. The program has strengthened capacity of immunization program in countries and supported the surveillance of acute flaccid paralysis (47). WHO's financial and technical support have been instrumental in initiating polio as a top public health priority and will be needed for eradicating that final 1%.


Additional Information:

For weekly reports on the status of polio worldwide, please visit:

To learn even more, an engaging talk on the public health successes and challenges in fighting polio [23:09]




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