Infectious Diseases

Polio Resurgence: Uncovering the Reasons Behind the Recent Global Outbreaks

Adan Amer—McMaster Arts & Science 2021

Despite the tremendous medical and technological advancements achieved in modern human history, only one disease has been officially eradicated from the world – smallpox (1). Nevertheless, another life-threatening disease came painstakingly close to being eliminated. Polio, or poliomyelitis, is a disease caused by the wildtype poliovirus (colloquially referred to as WPV) (2). In this case, wild type refers to the naturally occurring virus – a distinction that becomes important later. WPV is commonly transmitted through contact with contaminated feces and infection commonly occurs in children under the age of 5 (3,4). Approximately 75% of people infected with the poliovirus show no symptoms, while a little less than 25% experience mild symptoms like fatigue, fever, muscle pain, and stiffness (1,3,5). In the most severe cases, 1 in 200 become paralyzed while 2-10% of those paralyzed by polio die because it immobilizes the muscles needed for breathing (5,6).

SOURCE: Shutterstock

Across 125 endemic countries, the disease had paralyzed 350,000 children annually up until 1988 when the World Health Assembly launched the Global Polio Eradication Initiative (GPEI) (5). This vaccination program was on a mission to globally eradicate all cases of polio by raising funds to implement national immunization programs in nations that could not stop the transmission of the virus on their own (7). Two different types of vaccines were used. First, an injectable vaccine that contained the inactive form of the poliovirus (abbreviated to IPV), and an oral vaccine with a live, but weakened, form of the virus (OPV). The OPV was pivotal for preventing transmission in endemic areas with unsanitary conditions because it causes the virus to replicate in the gut and build immunity in the intestines. This then prevents it from being shed through feces into the environment (1). It’s also cheap to make and easy to administer on a community-wide level since there are no injections necessary (2). On the other hand, IPV, which builds immunity in the bloodstream and prevents the virus from causing paralysis, is useful in extremely sanitary areas like North America because children there have lower immunity to the virus (1).

SOURCE: Sebastian Meyer/WHO Iraq

For the most part, the GPEI had succeeded. WPV cases declined by over 99% since 1988, with only 175 cases reported in 2019 (5). The WHO had certified the global eradication of 2 out of the 3 wildtype strains in 2015 and 2019 (8).

So, how come polio hasn’t joined smallpox on the list of fully eradicated diseases? Well, there has been a recent and concerning phenomenon where the virus in the OPV is being transmitted from a vaccinated host to a susceptible person with no immunity to polio (6). This is a rare occurrence, about 1 in 2.5 million cases, yet it was common enough to spark outbreaks of vaccine-derived poliovirus (VDPV) in about 33 countries as of 2020 (2). More recently, wastewater studies in London and throughout the state of New York showed circulating cases of VDPV in July 2022 (5).

The problem here is two-pronged: there is not enough immunization coverage in these areas, and the vaccines being administered don’t prevent outbreaks. The WHO recommends a vaccination coverage of more than 95% against polio, yet they estimate that the coverage in the UK and US were 93% and 92% respectively (5). Some of the VDPV outbreaks occurred in areas with inadequate coverage. Unvaccinated people are at risk of having the weakened virus replicate in their gut and undergo mutations that revert it back to a harmful form (2). Second, IPVs are primarily used in these regions over OPVs after the latter was withdrawn from routine immunizations in 2016 (8). IPVs can prevent the virus from becoming paralytic in the host, but they do not prevent its transmission to those who are more susceptible (1).

Currently, researchers are at odds with each other on how to solve this health crisis. Some suggest reintroducing OPVs to all regions to increase coverage and prevent transmission (1), while others want to discontinue its use and shift towards the more resource-intensive IPV to mitigate additional VDPV infections (2,4). One silver lining is that by arriving on the cusp of another contentious vaccination topic, we’re primed to make advancements in these difficult, yet crucial conversations.


1. Kimball S. How polio came back to New York for the first time in decades, silently spread and left a patient paralyzed [Internet]. CNBC. [cited 2022 Nov 26]. Available from:

2. Sharfstein J. Polio in the U.S. | Bloomberg School of Public Health [Internet]. Johns Hopkins. 2022 [cited 2022 Nov 27]. Available from:

3. CDC. What is Polio? [Internet]. Centers for Disease Control and Prevention. 2022 [cited 2022 Nov 27]. Available from:

4. Duintjer Tebbens RJ, Thompson KM. Polio endgame risks and the possibility of restarting the use of oral poliovirus vaccine. Expert Rev Vaccines. 2018 Aug 9;17(8):739–51.

5. WHO. Detection of circulating vaccine derived polio virus 2 (cVDPV2) in environmental samples– the United Kingdom of Great Britain and Northern Ireland and the United States of America [Internet]. World Health Organization. 2022 [cited 2022 Nov 25]. Available from:

6. PAHO. The history of Polio – from eradication to re-emergence [Internet]. Pan American Health Organization. [cited 2022 Nov 27]. Available from:

7. Thompson KM, Kalkowska DA. Reflections on Modeling Poliovirus Transmission and the Polio Eradication Endgame. Risk Anal. 2021 Feb;41(2):229–47.

8. WHO. Standard operating procedures: responding to a poliovirus event or outbreak, version 4 [Internet]. World Health Organization. [cited 2022 Nov 26]. Available from:

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