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Public Health

Vetted resources and information on current public health events.

What is Polio?

Polio, or poliomyelitis, is a disabling and life-threatening disease caused by the poliovirus. It is extremely contagious and spreads from person-to-person contact.

Approximately 75% of people infected with the poliovirus are asymptomatic, or develop no visible symptoms, and the other 25% can experience flu-like symptoms. However, a small proportion of infected individuals (approximately 0.1%) develop more serious symptoms that affect the brain and spinal cord, including parasthesia (feeling of pins and needles), meningitis (infection of the spinal cord), and paralysis (can't move parts of the body). Paralysis can lead to permanent life-long disability and death.

Types of Polioviruses

WPV - Wild Poliovirus

The wild variant of polio, a highly infectious viral disease that can invade the nervous system to cause paralysis. Wild polio is endemic in just two countries – Pakistan and Afghanistan – though a case linked to Pakistan was also detected in Malawi in February 2022.

There are three types of wild poliovirus variants, Type 1 (WPV1), Type 2 (WPV2), and Type 3 (WPV3).

Wild Poliovirus Type 1 (WPV1) is the only remaining wild type poliovirus circulating globally. Type 2 was eradicated in 2015 and Type 3 was eradicated in 2019.

VDPV - Vaccine Derived Poliovirus

VDPVs are Oral Polio Vaccine (OPV) virus strains that are >1% divergent (types 1 and 3) or >0.6% divergent (type 2) from the corresponding OPV strain in the VP1 genomic region.

Also referred to as Sabin Type 2 virus or Sabin 2 strain

cVDPV - Circulating Vaccine Derived Poliovirus

Vaccine derived poliovirus isolates for which there is evidence of person-to-person transmission in the community. cVDPVs can emerge if the weakened live virus contained in oral polio vaccine, shed by vaccinated children, is allowed to circulate in under-immunized populations for long enough to genetically revert to a version that causes paralysis.

cVDPV2 - Circulating Vaccine Derived Poliovirus Type 2

Also referred to as Sabin-like type 2 virus (as it is a mutated form of the original Sabin type 2 OPV)

  • 90% of cVDPV outbreaks are type 2
  • Wild form of type 2 no longer circulates and was removed from the tOPV vaccine
  • The switch from tOPV to bOPV has lead to an increase in cVDPV2 outbreaks

History of Polio

Polio was once one of the most feared diseases in the U.S. In the early 1950s, before polio vaccines were available, polio outbreaks caused more than 15,000 cases of paralysis each year.

Following introduction of vaccines—specifically, the trivalent inactivated poliovirus vaccine (IPV) in 1955 and the trivalent oral poliovirus vaccine (OPV) in 1963—the number of polio cases fell rapidly to less than 100 in the 1960s and fewer than 10 in the 1970s.

First Polio Vaccines: 1950s

In 1935 Maurice Brodie had attempted to create a formalin inactivated "vaccine" by exposing the virus to formaldehyde. After initial attempts in 20 primates and 3,000 children, the results were poor and his vaccine was never used again.

In 1955, Jonas Salk developed an inactivated poliovirus vaccine, thus beginning widespread immunization.

In 1960, Albert Sabin developed a live attenuated oral vaccine. In 1955 there were 28,000 reported polio cases. Remarkably, one year later the number of cases dropped to 15,000.


The Pan American Health Organization (PAHO), which serves as the regional office of the World Health Organization for the Americas, announced a campaign to achieve polio elimination in the Americas by 1990. Its original goal of 1990 would not be met, but the last case of wild-type paralytic polio reported in the Americas, a three-year-old Peruvian boy, Luis Fermín, was in 1991, and the region of the Americas was certified polio free in 1994.


Fourteen years after the launch of the global eradication program, the World Health Organization declared polio eliminated in Europe on June 6, 2002. The last case of wild polio in Europe occurred in a young boy. Melik Minas, who lived in Turkey, contracted polio in November 1998. Minas, who had not been vaccinated, was paralyzed as a result of the infection—although he did partially recover.

Southeast Asia:

The World Health Organization certified the South-East Asia Region polio-free on March 27, 2014. The region includes Bangladesh, Bhutan, Democratic People’s Republic of Korea, India, Indonesia, Maldives, Myanmar, Nepal, Sri Lanka, Thailand, and Timor-Leste. In 2011, India was the last of these countries to report a case of disease from wild poliovirus.

WHO Declares Polio a Public Health Emergency of International Concern

On 5 May 2014 the WHO Director-General declared the international spread of poliovirus in 2014 a Public Health Emergency of International Concern (PHEIC) under the International Health Regulations (IHR 2005), issued Temporary Recommendations to reduce the international spread of poliovirus, and requested a reassessment of this situation by the Emergency Committee every three months. The 33rd meeting of the Emergency Committee was held in October 2022.

Based on the committee’s advice and the reports made by affected States Parties, the Director-General extended the following Temporary Recommendations under the IHR (2005), effective October 2022.

How it Spreads

Poliovirus is only found in humans. It enters the body through the mouth and lives in a person's throat and intestines.

Poliovirus spreads through:

  • Contact with the feces (poop) of an infected person
  • Droplets from a sneeze or cough of an infected person (less common)

You can get infected with poliovirus if:

  • You have picked-up minute pieces of feces on your hands, and you touch your mouth.
  • You put in your mouth objects like toys that are contaminated with feces.

An infected person may spread the virus to others immediately before and up to 2 weeks after symptoms appear.

  • The virus can live in an infected person’s feces for many weeks. It can contaminate food and water in unsanitary conditions.
  • People who don’t have symptoms can still pass the virus to others and make them sick.

Polio Vaccination

Poliovirus Vaccines

There are two types of poliovirus vaccines administered globally.

  • Inactivated poliovirus vaccine (IPV) given as an injection in the leg or arm, depending on the patient’s age. Only IPV has been used in the United States since 2000
  • Oral poliovirus vaccine (OPV) is still used throughout much of the world.

The first polio vaccine was available in the United States in 1955. Since 2000, the inactivated polio vaccine (IPV) is the only polio vaccine that is given in the United States. It is given by shot in the arm or leg, depending on the person’s age. The oral polio vaccine (OPV) is still used in other countries.

The polio vaccine protects children by preparing their bodies to fight the poliovirus. Almost all children (99 children to 100 out of 100) who get all the recommended doses of the inactivated polio vaccine will be protected from polio.

Poliovirus Vaccine Schedule
  • 2 months old
  • 4 months old
  • 6 through 18 months old
  • 4 through 6 years old

Oral Poliovirus Vaccine (Sabin)

A safe and effective vaccine that contains a combination of one, two or three strains of live, weakened poliovirus and is easily delivered via two oral droplets. Discovered by Sabin in 1961. Provides gut immunity, required to stop transmission. There have been four different formulations of the oral poliovirus vaccine (OPV).


mOPV - Monovalent Oral Polio Vaccine
  • An oral polio vaccine that provides protection against type 2 polio virus, one of the three polio strains.
bOPV - Bivalent Oral Polio Vaccine
  • An oral polio vaccine that provides protection against type type 1 and type 3 polio virus
tOPV - Trivalent Oral Polio Vaccine*
  • An oral polio vaccine that provides protection against all 3 types of polio virus. Following the certification in 2015 of WPV2 eradication, the type 2 component in tOPV was removed since 90% of cVDPV outbreaks derived from the type 2 component of tOPV.
nOPV2 - Novel Oral Polio Vaccine Type 2
  • A next generation version of mOPV2 that clinical trials have shown is more genetically stable and could help stop cVDPV2 outbreaks in a more sustainable way.

Inactivated Polio Vaccine (Salk)

A safe and effective vaccine that contains inactivated versions of all three poliovirus strains. It protects against paralysis and is delivered via an injection by a trained health worker. Discovered by Salk in 1955. Does NOT provider gut immunity per se, but boosts it in OPV primed populations.

The Future of Poilo Vaccination

The IPV does not stop transmission of the virus.

Thus the OPV must be used when there is an poliovirus outbreak that needs to be contained, even in countries (like the US), that rely exclusively on the IPV for routine immunization programs.

Once polio has been eradicated, use of all OPV will need to be stopped to prevent re-establishment of transmission due to VDPVs. Countries that use bOPV have added a single dose of IPV to protect against WPV2.

Global Polio Eradication Initiative (GPEI)

The goal of the Global Polio Eradication Initiative is to eradicate polio worldwide. The GPEI is a public-private partnership led by national governments with six core partners:

  • World Health Organization (WHO)
  • Rotary International
  • US Centers for Disease Control and Prevention (CDC)
  • United Nations Children’s Fund (UNICEF)
  • Bill & Melinda Gates Foundation
  • Gavi, the Vaccine Alliance.

The Global Polio Eradication Initiative (GPEI) is financed by a wide range of public and private donors, who help meet the costs of the Initiative’s eradication activities. The requirements for 2019-2023 are projected to be approximately US$ 4.2 billion.

Polio Today

To eradicate polio, the Global Polio Eradication Initiative brings together the latest scientific knowledge on the virus and tracks the status of the virus every week. We look back on the history of the poliovirus to make the most of lessons learned and plan ahead for a world without polio.

Polio Now

Global WPV1 and cVDPV Cases in the past 12 months (map).

cVDPV Outbreaks

A vaccine-derived poliovirus (VDPV) is a strain of the weakened poliovirus that was initially included in oral polio vaccine (OPV) and that has changed over time and behaves more like the wild or naturally occurring virus. This means it can be spread more easily to people who are unvaccinated against polio and who come in contact with the stool or respiratory secretions, such as from a sneeze, of an infected person. These viruses may cause illness, including paralysis.

For this reason, the global eradication of polio requires stopping all OPV in routine immunization, as soon as possible after the eradication of wild poliovirus (WPV) transmission.  To protect against all three types of WPV, the United States exclusively has used IPV since 2000. For more information on OPV cessation, please visit the Global Polio Eradication Initiative’s websiteexternal icon.

If a population is seriously under-immunized, there are enough susceptible children for the excreted vaccine-derived polioviruses to begin circulating in the community. If the vaccine-virus is able to circulate for a prolonged period of time uninterrupted, it can mutate and, over the course of 12-18 months, reacquire neurovirulence. These viruses are called circulating vaccine-derived polioviruses (cVDPV).

The lower the population immunity, the longer these viruses survive. The longer they survive, the more they replicate, change, and exchange genetic material with other enteroviruses as they spread through a community.

If a population is fully immunized against polio, it will be protected against the spread of both wild and vaccine strains of poliovirus.

Episodes of circulating vaccine-derived poliovirus are rare. Over the past ten years – a period during which more than 10 billion doses of oral polio vaccine were given worldwide – cVDPV outbreaks resulted in fewer than 800 cases cases.  In the same period, in the absence of vaccination with OPV, more than 6.5 million children would have been paralysed by wild poliovirus.