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What is Polio?

Updated August 15, 2022


Polio (poliomyelitis) is a disease caused by the poliovirus which in rare cases affects the nervous system and can lead to paralysis or death. The term ‘poliomyelitis’ used to describe the effects that the disease has on the body’s spinal cord comes from the Greek words of polio (grey) and myelin (marrow). 

There are three types of poliovirus - type 1, 2, and 3 - and immunity to one type of poliovirus does not provide immunity to the additional types. The poliovirus is an enterovirus that is unique to humans and usually inhabits the gastrointestinal tract. 

Approximately 95 percent of all polio infections are asymptomatic. This means that most people who are exposed to the virus will have no clinical symptoms of illness. Between 4 and 8 percent of individuals exposed to the poliovirus will develop mild symptoms, which often include flu-like illness, respiratory tract infections, and gastroenteritis. Approximately one percent of polio cases present as aseptic meningitis, and symptoms generally include severe back, neck, or leg spasms. Full recovery usually occurs within 10 days.  For non-paralytic polio, the incubation period (time from exposure to the onset of illness) is generally between 3 and 6 days. 

Less than one percent of children exposed to polio will develop paralytic polio, the most severe form of polio. Paralytic polio usually begins within 1 and 18 days of initial infection, and paralysis symptoms usually progress for 2 to 3 days. In some cases, an asymptomatic period of 7 to 10 days may occur between the earliest symptoms of illness and the presentation of more severe symptoms. Severe symptoms can include excruciating muscle pain and spasms of the back or limbs, and the loss of superficial reflexes. Asymmetrical paralysis will follow, and the disease will progress to flaccid paralysis and loss of deep tendon reflexes. This condition will usually persist for days to weeks; however, most people will recover fully. If muscle weakness or paralysis persists for longer than 1 year, the condition will likely be permanent.  

There are 3 types of paralytic polio – spinal, bulbar, and bulbospinal. Spinal paralysis, the most common form and seen in nearly 80 percent of all cases of paralytic polio, is most often recognizable by asymmetric paralysis of the lower limbs. Bulbar paralysis, which occurs in approximately 1-2 percent of cases, involves the cranial nerves and is associated with higher fatality rates. Bulbospinal paralysis, a combination of both bulbar and spinal paralysis, accounts for approximately 19 percent of all cases of paralytic polio. 

Individuals who recover from paralytic polio may be at risk for post-polio syndrome, a condition which begins with a slow progressive new weakening of the muscles previously involved in the initial case of polio. Additional symptoms often include fatigue, muscle atrophy, skeletal deformities, and joint degradation. Symptoms of post-polio syndrome can range from mild, with minor impairments, to severe, where a person’s activities of daily living are significantly impacted. There are no available tests to definitively confirm a diagnosis of post-polio syndrome, and the prevalence and incidence rates of this condition are not known. It is estimated by researchers that this condition may affect between 25 to 40 percent of persons who have recovered from paralytic polio. 

Wild-type polio was declared eradicated in the U.S. in 1979; however, between 1980 and 1998, there were 152 reported cases of paralytic polio in the U.S.  One hundred and forty-four of these cases were confirmed as vaccine-acquired paralytic polio (VAPP), 6 were imported, and 2 cases were unknown.   

VAPP was associated with use of the oral polio vaccine (OPV) immediately after the vaccine’s introduction in the early 1960s.  When OPV was in use in the United States, VAPP was estimated to occur at a rate of one case per 2.4 million doses, or one case per 750,000 doses, if OPV was administered as the first dose. On June 17, 1999, the CDC’s Advisory Committee on Immunization Practices (ACIP) voted to stop the use of OPV in the U.S., by January of 2000.  OPV, however, remains in use globally, and is the primary vaccine used in global campaigns aimed at eradicating polio. 

In recent years, vaccine-derived poliovirus (VDPV) has emerged to cause cases and outbreaks of paralytic polio. OPV is a live attenuated (weakened) virus vaccine and after it is administered, the virus replicates in the intestine and is excreted in the stool. This excreted live vaccine virus has the potential to circulate in the environment, undergo genetic changes, and cause paralytic polio. VDPVs are classified into 3 categories:    

  • Circulating VDPV (cVDPV) – this occurs in communities where individuals remain susceptible to either wild-type polio or VDPV because of inadequately administrated polio vaccination campaigns or due to a lack of immunity to polio;
  • Immunodeficiency (iVDPV) – these strains are found in individuals with primary immunodeficiency (PID)
  • Ambiguous VDPV (aVDPV) – these are strains isolated from individuals who are not immunodeficient or else found in sewage where the source is not known.

Type 2 wild-type polio was declared eradicated worldwide in 2015   and on October 24, 2019, the Global Polio Eradication Initiative (GPEI) announced the global eradication of type 3 wild-type polio. 

In September 2015, following the announcement that type 2 wild-type polio had been eradicated globally, public health officials moved quickly to stop use of the trivalent OPV (vaccine containing vaccine-strain poliovirus types 1, 2, and 3) and replaced it with a bivalent OPV containing only vaccine-strain poliovirus types 1 and 3.  This initiative was implemented to stop the spread of type 2 VDPV (cVDPV2), which had caused multiple polio outbreaks in several countries.  Since 2000, there have been 1,085 cases of paralytic polio associated with cVDPV and most cases  - 932 (86 percent) – have been caused by cVDPV2. 

Despite removing type 2 polio from OPV in the spring of 2016,  cVDPV2 associated paralytic polio cases have continued to occur. In 2019, cVDPV2 outbreaks were reported in the Philippines, and several African countries.  This has prompted health officials to reintroduce a type 2 OPV and since April 2016, approximately 300 million doses have been administered in regions experiencing outbreaks of cVDPV2. Problems associated with the reintroduction of the live type 2 OPV have included the risk that its use will potentially lead to additional cases and outbreaks of cVDPV2.  

Type 1 wild-type polio is the only poliovirus strain currently circulating and in 2018, there were 33 cases detected globally – 12 in Pakistan and 21 in Afghanistan.  However, in 2018, there were 6,732 cases of VAPP associated with the use of the Sabin OPV and 104 cases of paralysis from VDPV. These cases were detected after testing was completed on the stool specimens of 190,055 persons diagnosed with acute flaccid paralysis (AFP). 

AFP or acute flaccid myelitis (AFM) is often referred to as a “polio-like” illness. This neurological disease affects the grey matter of the spinal cord and causes weakness in the reflexes and muscles. Symptoms of AFM include sudden loss of reflexes and muscle tone and extremity weakness. Additional symptoms may include swallowing impairments, slurred speech, eyelid and facial drooping, and eye movement disorder. AFM can affect the respiratory muscles and when this occurs, mechanical ventilation is required. There is no specific treatment for AFM and long-term outcomes for individuals affected by this disease are not known at this time. 

The exact cause of AFM is currently unknown, but researchers are focusing on the theory that a virus is responsible for the disease. AFM is most commonly seen in children, but public health officials believe that this disease can affect persons of any age. 

The CDC began tracking AFM in the U.S. in August 2014 and as of January 2020, 603 confirmed cases have been reported.  In the U.S., cases of AFM appear to occur more frequently between August and October, every 2 years. 

On October 3rd, 2019, the World Health Organization (WHO) reported a significant increase in wild-type poliovirus type 1 (WPV1), from 33 cases in 2018, to 73 cases. Multiple cVDPV2 outbreaks in Africa were also reported by health officials and noted to be at the highest levels in history. Additionally, cVDPV2 had also been detected in the Philippines and China and had spread undetected for approximately a year in China and even longer in the Philippines.  

IMPORTANT NOTE: NVIC encourages you to become fully informed about Polio and the Polio vaccine by reading all sections in the Table of Contents, which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child. This information is for educational purposes only and is not intended as medical advice.


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