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According to poliovirus researchers, approximately 95 percent of all polio cases 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 1 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.
The CDC reports that 72 percent of poliovirus infections among children occur asymptomatically and that 24 percent of children infected with poliovirus will develop a non-specific illness with symptoms that frequently include sore throat and low-grade fever. This non-specific illness is referred to as abortive poliomyelitis and when this occurs, no laboratory or clinical findings of central nervous system impairments are noted.
Less than 1 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 with the virus, 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 symptom 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.
Between 2 and 5 percent of paralytic polio cases in children are fatal, while the death rate among adults ranges from 15 to 30 percent.
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, and muscle atrophy, skeletal deformities, and joint degradation. Symptoms of post-polio syndrome can range from mild, with impairments, to severe, impacting a person’s activities of daily living. 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 may affect between 25 to 40 percent of persons who have recovered from paralytic polio.
The live oral poliovirus vaccine (OPV) can also cause paralytic polio. When this occurs, it is referred to as vaccine-acquired paralytic poliovirus (VAPP). VAPP was acknowledged immediately following the introduction of the live virus OPV and is currently responsible for most cases of paralytic polio globally.
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 through the stool. This excreted live vaccine virus can circulate in the environment, undergo genetic changes, and cause paralytic polio. VDPVs have been classified into 3 separate categories:
- Circulating VDPV (cVDPV) – this occurs in communities where individuals remain susceptible to either wild-type 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.
Paralysis caused by wild-type poliovirus, OPV, and VDPV cannot be distinguished through clinical symptoms.
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.