No, Not Everyone Infected with Polio Develops Paralysis
Most poliovirus infections are asymptomatic, and paralytic polio occurs in only approximately 0.1-1% of all poliovirus infections, with a ratio of inapparent infection to paralytic disease ranging from 100:1 to 1,000:1. 1
Disease Manifestation Spectrum
The vast majority of people infected with poliovirus will never develop paralysis:
- Approximately 95% of poliovirus infections are completely asymptomatic, meaning infected individuals have no symptoms at all 2
- When symptoms do occur, they typically manifest in phases, starting with nonspecific febrile illness (fever and general malaise) 1
- Only a small fraction progress to aseptic meningitis or paralytic disease 1
Paralytic Polio: The Rare Outcome
The development of paralysis is the exception, not the rule:
- Less than 1% of poliovirus infections result in acute flaccid paralysis 3
- The CDC reports that paralytic polio occurs in approximately 0.1-1% of all infections 1
- This means that for every 100 to 1,000 people infected with poliovirus, only 1 person will develop paralysis 1
Clinical Progression When Paralysis Occurs
When the rare paralytic form does develop, it follows a characteristic pattern:
- Rapid progression to maximum paralysis occurs within 2-4 days 1
- Fever and muscle pain accompany the progression 1
- Asymmetric spinal paralysis is more severe proximally than distally 1
- Absent or diminished deep tendon reflexes are characteristic findings 1
- Bulbar paralysis can affect respiration and swallowing in severe cases 1
Important Clinical Caveat
The risk of paralysis is not uniform across all infected individuals:
- In historical epidemics, the highest incidence among children 0-2 years was 2%, while among those over 10 years it reached 25% 4
- Certain populations are more susceptible, including hypogammaglobulinemics, some neonates, and pregnant women 4
- Intramuscular injections and strenuous exercise during the viremic phase can increase the risk or severity of paralysis 4