Disease Risk – Polio

 

Risk to a child from Polio if not vaccinated until after the age of 5:

Polio is an infectious disease caused by a virus that infects the throat and intestinal tract. It is most often spread through person-to-person contact with the stool of an infected person, and may also be spread through oral/nasal secretions. The polio vaccine was introduced in 1955 [3].

Wild-type polio has not been eradicated in four countries:  Nigeria, Pakistan, Afghanistan, and India (most of India is polio-free; persistent transmission of poliovirus is localized in areas of western Uttar Pradesh and central Bihar), with a further four countries known to have (Angola, Chad and Democratic Republic of the Congo) or suspected of having (Sudan) re-established transmission of poliovirus [1].

Rate of Long Term Sequelae or Injury: Up to 95% of all polio infections are asymp­tomatic. Estimates of the ratio of asymptomatic to paralytic illness vary from 50:1 to 1,000:1 (usually 200:1, or 0.5%). Many persons with paralytic poliomyelitis recover completely and, in most, muscle function returns to some degree. Weakness or paralysis still present 12 months after onset is usually permanent.[2]

Incidence Rates:  In the immediate pre-vaccine era, improved sanitation allowed less frequent exposure and increased the age of primary infection. Boosting of immunity from natural exposure became more infrequent and the number of susceptible persons accumulated, ultimately resulting in the occurrence of epidemics, with 13,000 to 20,000 para­lytic cases reported annually [2].  For the purposes of this analysis, the incidence rate of paralytic polio in the pre-vaccine era was calculated to be a midpoint of 16,500 annual cases divided by the average population in the years 1950 – 1955 [4], which equates to 1.04 per 10,000 or 1 in 9,586.

Polio was eradicated from the United State in 1979, and from the Western hemisphere in 1991.  From 1980 through 1999, there were 152 confirmed cases of paralytic polio reported in the USA.  Of the 152 cases, 6 cases were imported from outside the USA, 2 were indeterminate, and the remaining 144 cases were vaccine-induced polio from the Oral Polio Vaccine (OPV).  OPV was replaced in the USA by the Inactivated Polio Vaccine (IPV) in 2000.  In 2009, only 1,579 confirmed cases of polio were reported globally and polio was endemic in four countries.[2]  Vaccine-induced polio is no longer a risk in the USA since the OPV has not been used there since 2000.  For the purposes of this analysis, the incidence rate of paralytic polio in the post-vaccine era was calculated to be the average cases per year in 1980 – 1999 (8/20) divided by the average USA population in that time period [4], resulting in a 0.000016 per 10,000 or 1 in 623,000,000.

Case Fatality Rate: The death-to-case ratio for paralytic polio is generally 2%–5% among children. [2].

Incremental Risk in Population with Low Rates of Vaccination (if a child does not vaccinate by age 5):

Assuming that polio would travel from one of the four endemic countries to the USA and reach similar incidence levels as in the pre-vaccine era, the cumulative risk of paralytic polio in the first five years of life to an unvaccinated child was calculated to be 5.2 per 10,000 or 1 in 1,918.  The cumulative risk of death was calculated to be 0.18 per 10,000 or 1 in 54,776.

Incremental Risk in Highly Vaccinated Population (if a child does not vaccinate by age 5):  

The cumulative risk of paralytic polio in the first five years of life to an unvaccinated child was calculated to be 0.00008 per 10,000 or 1 in 125 million.  The cumulative risk of death was calculated to be 0.000003 per 10,000 or 1 in 3.6 billion.

 

 

References:

[1] Global Polio Eradication Initiative.  http://www.cdc.gov/vaccines/vpd-vac/polio/dis-faqs.htm

[2] Centers for Disease Control and Prevention: Epidemiology and Prevention of Vaccine-Preventable Disease. Atkinson W, Wolfe S, Hamborsky J, McIntyre L. eds. 11th edition. Washington D.C.: Public Health Foundation, 2009.

[3] Centers for Disease Control and Prevention: Polio Disease – Questions and Answers.  http://www.cdc.gov/vaccines/vpd-vac/polio/dis-faqs.htm

[4] http://www.census.gov/popest/archives/1990s/popclockest.txt