Polio epidemiology and demographics

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]; Tarek Nafee, M.D. [3]

Overview

Following the poliomyelitis eradication efforts, started by the GPEI (Global Polio Eradication Initiative) in 1988, the number of reported cases has fallen by over 99%, to the point that in 2015 only 74 cases were reported. In 2016 only 3 countries remain polio-endemic: Nigeria, Pakistan, and Afghanistan. The incidence of poliomyelitis is higher in children under 5 years of age, however, the death-to-case ratio for paralytic poliomyelitis is higher among adults. Incidence and prevalence of poliomyelitis do not vary by gender. The last case of wild-virus poliomyelitis acquired in the United States was in 1979. Of the 3 strains of wild poliovirus worldwide, wild poliovirus type 2 was eradicated in 1999, and case numbers of wild poliovirus type 3 are down to the all-time lowest levels. The last reported case of type 3 was in Nigeria in November 2012.

Prevalence

• In the immediate prevaccine era, improved sanitation led to less frequent exposure and increased the age of primary infection. Boost of immunity from natural exposure became less frequent, and the number of susceptible persons accumulated, ultimately resulting in the occurrence of epidemics, with 13,000 to 20,000 paralytic cases reported annually.

• Since the GPEI, launched in 1988, the number of cases has fallen by over 99%.

• In 2016, only 3 countries in the world remain polio-endemic: Nigeria, Pakistan, and Afghanistan.

Polio virus progress 1988 - 2012 Data from The World Health Organization (WHO)/ Global Polio Eradication Initiative

2017 Syria outbreak

• On Thursday June 8th, WHO announced a vaccine related polio outbreak in Syria.
• In Deir El-zor governorate, 2 children were paralyzed by a virus strain related to the used vaccine and the virus was also isolated from other child who was not paralyzed.
• Syria witnessed an outbreak in 2013 and 2014  but this outbreak was related to wild poliovirus not the vaccine strains.
• Emergency vaccination of 200,000 children is planned in an effort to contain the outbreak.

Incidence

• Poliomyelitis cases have decreased by over 99% since 1988, from an estimated 350 000 cases in more than 125 endemic countries then, to 406 reported cases in 2013. These included only 160 cases in endemic countries; international spread of the virus from endemic areas, into poliomyelitis-free areas accounted for the remainder.

• In 2015, only 74 cases were reported worldwide.

• The year-to-date incidence of polio in 2016 shows a 59.5% reduction from 2015. The precise figures are demonstrated in the table below:

Number of Cases of Polio Worldwide, by Type (2015-2016)

Year	Wild Polio Virus (WPV)	Circulating Vaccine Developed Polio Virus (cVDPV)
2015	51	23
2016	27	3
Data from WHO Global Polio Eradication Initiative

Age

• Poliomyelitis affects mainly children under 5 years of age . However, the death-to-case ratio for paralytic poliomyelitis is generally 2-5% among children and up to 15-30% for adults (depending on age).

Gender

• The prevalence and incidence of poliomyelitis do not vary by gender.
• Female gender carries a higher risk of developing "postpolio syndrome".

Developing Countries

• In 2016, only parts of 3 countries in the world remain endemic for the disease - the smallest geographic area in history. Of the 3 strains of wild poliovirus (type 1, type 2, and type 3), wild poliovirus type 2 was eradicated in 1999, and case numbers of wild poliovirus type 3 are down to the lowest-ever levels with the last case reported in November 2012 from Nigeria.
• Polio remains endemic in three countries – Afghanistan, Nigeria and Pakistan.
• As of 2016, the following countries are considered outbreak regions (those affected by either re-infection due to an imported poliovirus, or the emergence of a circulating vaccine-derived poliovirus):
  • Guinea
  • Lao People's Democratic Republic,
  • Madagascar
  • Myanmar

Developed Countries

• Poliomyelitis reached a peak in the United States in 1952, with more than 21,000 paralytic cases. However, following introduction of effective vaccines, poliomyelitis incidence declined rapidly. A poliomyelitis eradication program conducted by the Pan American Health Organization led to elimination of polio in the Western Hemisphere in 1991.
• The Global Polio Eradication Program has dramatically reduced poliovirus transmission throughout the world. The last case of wild-virus poliomyelitis acquired in the United States was in 1979, and global polio eradication may be achieved within the next decade.
• Since 1991, when the last wild-virus-associated indigenous case was reported from Peru, no additional cases of poliomyelitis have been confirmed despite intensive surveillance. In September 1994, an international commission certified the Western Hemisphere to be free of indigenous wild poliovirus.
• From 1980 through 1999, a total of 152 confirmed cases of paralytic poliomyelitis were reported, an average of 8 cases per year. Six cases were acquired outside the United States and imported. The last imported case was reported in 1993. Of the total, 144 (95%) cases were vaccine-associated paralytic polio (VAPP) caused by live oral polio vaccine.
• In order to eliminate VAPP from the United States, ACIP recommended in 2000 that IPV to be used exclusively in the United States. The last case of VAPP acquired in the United States was reported in 1999. In 2005, an unvaccinated U.S. resident was infected with polio vaccine virus in Costa Rica and subsequently developed VAPP. Also in 2005, several asymptomatic infections with a vaccine-derived poliovirus were detected in unvaccinated children in Minnesota. The source of the vaccine virus has not been determined, but it appeared to have been circulating among humans for at least 2 years based on genetic changes in the virus.