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== Epidemiology and Demographics ==
==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.


===Occurrence===
==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.<ref name=CDC>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/polio.html#epi }}</ref>


At one time poliovirus infection occurred throughout the world. Transmission of wild poliovirus was interrupted in the United States in 1979, or possibly earlier. A polio 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. In 2005, only 1,948 confirmed cases of polio were reported globally and polio was endemic in six countries.
* Since the GPEI, launched in 1988, the number of cases has fallen by over 99%.  


Poliovirus infection typically peaks in the summer months in temperate climates. There is no seasonal pattern in tropical climates.
* '''In 2016, only 3 countries in the world remain polio-endemic: Nigeria, Pakistan, and Afghanistan.'''<ref name=WHO2>{{cite web | title = Poliomyelitis | url = http://www.who.int/mediacentre/factsheets/fs114/en/ }}</ref>


===Transmission===
[[image:Polioprogress.png|800px|thumb|center|Polio virus progress 1988 - 2012 <SMALL><SMALL>''[http://www.polioeradication.org/Dataandmonitoring.aspx  Data from The World Health Organization (WHO)/ Global Polio Eradication Initiative]''<ref name="WHO">{{Cite web | title = World Health Organization (WHO)| url = http://www.polioeradication.org/Dataandmonitoring.aspx}}</ref></SMALL></SMALL>]]


Person-to-person spread of poliovirus via the fecal-oral route is the most important route of transmission, although the
==2017 Syria outbreak==
oral-oral route may account for some cases.
*On Thursday June 8th, WHO announced a vaccine related polio outbreak in Syria.<ref name="urlPolio outbreak is reported in Syria, health officials say">{{cite web |url=https://www.statnews.com/2017/06/08/polio-outbreak-syria-who/ |title=Polio outbreak is reported in Syria, health officials say |format= |work= |accessdate=}}</ref>
*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.


Poliomyelitis is highly contagious and spreads easily from human-to-human contact.<ref name=Kew_2005>{{cite journal |author=Kew O, Sutter R, de Gourville E, Dowdle W, Pallansch M |title=Vaccine-derived polioviruses and the endgame strategy for global polio eradication |journal=Annu Rev Microbiol |volume=59 |issue= |pages=587–635 |year=2005 |pmid=16153180}}</ref> In endemic areas, wild polioviruses can infect virtually the entire human population.<ref name=McGraw>{{cite book |author = Parker SP (ed.) | title = McGraw-Hill Concise Encyclopedia of Science & Technology |publisher=McGraw-Hill |location=New York |year=1998 | isbn=0-07-052659-1| page= 67}}</ref> It is seasonal in temperate climates, with peak transmission occurring in summer and autumn.<ref name=Kew_2005 /> These seasonal differences are far less pronounced in tropical areas.<ref name=McGraw /> The time between first exposure and first symptoms, known as the [[incubation period]], is usually 6 to 20&nbsp;days, with a maximum range of 3 to 35&nbsp;days.<ref name=Racaniello>{{cite journal |author=Racaniello V |title=One hundred years of poliovirus pathogenesis |journal=[[Virology (journal)|Virology]] |volume=344 |issue=1 |pages=9–16 |year=2006 |pmid = 16364730}}</ref> Virus particles are excreted in the [[feces]] for several weeks following initial infection.<ref name=Racaniello /> The disease is [[Transmission (medicine)|transmitted]] primarily via the [[fecal-oral route]], by ingesting contaminated food or water. It is occasionally transmitted via the oral-oral route,<ref name= Ohri>{{cite journal |last= Ohri |first=Linda K. |coauthors= Jonathan G. Marquess |year=1999 |title= Polio: Will We Soon Vanquish an Old Enemy? |journal= Drug Benefit Trends |volume= 11 |issue= 6|pages=41–54 |id= |url=http://www.medscape.com/viewarticle/416890 |accessdate= 2007-11-06 }} (Available free on [[Medscape]]; registration required.)</ref> a mode especially visible in areas with good sanitation and hygiene.<ref name=Kew_2005 /> Polio is most infectious between 7–10 days before and 7–10 days after the appearance of symptoms, but transmission is possible as long as the virus remains in the saliva or feces.<ref name= Ohri/>
==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.  


Factors that increase the risk of polio infection or affect the severity of the disease include [[immune deficiency]],<ref>{{cite journal |author=Davis L, Bodian D, Price D, Butler I, Vickers J |title=Chronic progressive poliomyelitis secondary to vaccination of an immunodeficient child |journal=[[New England Journal of Medicine|N Engl J Med]] |volume=297 |issue=5 |pages=241–5 |year=1977 |pmid = 195206}}</ref> [[malnutrition]],<ref>{{cite journal |author=Chandra R |title=Reduced secretory antibody response to live attenuated measles and poliovirus vaccines in malnourished children| url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=1131622|journal=[[British Medical Journal|Br Med J]] |volume=2 |issue=5971 |pages=583–5 |year=1975 |pmid=1131622}}</ref> [[tonsillectomy]],<ref>{{cite journal |author=Miller A |title=Incidence of poliomyelitis; the effect of tonsillectomy and other operations on the nose and throat | url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=12978882 |journal=Calif Med |volume=77 |issue=1 |pages=19–21 |year=1952 |pmid=12978882}}</ref> physical activity immediately following the onset of paralysis,<ref>{{cite journal |author=Horstmann D |title=Acute poliomyelitis relation of physical activity at the time of onset to the course of the disease |journal=[[Journal of the American Medical Association|J Am Med Assoc]] |volume=142 |issue=4 |pages=236–41 |year=1950 |pmid=15400610}}</ref> skeletal muscle injury due to [[intramuscular injection|injection]] of vaccines or therapeutic agents,<ref>{{cite journal |author=Gromeier M, Wimmer E |title=Mechanism of injury-provoked poliomyelitis |url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=9573275| journal=J. Virol. |volume=72 |issue=6 |pages=5056–60 |year=1998 |pmid=9573275 |doi=}}</ref> and [[pregnancy]].<ref name= Evans_1960>{{cite journal |author=Evans C |title=Factors influencing the occurrence of illness during naturally acquired poliomyelitis virus infections | url=http://mmbr.asm.org/cgi/reprint/24/4/341.pdf  | format = PDF | journal=Bacteriol Rev |volume=24 |issue=4 |pages=341–52 |year=1960 |pmid=13697553}}</ref> Although the virus can cross the [[placenta]] during pregnancy, the fetus does not appear to be affected by either maternal infection or polio vaccination.<ref name=UK>{{cite book |author=Joint Committee on Vaccination and Immunisation (Salisbury A, Ramsay M, Noakes K (eds.) |title = Chapter 26:Poliomyelitis. ''in:'' Immunisation Against Infectious Disease, 2006  | url=http://www.immunisation.nhs.uk/files/GB_26_polio.pdf  | format = PDF |publisher=Stationery Office |location=Edinburgh |year=2006 |pages = 313–29 |isbn = 0-11-322528-8}}</ref> Maternal antibodies also cross the [[placenta]], providing [[passive immunity]]  that protects the infant from polio infection during the first few months of life.<ref>{{cite journal |author=Sauerbrei A, Groh A, Bischoff A, Prager J, Wutzler P |title=Antibodies against vaccine-preventable diseases in pregnant women and their offspring in the eastern part of Germany |journal=Med Microbiol Immunol |volume=190 |issue=4 |pages=167–72 |year=2002 |pmid=12005329}}</ref>
*'''In 2015, only 74 cases were reported worldwide.'''<ref name="WHOPolio> WHO Global Polio Eradication Initiative - Polio today. http://polioeradication.org/polio-today/polio-now/ Accessed on October 24, 2016. </ref>


===Reservoir===
*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:
{|
|+ style="background: #4479BA;" | {{fontcolor|#FFF|'''Number of Cases of Polio Worldwide, by Type (2015-2016)'''}}
! style="background: #4479BA; width: 150px;" | {{fontcolor|#FFF|Year}}
! style="background: #4479BA; width: 150px;" | {{fontcolor|#FFF|Wild Polio Virus (WPV)}}
! style="background: #4479BA; width: 150px;" | {{fontcolor|#FFF|Circulating Vaccine Developed Polio Virus (cVDPV)}}
|-
| style="padding: 5px 5px; background: #F5F5F5;" align="center" | 2015
| style="padding: 5px 5px; background: #F5F5F5;" align="center" | 51
| style="padding: 5px 5px; background: #F5F5F5;" align="center" | 23
|-
| style="padding: 5px 5px; background: #F5F5F5;" align="center" | 2016
| style="padding: 5px 5px; background: #F5F5F5;" align="center" | 27
| style="padding: 5px 5px; background: #F5F5F5;" align="center" | 3
|-
| colspan="3" style="padding: 5px 5px; background: #F5F5F5;" | <SMALL><SMALL> Data from WHO Global Polio Eradication Initiative <ref name="WHOPolio> WHO Global Polio Eradication Initiative - Polio today. http://polioeradication.org/polio-today/polio-now/ Accessed on October 24, 2016. </ref></SMALL></SMALL>
|}


Humans are the only known reservoir of poliovirus, which is transmitted most frequently by persons with inapparent infections. There is no asymptomatic carrier state except in immune deficient persons.
==Age==
* Poliomyelitis affects mainly children under 5 years of age <ref name=WHO>{{cite web | title = Poliomyelitis | url = http://www.who.int/mediacentre/factsheets/fs114/en/ }}</ref>.  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).<ref name=CDC2>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/polio.pdf }}</ref>


===Secular Trends in the United States===
==Gender==
* The [[prevalence]] and [[incidence]] of poliomyelitis do not vary by gender.<ref name=CDC>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/polio.html#epi }}</ref>
* Female gender carries a higher risk of developing "postpolio syndrome".<ref name=CDC>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/polio.html#epi }}</ref>
==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.<ref name=WHO2> [[http://www.who.int/mediacentre/factsheets/fs114/en/| WHO Polio factsheet. Last Updated April 2016. Accessed on October 24th, 2016]]</ref>
* 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)<ref name="PE">[http://polioeradication.org/where-we-work/polio-outbreak-countries | WHO Global Polio Eradication Initiative. Accessed on October 24th, 2016]]</ref>:
**Guinea
**Lao People's Democratic Republic,
**Madagascar
**Myanmar


Before the 18th century, polioviruses probably circulated widely. Initial infections with at least one type probably occurred in early infancy, when transplacentally acquired maternal antibodies were high. Exposure throughout life probably provided continual boosting of immunity, and paralytic infections were probably rare. (This view has been recently challenged based on data from lameness studies in developing countries.)
==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.
In the immediate prevaccine 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.
* 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.<ref name=CDC>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/polio.html#epi }}</ref><ref name=CDC2>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/polio.pdf }}</ref>
 
* 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]]. <ref name=CDC2>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/polio.pdf }}</ref>
In the early vaccine era, the incidence dramatically decreased after the introduction of inactivated polio vaccine (IPV) in 1955. The decline continued following oral polio vaccine (OPV) introduction in 1961. In 1960, a total of 2,525 paralytic cases were reported, compared with 61 in 1965.
* 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.<ref name=CDC>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/polio.html#epi }}</ref>
 
* 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.<ref name=CDC>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/polio.html#epi }}</ref><ref name=CDC2>{{cite web | title = Poliomyelitis | url = http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/polio.pdf }}</ref>
The last cases of paralytic poliomyelitis caused by endemic transmission of wild virus in the United States were in 1979, when an outbreak occurred among the Amish in several Midwest states. The virus was imported from the Netherlands. 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. Two cases were classified as indeterminant (no poliovirus isolated from samples obtained from the patients, and patients had no history of recent vaccination or direct contact with a vaccine recipient). The remaining 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 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. No VAPP has been reported from this virus.
 
[[Image:Polio USA, 1950-2004.jpg|center|frame|Poliomyelitis in the U.S., 1950-2204]]
{{clr}}
 
[[Image:Polio USA, 1980-2004.jpg|center|frame|Poliomyelitis in the U.S., 1980-2204]]
{{clr}}




==References==
==References==
{{Reflist|2}}
{{Reflist|2}}


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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.[1]
  • 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.[2]
Polio virus progress 1988 - 2012 Data from The World Health Organization (WHO)/ Global Polio Eradication Initiative[3]

2017 Syria outbreak

  • On Thursday June 8th, WHO announced a vaccine related polio outbreak in Syria.[4]
  • 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.[5]
  • 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 [5]

Age

  • Poliomyelitis affects mainly children under 5 years of age [3]. 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).[6]

Gender

  • The prevalence and incidence of poliomyelitis do not vary by gender.[1]
  • Female gender carries a higher risk of developing "postpolio syndrome".[1]

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.[2]
  • 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)[7]:
    • 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.[1][6]
  • 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. [6]
  • 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.[1]
  • 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.[1][6]


References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 "Poliomyelitis".
  2. ↑ 2.0 2.1 "Poliomyelitis".
  3. ↑ 3.0 3.1 "World Health Organization (WHO)".
  4. ↑ "Polio outbreak is reported in Syria, health officials say".
  5. ↑ 5.0 5.1 WHO Global Polio Eradication Initiative - Polio today. http://polioeradication.org/polio-today/polio-now/ Accessed on October 24, 2016.
  6. ↑ 6.0 6.1 6.2 6.3 "Poliomyelitis" (PDF).
  7. ↑ | WHO Global Polio Eradication Initiative. Accessed on October 24th, 2016]

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