What is Poliomyelitis?

Poliomyelitis is a disease caused by the poliovirus that in rare cases affects the nervous system and can lead to paralysis or death. The term "polio," used to describe the effects of the disease on the spinal cord, comes from the Greek words "polio" (gray) and "myelin" (marrow).⁽¹⁾

There are three types of poliovirus - types 1, 2 and 3 - and immunity to one type of poliovirus does not provide immunity to other types. Poliovirus is an enterovirus unique to humans and usually inhabits the gastrointestinal tract.⁽²⁾

About 95% of all polio infections are asymptomatic. This means that most people exposed to the virus do not have clinical symptoms of illness. Between 4 and 8 percent of people exposed to poliovirus develop mild symptoms, which often include flu-like illnesses, respiratory tract infections, and gastroenteritis. About 1% of polio cases present as aseptic meningitis, and symptoms generally include severe spasms in the back, neck, or legs. Complete healing usually occurs within 10 days.⁽³⁾ For nonparalytic polio, the incubation period (time from exposure to onset of the disease) is generally 3 to 6 days.⁽⁴⁾

Fewer than 1% of children exposed to polio will develop paralytic polio, the most severe form of polio. Paralytic polio usually begins between 1 and 18 days after the initial infection, and symptoms of paralysis usually progress for 2 to 3 days. In some cases, an asymptomatic period of 7-10 days may elapse between the first symptoms of the disease and the appearance of the most serious symptoms. 

In recent years, vaccine-derived poliovirus (VDPV) has emerged as a cause of paralytic polio cases and outbreaks. OPV is an attenuated (weakened) live virus vaccine, and after its administration, the virus replicates in the intestine and is excreted in the feces. This excreted live vaccine virus has the potential to circulate in the environment, undergo genetic changes, and cause paralytic polio. VDPVs are classified into 3 categories:^(5-6)

• Circulating VDPV (cVDPV) – occurs in communities where individuals remain susceptible to wild-type polio or VDPV due to inadequately administered polio vaccination campaigns or 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 non-immunodeficient individuals or found in wastewater of which the source is unknown.

Wild-type polio type 2 was declared eradicated worldwide in 2015⁽⁷⁾ and on October 24, 2019, the Global Polio Eradication Initiative (GPEI) announced the global eradication of wild-type polio type 3.⁽⁸⁾

In September 2015, following the announcement of the eradication of wild-type polio globally, public health officials moved quickly to discontinue the use of trivalent OPV (vaccine containing vaccine strains of poliovirus types 2, 1 and 2) and replace it with a bivalent OPV containing only the vaccine strains of poliovirus types 3 and 1.⁽⁹⁾ This initiative was implemented to stop the spread of VDPV type 2 (cVDPV2), which had caused multiple polio epidemics in several countries.⁽¹⁰⁾ Since 2000, there have been 1.085 cases of paralytic polio associated with cVDPV, and the majority of cases – 932 (86%) – were caused by cVDPV2.⁽¹¹⁾

Despite the elimination of type 2 polio from the OPV in spring 2016,⁽¹²⁾ Cases of paralytic polio associated with cVDPV2 continued to occur. In 2019, cVDPV2 outbreaks were reported in the Philippines and several African countries.⁽¹³⁾ This prompted health officials to reintroduce a type 2 OPV, and as of April 2016, approximately 300 million doses have been administered in regions where cVDPV2 outbreaks have occurred. Issues associated with the reintroduction of live OPV type 2 have included the risk that its use could potentially lead to additional cVDPV2 cases and outbreaks.⁽¹⁴⁾

Wild-type polio type 1 is the only strain of poliovirus currently circulating, and 2018 cases were detected globally in 33, including 12 in Pakistan and 21 in Afghanistan.⁽¹⁵⁾ However, in 2018, 6.732 cases of VAPP associated with the use of Sabin OPV and 104 cases of VDPV paralysis were recorded. These cases were identified after testing was completed on stool samples from 190.055 people diagnosed with acute flaccid paralysis (AFP).⁽¹⁶⁾

References:

1. CDC Poliomyelitis – Poliovirus Epidemiology and Prevention of Vaccine-Preventable Diseases (The Pink Book). 13th ed. 2015.
2. Ibid
3. Mehndiratta MM, Mehndiratta P, Pande R Poliomyelitis Historical Facts, Epidemiology, and Current Challenges in Eradication Neurohospitalist. 2014 Oct; 4(4): 223–229
4. CDC Poliomyelitis – Clinical Features Epidemiology and Prevention of Vaccine-Preventable Diseases (The Pink Book). 13th ed. 2015.
5. WHO What is vaccine-derived polio? April 2017
6. Guo J, Bolivar-Wagers S, Srinivas N et al Immunodeficiency-related vaccine-derived poliovirus (iVDPV) cases: a systematic review and implications for polio eradication. Vaccines. 2015 Mar 3;33(10):1235-42
7. CDC Progress Toward Polio Eradication — Worldwide, January 2017–March 2019 MMWR 68 (20): 458-462
8. Global Polio Eradication Initiative (GPEI) TWO OUT OF THREE WILD POLIOVIRUS STRAINS ERADICATED Oct. 24, 2019
9. CDC Cessation of Trivalent Oral Poliovirus Vaccine and Introduction of Inactivated Poliovirus Vaccine — Worldwide, 2016 MMWR Sep 9, 2016; 65(35);934–938
10. CDC Update on Vaccine-Derived Polioviruses — Worldwide, January 2015–May 2016 MMWR Aug. 5, 2016; 65 (30); 763-769
11. CDC Update on Vaccine-Derived Poliovirus Outbreaks — Worldwide, January 2018–June 2019 MMWR Nov. 15, 2019; 68(45);1024–1028
12. CDC Cessation of Trivalent Oral Poliovirus Vaccine and Introduction of Inactivated Poliovirus Vaccine — Worldwide, 2016 MMWR Sep 9, 2016; 65(35);934–938
13. WHO Emergencies Preparedness, response – Poliomyelitis – Disease outbreak news No Date (accessed Jan. 30, 2020)
14. CDC Update on Vaccine-Derived Poliovirus Outbreaks — Worldwide, January 2018–June 2019 MMWR Nov. 15, 2019; 68(45);1024–1028
15. CDC Surveillance to Track Progress Toward Polio Eradication — Worldwide, 2017–2018 MMWR 68 (13): 312-318
16. Ibid