Progress Toward Polio Eradication Worldwide, 2016–Early 2018

This report describes the state of polio eradication as of April 2018.

When the World Health Assembly resolved in 1988 to eradicate polio, an estimated 350,000 cases occurred in 125 countries. By 2018, wild poliovirus (WPV) was still circulating uninterrupted in only three countries, Afghanistan, Nigeria and Pakistan, and type 1 (WPV1) was the only type confirmed to be circulating. Wild type 2 was certified eradicated in 2015, after no detection since 1999, and type 3 had not been found since 2012.

Wild polio cases

Cases fell 41% from 2016 to 2017. No wild polio cases had occurred outside these three countries since 2014.

• Afghanistan: cases rose slightly, from 13 in four districts to 14 in nine. In 2016, 54% came from Paktika province in the southeast; in 2017, half came from Kandahar province in the south. The 2018 cases were in Kandahar, Nangarhar and Kunar provinces. More children in the south and east were out of reach of vaccinators because of insecurity in 2017, despite constant negotiations for access.
• Pakistan: cases fell 60%, continuing a decline since 2014, helped by intensified campaigns, efforts to reach unvaccinated children, community-based local vaccinators and better worker safety. Virus was still found in sewage in five provinces (Balochistan, Islamabad, Khyber Pakhtunkhwa, Punjab and Sindh), which could mean surveillance gaps or waning transmission. Large movements of people across the border with Afghanistan in both directions remained a challenge.
• Nigeria: no wild polio since the four cases of 2016. But the virus had gone undetected there from mid-2014 to mid-2016, and insurgent-held areas of Borno State remained hard to reach for vaccination and surveillance, so hidden transmission could not be ruled out. Neighboring Lake Chad basin countries (Cameroon, Chad and Niger) had improved campaign planning, but doubts remained about campaign quality.

Vaccination

Worldwide, 85% of infants received three doses of polio vaccine in 2016, ranging by WHO region from 73% in Africa to 95% in the Western Pacific. In the countries with endemic polio, coverage with three doses of oral polio vaccine (OPV) was 60% in Afghanistan, 72% in Pakistan and 49% in Nigeria, and far lower in areas where the virus circulated and mobile or conflict-affected children were repeatedly missed.

After wild type 2 was certified eradicated, countries switched together, by the end of April 2016, from trivalent OPV (types 1, 2 and 3) to bivalent OPV (types 1 and 3). From 2015, countries using OPV added an injected inactivated polio vaccine (IPV), usually at 14 weeks, though global shortages delayed some countries until 2018.

Mass vaccination campaigns delivered about two billion doses in 186 campaigns in 2016 and about 1.79 billion in 172 campaigns in 2017. In 2017, 95% of those doses were bivalent OPV.

Vaccine-derived polio

Where too few children are vaccinated, the weakened virus in OPV can occasionally spread and regain the ability to paralyze, causing outbreaks of circulating vaccine-derived poliovirus (cVDPV). About 90% of such cases since 2006 have been type 2.

• 2016: five cases in three countries: three type 1 cases continuing a Laos outbreak that began in 2015, and one type 2 case each in Nigeria and Pakistan.
• 2017: 96 type 2 cases, 74 in Syria (the last in September 2017) and 22 in the Democratic Republic of the Congo, where four more occurred in early 2018.
• Sewage findings: type 2 virus in Mogadishu, Somalia, in late 2017 and early 2018, with related virus in Nairobi, Kenya, pointed to long-term, widespread transmission; type 3 was found in Mogadishu sewage in March 2018; and type 2 was detected in sewage in two Nigerian states in early 2018. None had caused known cases yet, and response vaccination was under way or planned.

Coverage with three doses was 74% in Congo, 48% in Syria, 47% in Somalia and 83% in Laos, and lower still in the affected areas. Efforts to raise type 2 immunity before the 2016 switch had not reached every persistently unvaccinated child, which is why some type 2 outbreaks emerged afterward.

Surveillance

Polio is detected by investigating cases of acute flaccid paralysis and testing stool. Surveillance is judged sensitive enough if it finds at least 2 non-polio paralysis cases per 100,000 children under 15 each year (1 in regions already certified polio-free) and complete enough if at least 80% of cases have adequate stool specimens. In 2017, Afghanistan, Congo, Nigeria and Pakistan met both standards and Syria did not, though Nigeria and Congo still faced serious access problems. Sewage testing supplemented this in many countries. Four of WHO's six regions (the Americas, Europe, South-East Asia and the Western Pacific) had been certified free of wild polio; certifying Africa and the Eastern Mediterranean would require better surveillance in key areas.

What remained

Much recent progress in reaching missed children came from recruiting trusted community volunteers invested in their own areas. The authors called for better-quality vaccination and surveillance, new approaches to persistent obstacles, and vigilance everywhere, with high immunity and sensitive surveillance, until polio is eradicated.

Sources

• Khan F, Datta SD, Quddus A, et al. Progress Toward Polio Eradication — Worldwide, January 2016–March 2018. MMWR Morb Mortal Wkly Rep 67(18). https://www.cdc.gov/mmwr/volumes/67/wr/mm6718a4.htm
• WHO/UNICEF polio vaccine coverage estimates: http://apps.who.int/immunization_monitoring/globalsummary/timeseries/tswucoveragepol3.html
• The report was written jointly by CDC and WHO; its figure is not reproduced here.