How Well the World Watched for Polio, 2018–2019

Since the Global Polio Eradication Initiative (GPEI) began in 1988, polio cases worldwide have fallen by approximately 99.99%. Only two countries, Afghanistan and Pakistan, have never interrupted transmission of wild poliovirus (WPV). Finding the virus wherever it still circulates depends on surveillance, and this CDC report assesses it for 2018–2019.

How polio is tracked

• Acute flaccid paralysis (AFP) surveillance is the main tool: cases in children under 15 are reported, and their stool is tested in World Health Organization (WHO)–accredited laboratories for wild poliovirus and for vaccine-derived polioviruses (VDPVs) — vaccine strains that have genetically reverted and regained the ability to cause paralysis. Where immunity is low, VDPVs can spread and cause outbreaks; these are circulating VDPVs (cVDPVs).
• Environmental surveillance — regularly collecting and testing sewage — adds a picture of how widely and how long poliovirus is circulating.

Two indicators measure AFP surveillance quality:

1. a nonpolio AFP rate of at least 2 per 100,000 children under 15 per year, considered sensitive enough to detect circulating poliovirus;
2. adequate stool specimens from at least 80% of AFP patients: two samples at least 24 hours apart, within 14 days of paralysis onset, reaching an accredited lab in good condition with the cold chain maintained.

The report focuses on priority countries — those with WPV or VDPV outbreaks, or at high risk of importation because they border one. A surge in VDPV outbreaks raised their number from 31 in 2018 to 40 in 2019. In 2019 only 25 (63%) met both indicators nationally, and performance within countries varied widely.

Combined AFP surveillance indicators in subnational areas of the 40 priority countries, 2019. CDC.

Region by region

Sewage finds what cases miss

Environmental surveillance detected poliovirus where no AFP cases were found — in Cameroon, Côte d'Ivoire and Kenya — and confirmed cVDPV circulation well before any case in China, Central African Republic, Ghana, Malaysia, Nigeria, the Philippines and Somalia. In 2019 Iran found WPV1 in sewage with no detected AFP cases. Nigeria's sewage yielded 45 cVDPV2 isolates in 2018 and 60 in 2019. In Afghanistan six WPV1 genetic clusters turned up in sewage from seven provinces in 2018; in Pakistan eight clusters came from 28 districts in four provinces and the Islamabad Capital Territory, and detections rose in 2019, most of all in Sindh.

The laboratory network

The Global Polio Laboratory Network has 145 laboratories across WHO's six regions: all isolate poliovirus, 134 tell wild, vaccine and vaccine-derived strains apart (intratypic differentiation), and 28 do genomic sequencing. Accreditation, with onsite reviews and proficiency testing, is annual.

• Stool specimens tested: 190,055 in 2018 and 219,049 in 2019.
• WPV1 was isolated from 33 specimens in 2018 and 156 in 2019.
• cVDPVs came from 105 AFP patients in 2018 and 437 in 2019. Specimens with cVDPV rose from 65 to 303 in the African region (366%), from 13 to 50 in the Eastern Mediterranean (284%), from one to 10 in South-East Asia, and from 26 to 74 in the Western Pacific (185%).
• Every region met the timeliness target for virus isolation in both years.
• Only the South Asia genotype of WPV1, the sole one circulating since 2016, was found, in Afghanistan and Pakistan. "Orphan" WPV1 isolates from AFP patients — 98.5% or less genetically identical to other isolates, a sign of possible surveillance gaps — went from none in 2018 to five in 2019 (two in Afghanistan, three in Pakistan). Sequencing identified 7 new cVDPV2 emergences in 6 countries in 2018 and 39 in 19 countries in 2019.

What it means

The share of 2019 priority countries meeting both indicators fell from 83% in 2018 to 63% in 2019, and almost every country had serious gaps below the national level. Falling stool adequacy points to trouble detecting and investigating cases quickly, or moving specimens. Competing priorities, thin logistics and heavy workloads all play a part, and the COVID-19 pandemic threatened to make things worse by reducing access to care and immunization.

The authors note limits: insecurity and hard-to-reach groups affect local indicators; a high nonpolio AFP rate is not proof of sensitive surveillance, since not all reported cases fit the definition and some are missed; and timeliness depends on learning the real date paralysis began. Their conclusion is that case detection, investigation, reporting, monitoring and supervision must all be strengthened, with continuous checks at national and local levels, to finish eradication.

Sources

Based on Lickness JS, Gardner T, Diop OM, et al., "Surveillance to Track Progress Toward Polio Eradication — Worldwide, 2018–2019," MMWR Vol. 69, No. 20, CDC; a work of the United States government in the public domain. The report gives the South-East Asia region's rise from one to 10 cVDPV specimens as "90%"; no percentage is given here.