Is the World Watching Closely Enough for Polio? Surveillance in 2017–2018

Summary

• Sensitive surveillance for acute flaccid paralysis (AFP) is the cornerstone of polio eradication.
• In 2018, 26 of 31 countries the Global Polio Eradication Initiative ranks as high priority (84%) met AFP surveillance targets nationally — but performance varied widely within countries, and there was no improvement from 2017.
• Stronger monitoring, supervision, and better specimen collection and transport are needed to prove that poliovirus has stopped circulating.

How polio is tracked

When the initiative began in 1988, polio was reported in 125 countries. Since then only Afghanistan, Nigeria and Pakistan have had uninterrupted wild poliovirus transmission. The main tool is testing the stool of children under 15 with AFP for wild and vaccine-derived poliovirus, in labs of WHO's Global Polio Laboratory Network. Sewage testing adds to it, and genetic sequencing shows how the virus moves and where surveillance has gaps.

Two targets measure AFP surveillance:

• a non-polio AFP rate of at least 2 per 100,000 children under 15 a year — enough to detect circulating poliovirus;
• adequate stool specimens — two, at least 24 hours apart, within 14 days of paralysis, reaching a WHO-accredited lab cold and intact — from at least 80% of AFP cases.

The 31 high-priority countries have a high risk of transmission and limited capacity to deal with it; some are in regions with endemic polio, others in polio-free regions.

Region by region

• Africa: no wild polio in 2017–2018, but vaccine-derived type 2 cases — 22 in 2017, all in the Democratic Republic of the Congo; 65 in 2018 (34 Nigeria, 20 DR Congo, 10 Niger, one Mozambique).
• Eastern Mediterranean: wild type 1 cases rose in Afghanistan (14 to 21) and Pakistan (eight to 12). Syria had all 74 vaccine-derived type 2 cases in 2017; in 2018 Somalia had 12 vaccine-derived cases of types 2 and 3.
• Western Pacific: 26 vaccine-derived type 1 cases in Papua New Guinea in 2018. It missed both targets in 2017; its AFP rate improved with the outbreak response, but specimen collection stayed low.
• South-East Asia: one vaccine-derived type 1 case in Indonesia in 2018 — which met both targets nationally but had substantial gaps below the national level.

In both big regions, national figures masked weak performance in parts of countries.

Surveillance quality below the national level in the 31 high-priority countries, 2017. Figure from the CDC report.

Sewage testing

Sites in Afghanistan, Nigeria and Pakistan grew from 143 to 185.

• Nigeria: nothing in 2017, but 46 vaccine-derived type 2 isolates in 2018 — some weeks before cases were confirmed.
• Pakistan: 16% of samples from 19 districts positive for wild type 1 in 2017, and 20% from 27 districts in 2018. Afghanistan's positive samples came from five provinces in 2017 and seven in 2018.
• 2018 also brought detections in Kenya, Somalia and Papua New Guinea. Sewage testing now runs in 44 countries without wild virus transmission, 24 of them in Africa.

The laboratory network

146 quality-assured labs in all six WHO regions follow standard methods and timeliness targets, checked by yearly accreditation. They tested 201,546 stool specimens in 2017 and 190,055 in 2018, finding wild type 1 in 22 and 33 patients and vaccine-derived virus in 96 and 104. All regions met the isolation and typing targets; all met the overall onset-to-result target except Europe and the Western Pacific in 2018.

• Only the South Asia genotype of wild type 1 has circulated since 2016, crossing often between Afghanistan and Pakistan.
• "Orphan" isolates — distantly related to others, a sign of missed transmission — fell from three to zero, suggesting AFP surveillance gaps may be closing; but genetic diversity stayed constant for 3 years as many lineages persist.
• Sequencing identified nine new vaccine-derived emergences in seven countries in 2018.

What it means

• In most African countries evaluated, the main weakness was too few adequate specimens, usually because cases were found late.
• Nigeria found no wild virus from August 2014 to July 2016, then cases in Borno State — where large insurgent-held areas had no effective surveillance in 2013–2016. More areas are now reachable, and community informants report from those still inaccessible. Africa will be considered for wild-polio-free certification in early 2020, which demands a careful look at surveillance quality.
• The vaccine-derived viruses in Indonesia and Papua New Guinea had circulated for years before detection — surveillance must stay sensitive everywhere.
• Sewage testing has caught vaccine-derived virus before paralysis cases and shown wild virus still circulating in Afghanistan and Pakistan; it will be needed long term.
• Technical and financial support has gone to all 31 countries under the Global Polio Surveillance Action Plan, 2018–2020; linking AFP surveillance with surveillance for other diseases would make the most of field capacity.

Limits: insecurity and hard-to-reach populations can distort the indicators, and a high AFP rate doesn't guarantee sensitivity — some reported cases aren't true AFP, and weak systems miss real ones.

Sources

Based on Jaymin C. Patel, Ousmane M. Diop, Tracie Gardner and others (CDC and the World Health Organization), "Surveillance to Track Progress Toward Polio Eradication — Worldwide, 2017–2018," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain.