Challenging epidemiolgical developments
Since 2000, secondary epidemics—caused by either a direct spread of WPV from the remaining endemic countries to neighbouring countries (although no longer in the past 5 years) or by cVDPV—were reported from about 30 countries formerly certified as polio-free, most recently from the Democratic Republic of Congo (DRC), Papua New Guinea, Somalia and Syria.4 13 18 19 A total of 21 and 12 WPV cases were reported from Afghanistan and Pakistan, respectively, in 2018; by 26 June 2019, the number of new WPV cases reported had reached already 37 (10 in Afghanistan, 27 in Pakistan).4 Thus, polio remains endemic in Afghanistan and Pakistan, two countries which share a porous border. Nigeria, while not certified as polio-free, has so far reported no new WPV cases in 2019.4 Regarding cVDPV cases, a total of 104 were reported for the whole of 2018 (DRC 20, Indonesia 1, Mozambique 1, Niger 10, Nigeria 34, Papua New Guinea 26, Somalia 12); by 26 June 2019, the total of number of new cVDPV stood at 20 (Angola 1, DRC 5, Ethiopia 1, Niger 1, Nigeria 9, Somalia 3).4
Important reasons for continuing transmission are first, the weak health systems and correspondingly low routine childhood immunisation coverage in many countries still at risk of polio due to ongoing political instability, underdevelopment and poverty, compounded by the technical challenges of the GPEI; and second, the perception that polio eradication is a priority of wealthier ‘Western’ countries, not of the people living in the countries where elimination proves to be the hardest, as has been shown in Pakistan.20 21 While there have been repeated OPV campaigns in the remaining endemic countries for many years (eg, 6–8 campaigns per year in the critical provinces of Pakistan and Nigeria), routine health services including immunisation services had in the past been largely neglected, even disrupted by vertical polio campaigns,22 and overall vaccination coverage remains low in several countries and regions.23
The current DRC epidemic has emerged in different provinces as independent cVDPV type 2 outbreaks, which now threatens to spread to other neighbouring countries and may endanger the whole of sub-Saharan Africa (SSA).3 5 By 2016, 155 countries had already replaced trivalent OPV with a bivalent (types 1 and 3) vaccine; the DRC outbreak thus demonstrates the weaknesses in polio surveillance systems and—if not contained—may cause a general move back to the trivalent OPV.3 4 10 24
The GPEI faces further technical challenges which incur at least a theoretical risk of future outbreaks. They include the decades-long excretion of polio-related viruses in persons with a B-cell defect (this risk may only be minute as no resulting outbreaks have been identified since the switch from trivalent OPV), the risk of ongoing circulation of polio viruses in populations with high IPV coverage due to low mucosal immunity, the possibility of an accidental spread of unknowingly stored polioviruses from laboratories, or even a deliberate spread of de-novo synthesised polioviruses.6 25 Failure to contain poliovirus would be a greater risk than with smallpox virus because resulting outbreaks are less easily identified and thus contained.
And sadly, even successful eradication of poliovirus may not mean an end of polio-like illness. Other viruses from the same family (eg, enteroviruses D68, D71) may produce flaccid paralysis resembling poliomyelitis, with outbreaks reported from a number of industrialised countries in recent years.26 27 The existence of other causes of disease does not mean that eradication of one cause should not be attempted. However, it would bring about the challenge of explaining to the world community why outbreaks presenting with the clinical symptoms of a disease eradicated at substantial cost continue to occur.