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What is Happening with Polio?

polio vaccine
Photo: Getty Images

[This text has been written by Adelaida Sarukhan, scientific writer at ISGlobal, and Quique Bassat, researcher at ISGlobal and Head of the Malaria Programme]

We have been hearing about poliomyelitis ("polio" for short) in the media for a couple of months now. Last month (September 10), the state of New York declared a state of emergency after detecting the virus in sewage of several counties (in fact, as we will explain later, what they found is a version of the virus derived from the oral vaccine, but capable of producing paralysis). What is happening to a disease that we thought was practically extinct?

Let's start by reviewing some basic facts about polio, a candidate disease for eradication:

  • Transmission: poliovirus is transmitted by water (particularly in places with open sewage) or contaminated food. It can also be transmitted through respiratory droplets. The virus can survive for several weeks outside the body.
  • Symptoms: Three out of four infections are asymptomatic, making it difficult to monitor the disease. In those who do become ill, symptoms are usually mild and difficult to differentiate from other common viruses (fever, sore throat, muscle weakness). However, in about 1% of cases, the virus can reach the nervous system and cause paralysis. 
  • Treatment: There is no cure for polio. Hence the importance of vaccination.
  • Vaccine: There are two types of polio vaccine currently in use. The oral vaccine (OPV), developed by Albert Sabin in 1961, consists of a live but attenuated virus (it can replicate, but cannot cause disease). The intramuscular vaccine (IPV), developed by Jonas Salk in 1954, contains inactivated virus (unable to infect or replicate). Both have played a key role in the eradication campaign, a global effort that has reduced the number of cases - and their geographic spread - to almost anecdotal levels.

Disease surveillance is complicated by the fact that, of 100 infections, most present no (green)
or few 
(yellow) symptoms. Only one develops clear neurological symptoms.

How close are we to its eradication?

We are very close. In 2021, only 6 cases of wild polio were reported, compared to about 350,000 cases in 1998, the year the global polio eradication campaign (GPEI) was launched, when the virus was circulating in over 125 countries. Currently, wild poliovirus (specifically, the type 1 or WPV1 strain) circulates in only two countries- Afghanistan and Pakistan (Africa was certified polio-free in 2020). The other two wild poliovirus strains (type 2 and 3) are considered eradicated. These achievements are enormous and show that polio eradication (i.e. its elimination from the entire planet) is within reach - it would be the second human disease to be eradicated, after smallpox.

But... as is often the case, the last mile is the hardest. The COVID-19 pandemic has hampered vaccination campaigns around the globe, and the conflict in Afghanistan complicates matters further. In February of this year, Malawi confirmed a case of wild polio, and its neighbour Mozambique soon followed suit. Both cases were imported from Pakistan, so Africa still retains its polio-free status. In total, 19 cases of wild poliovirus have been reported so far in 2022.

However, the most concerning (and unforeseen) challenge for eradication are the cases caused by vaccine-derived virus (VDPV) - 223 cases in 15 countries (almost all in Africa) so far in 2022 - 93% of which are attributed to type 2 virus (VDPV2). For this reason, in 2014 the WHO declared that the increase in circulating VDPV2 cases represents a Public Health Emergency of International Concern (PHEIC) - an emergency that remains in effect to date.

What does vaccine-derived polio mean?

OPV has been the vaccine of choice for low- and middle-income countries because it is much easier to deliver than an intramuscular one. Moreover, because it is oral, it induces both systemic and mucosal immunity, thus helping to interrupt virus transmission. Vaccinated individuals excrete the attenuated virus in faeces for about two weeks after vaccination, which brings an additional advantage - the unvaccinated population can ingest the virus through contaminated water and thereby also develop immunity (a kind of "contagious vaccine"). But excretion of the attenuated virus also has a disadvantage (quite unexpected when Sabin developed the vaccine): when passed from one person to another in a community with low vaccination coverage, the virus can mutate sufficiently to revert and cause disease and paralysis again. This is called vaccine-derived poliovirus (VDPV), and circulating VDPV (cVDPV) has been recently detected in sewage from several countries that had already eliminated the disease, including the US, UK and Israel. These viruses were probably introduced by a traveller who received the oral vaccine or who became infected with VDPV2 in another country.

Does vaccine-derived poliovirus pose a risk?

In high- and middle-income countries where IPV has long been used (to avoid vaccine-derived poliovirus), vaccinated persons are fully protected against VDPVs, including VDPV2 (the IPV contains all three types). But when VDPV circulates in communities with low vaccine coverage, such as some Orthodox Jewish communities in New York, the danger of a polio outbreak in unvaccinated people increases. In Europe, the latest ECDC report warns that Poland, Romania and Ukraine are at risk of an outbreak due to low vaccination rates.

In countries that still use OPV there is an additional challenge, and that is that there are children who are vaccinated but are unprotected against type 2 VDPV. This is because about four years ago there was a switch from the trivalent (types 1, 2 and 3) to the bivalent (types 1 and 3) oral vaccine. The continued circulation of VDPV2 in some countries represents a problem for which there are two solutions: reintroduce in those regions the oral vaccine with attenuated type 2 virus (i.e. fight fire with fire), or wait for the new oral type 2 vaccines (nOPV2) containing a more genetically stable virus that cannot revert. As of June 2022, 370 million doses of nOPV2 had already been administered in twenty African countries. WHO has also recommended introducing a dose of IPV in all countries using bivalent OPV, to boost immunity against type 2 virus.

A final effort

A last – and big - financial and human effort is needed to interrupt transmission of wild type 1 virus in Asia and prevent transmission of oral vaccine-derived type 2 virus in other countries. Deploying new vaccines and strengthening surveillance systems around the world will be key to covering that last mile to eradicate polio once and for all. In fact, last week over 3,000 scientists, physicians and public health experts from 113 countries launched a Declaration on Polio Eradication, calling for urgent action to end polio by 2026. Having come so close, the world cannot afford to stumble again over such a devastating disease.