Introduction

Poliovirus is an enterovirus of the Picornaviridae family. There are three types of poliovirus that differ slightly in capsid protein, and thus in receptor specificity and antigenicity. Wild serotype 1 (wPV-1) is currently only found in two endemic countries (Pakistan and Afghanistan), while serotypes 2 and 3 (wPV-2 and wPV-3, respectively) have been eradicated. The last case of wPV-2 was detected in 1999 and was declared eradicated in the world in September 2015. The last case of wPV-3 was detected in 2012 and it was declared eradicated in October 2019¹.

The history of polio developed in three, well-defined periods. The first period was endemic, occurring until the 19th century, in which wPV infection determined the severity of the disease. Subsequently, the epidemic period occurred in the 20th century with urbanization, until the pandemic of 1955-1957, in which the iron lungs improved survival rates, but survivors had serious sequelae. With the distribution of the oral poliovirus vaccine (OPV) in 1955, the post-vaccination period began, which managed to eradicate two of the three wPV serotypes.

In 1988, the World Health Organization (WHO) launched the polio eradication initiative. This initiative involved mass vaccination campaigns that achieved a significant reduction in the circulation of wPV. At the beginning of the campaign, approximately 350,000 cases of paralysis were estimated in more than 125 countries, due to the three wPV serotypes. As of September 22, 2020, the WHO has reported 120 cases of wPV-1². Although this serotype has not been eradicated, the progress of the campaign continues to be remarkable, achieving a reduction in cases due to the three serotypes of poliovirus greater than 99.9%. However, there is still a long and difficult road ahead to achieve wPV eradication.

Figure 1 presents the global evolution of poliomyelitis cases and the main events that have occurred over time. With the recognized leadership of Dr. Ciro de Quadros³, the American continent was the first to obtain the eradication certification of the three wPV serotypes in 1994, after the last case was reported in Peru in 1991. Other regions achieved the disease eradication more than a decade later: Western Pacific (2000), Europe (2002), Southeast Asia (2014) and, more recently, Africa⁴ (2020). To date, wPV is only endemic in Afghanistan and Pakistan, as it was already mentioned.

Source: prepared by the authors with data from https://ourworldindata.org/polio and from WHO (2020).

Figure 1 Global evolution of the frequency of poliomyelitis cases 1980-2020 and milestones in the eradication strategy. cVDPV: community circulating vaccine derived poliovirus; GPEI: Global Strategy for Poliomyelitis Eradication; bOPV: bivalent oral polio vaccine; WHO: World Health Organization; tOPV: trivalent oral polio vaccine; wP-2: wild poliovirus type 2; wP-3: wild poliovirus type 3. 

In April 2016, the WHO recommended the global transition from trivalent OPV (tOPV) to a bivalent vaccine (bOPV) that excluded serotype 2. This strategy aimed at reducing the circulation of vaccine-derived poliovirus 2 (VDPV-2). In addition, during the same period, it was recommended to start the sequential scheme with at least one dose of IPV –preferably the first dose– which has already been implemented in all the countries of the Americas region⁵. Vaccination schedules differ in terms of dates of administration and incorporate the IPV, either in the primary series between 1 and 6 months of age in addition to a booster, or, between 7 and 24 months (eg, Costa Rica) and between 4 and 7 years (eg, Uruguay).

As an example, the last case of polio due to wPV in Argentina was registered in 1984. In response to recommendations by the WHO⁶, the Latin American Society of Pediatric Infectious Diseases (SLIPE)^7,8, and national experts, Argentina's National Immunization Commission (CoNaIn) introduced the poliovirus containment plan through the use of bOPV as of May 2, 2016 with use of IPV at 2 and 4 months, in a sequential schedule⁹. Through Ministerial Resolution 814/2020, the exclusive use of IPV was implemented in the primary series in 2020, with a reinforcement at 5-6 years before school entry, as of June 1, 2020.

Oral polio vaccine (OPV)

OPV contains live attenuated viruses and is safe and effective in most children. It is estimated that more than one billion children have received OPV and that this has prevented more than 650,000 cases of polio in the world since 2010. Without a doubt, this vaccine was extremely important in the process of reducing polio worldwide. Its advantages include the ease of oral administration and low cost, which contributed to its extremely practical implementation.

Despite its safety and efficacy, OPV has some disadvantages. For example, infants require multiple doses to achieve reasonable protection. Furthermore, as it is a live attenuated virus vaccine, they are susceptible to mutations and re-acquire neurovirulence. Poliomyelitis due to VDPVs is indistinguishable from poliomyelitis due to wPV. Vaccine viruses are also excreted in the fecal route for over than two months and may infect other people in the community.

Depending on the intensity of the genetic divergence, mutant polioviruses can cause two types of acute flaccid paralysis (AFP) phenomena, namely:

Mutations and recombinations are practically out of the control of health systems because these mechanisms are spontaneous in circulating viruses¹⁰. Regardless of the degree of genetic divergence, VAPP and VDPV may cause poliomyelitis with the same symptoms, intensity and severity than wPV. The only proven way to avoid VAPP and VDPV cases is to exclusively use IPV, which does not contain live viruses and therefore has no potential for mutations.

Vaccine-associated paralytic poliomyelitis (VAPP)

VAPP cases are rare, and in the late 1980s, one case of VAPP was reported for every 0.5 to 2.2 million doses administered. All three types of Sabin viruses can cause VAPP and the risk is higher after the first dose of OPV and very high in immunocompromised people. Figure 2 presents VAPP cases by number of OPV doses applied. Data from Brazil, where the last case of wP paralysis was identified in 1989, show that in the period between 2005 and 2014, 94 cases of VAPP were identified in children under 5 years of age, caused by the Sabin type vaccine serotypes. 1 (19%), type 2 (14%), type 3 (31%) and the remaining isolates of more than one Sabin serotype¹¹.

Source: Modified by the author (LHFA) based on references ¹²-¹⁴ and WHO surveillance data before and after the switch from tOPV to bOPV.

Figure 2 Trend of VAPP cases in the Latin American and Caribbean Subregion, 1992-2018. bOPV: bivalent OPV (serotypes 1 and 3); WHO: World Health Organization; OPV: oral poliovirus vaccine; tOPV: trivalent OPV; VAPP: paralytic poliomyelitis associated with vaccination. 

Paralytic poliomyelitis by vaccine-derived poliovirus (VDPV)

VDPVs affect non- or under-immunized populations living in areas with inadequate basic sanitation. If vaccination coverage against poliovirus remains low in the community and sanitation is inadequate (eg, no provision of safe water or sanitary disposal of excreta), VDPV isolated from the environment can be transmitted to susceptible populations, leading to emergencies. of the three types of VDPV.

Surveillance for acute flaccid paralysis (AFP)

Surveillance for AFP is the primary way to detect poliovirus disease. The quality of such surveillance is critical to achieving polio eradication. Unfortunately, underreporting of cases has been documented in the surveillance systems in the countries of the region, both in the notification of AFP, and in the collection and delivery of samples in the recommended time, with the forms properly filled out and follow-up of the cases, according to the recommendations. PFA surveillance should be supplemented by environmental surveillance, thus ensuring that surveillance sources of VDPV and potentially imported wPV are covered. The improvement in the sensitivity of surveillance activities is based on five pillars: effective detection, investigation, notification, monitoring and supervision of cases³¹.

Since it was demonstrated that it was possible to eradicate the disease, notification of AFP has remained below the expected cases in most Latin American countries. Figure 3 presents the notification rates of AFP cases in the Americas between 2019 and 2020. For this period, a total of 1424 AFP cases were reported³².

Source: ^{Reference 32}.

Figura 3 Notification rates (per100.000 under 15 year-olds) of acute placcid paralysis in the Americas by country, 2019-2020. a. From epidemiological week 40 in 2019 to epidemiological week 39 in 2020. Abbreviations correspond to the ISO code of each country; CAR = Caribbean; United States does not report data to the surveillance system; Equador and Uruguay did not submit data for the informed period. 

AFP notification rates do not reach the recommended minimum (1 per 100,000 population < 15 years) in most countries on the continent³². Only 4 countries (Mexico, Honduras, Costa Rica and Cuba; Figure 3, green bars) report rates higher than 1, while other 6 countries (Panama, Colombia, El Salvador, Paraguay, Nicaragua, Venezuela; Figure 3, yellow bars) approximate this rate, with values higher than 0.80. Nine countries and the Caribbean region report rates below 0.80 (Figure 3, red bars). Similarly, the recommended minimum number of samples is not achieved during the first 48 hours (80%) after notification³². In itself, this situation is worrisome, and it is aggravated by the pandemic of the disease caused by SARS- CoV-2, which has suspended and redirected numerous essential health services in Latin America and the Caribbean. In addition, when the borders are reopened, there is the possibility of re-introduction of wPV and/or cVDPV, especially with the suboptimal pre-pandemic vaccine coverage, which has been aggravated by the pandemic.

Some poliovirus environmental surveillance studies in the Americas show the effectiveness of switching from OPV to IPV in reducing the circulation of polioviruses. Between 2003 and 2005, the province of Córdoba, Argentina, implemented the change from OPV to IPV. Analysis at the end of the implementation cycle revealed that VDPV represented 19% of the positive environmental samples. Upon returning to the use of OPV the following two months, the percentage of VDPV in the environmental samples increased to 100%²⁵. Another study of 31 environmental samples in Rio de Janeiro, Brazil, between 2011 and 2012 found enterovirus in 27 samples, of which 8 were found to be Sabin type (type 1 = 1; type 2 = 5; type 3 = 2), with no evidence of VDPV or recombinations²⁶. More recently, between 2016 and 2017 in Port-au-Prince and Gonaïves, Haiti, non-polio enterovirus and Sabin poliovirus type 1 and 2 were found, but no strains of VPDV or wPV were detected³³.

Vaccine coverage in Latin America

According to PAHO, Latin American countries present vaccination coverages that are too low and heterogeneous –between 81% and 95%– which does not allow achieving vaccine efficacy¹⁴. In practice, these figures result in large populations at risk of a new poliomyelitis outbreak. For example, Ecuador reports that approximately 70% of its children are not vaccinated with the first dose of IPV, which puts more than 100,000 children at risk for wPV-2³⁵. Using data from 2019, UNICEF³⁵ estimated that Venezuela, with 55% coverage with one dose of IPV, has 226,000 children at risk of polio. The same year, the coverage with a dose of IPV from Brazil and Mexico were relatively higher (86% and 84%, respectively) but, due to the population density of these countries, it was estimated that the total number of children at risk of poliomyelitis is 400,000 in Brazil and 348,000 in Mexico³⁵.

2019-2023 Endgame Strategy for the eradication of poliomyelitis

The Endgame Strategy is based in three objectives during the 2019-2023 period¹⁵:

Although OPV remains extremely useful in the polio elimination process in the Americas, it is the leading cause of potential silent circulation of VDPVs²⁰. Until a vaccine without risk of reversal is available, the Endgame Strategy¹⁵ involves the exclusive use of three or more IPV doses with high vaccination coverage (≥ 95%)³⁴. For this reason, SLIPE presented a recommendation of four doses of IPV (3 in the primary series + 1 booster), either as monovalent or polyvalent in combination with other vaccines of the immunization program of each country in the region⁷.

To date, all Latin American countries have introduced at least one dose of IPV (Table 2). Five countries (Costa Rica, Mexico, Chile, Uruguay, and Argentina) have implemented the SLIPE recommendation of at least 4 doses of IPV. Except for Mexico, the rest of these countries have stopped using IPV entirely.

In October 2020, the WHO Strategic Advisory Group of Experts (SAGE) on Immunization recommended that all countries in the world using one dose of IPV switch to using two doses of IPV to increase protection against wPV-2³⁷. In Latin America, this recommendation affects the vaccination schedules of Bolivia, Haiti, Nicaragua, the Dominican Republic and Venezuela (Table 2).

Conclusions and Recommendations

Three key elements in Latin America significantly increase the risk of polio outbreaks: continued use of OPV, low vaccine coverage, and inefficient AFP surveillance. Thus:

Control and improvement of these three key elements in Latin America continue to be a priority in the process of eradication of poliomyelitis. Governments should do everything possible to ensure the proper functioning of these pillars, including: