Polio outbreaks in the post-COVID-19 pandemic era: causes and solutions

ABSTRACT

The coronavirus disease 2019 (COVID‐19) pandemic has disrupted polio immunization programs worldwide. The consequences of these programs’ suspension were not fully presented during the COVID-19 pandemic, as some take time to present in a population. We conducted a narrative review to provide a perspective of current literature on the effect of the COVID-19 pandemic on efforts made for poliomyelitis eradication. An overview of potential risks of polio outbreaks and areas where wild and vaccine-derived polioviruses have been reported will be presented in this review. Decreased vaccination rate, human and financial resources diversion to tackle COVID-19, and polio surveillance suspension during the COVID-19 pandemic contributed to creating an immunity gap and increasing the risk of polio outbreaks in at-risk areas. Approaches for integrating immunization efforts with educating the general population, engaging religious leaders, and solving gender disparities to fill the gap that have been made during the pandemic. The path to control polio should engage different levels of policy-making, and governments of affected countries play crucial roles. Strong interdisciplinary collaboration and dedicated efforts are needed to inform policymakers and encourage the public to follow vaccination programs.

Introduction

Poliomyelitis is a viral disease caused by the poliovirus (PV), a positive-strand virus categorized into three serotypes, namely PV1, PV2, and PV3 [1,2]. The virus multiplies in the oropharyngeal cavity and gastrointestinal tracts. The nasopharyngeal secretions might be contaminated for a couple of weeks. Moreover, this virus is present in feces for several weeks after infection [3]. The disease is primarily seen in infants and young children. Importantly, kids in poor hygienic conditions are susceptible to getting affected at earlier ages than others [3]. Many individuals infected with the virus will not manifest signs or symptoms. Some have flu-like symptoms like fever, fatigue, vomiting, and headache that usually last 2 to 5 days and disappear. Only a small number of individuals, less than 1%, experience weakness or complete loss of movement in their arms, legs, or both. Furthermore, one in 200 infections leads to irreversible acute flaccid paralysis (AFP), with a Case Fatality Rate of 5 to 10%, that could cause permanent disability and death. Death might occur when their breathing muscles become incapacitated [4].

In January 2020, the World Health Organization (WHO) declared COVID-19 a public health emergency of international concern. To aid in controlling the pandemic being controlled, the Global Polio Eradication Initiative (GPEI) stopped vaccination programs worldwide in March 2020 [5,6]. From March to September 2020, circulating vaccine-derived poliovirus 2 (cVDPV2) was detected in countries in two areas of the world, Africa and the Eastern Mediterranean Region (EMR). Furthermore, Africa dealt with simultaneous COVID-19 and cVDPV2 outbreaks, often within the same sub-national regions [7]. The COVID-19 pandemic has affected several countries’ monitoring and detecting cVDPV surveillance systems. This disruption in surveillance systems could lead to delayed detection and response to cVDPV outbreaks, resulting in more widespread virus transmission [8]. The disruption in the circulation of WPV1 transmission in Africa was declared by WHO in 25 August 2020, when the disruption of all indigenous wild polioviruses was reported [9]. The COVID-19 pandemic has affected many aspects of health system and disease monitoring [10].

This review, therefore, aims to provide a perspective of current literature on the effect of the COVID-19 pandemic on efforts made for poliomyelitis eradication, present an overview of the countries with poliomyelitis cases, and the proposed solutions specific to the post-COVID-19 era.

Method

We performed a comprehensive search across multiple electronic databases, including PubMed, Google Scholar, Scopus, and Web of Science. The search focused on articles published from 2018 to the present, utilizing targeted keywords related to COVID-19 and polio outbreaks. The results, limited to articles written in English, revealed a notable connection between COVID-19 and polio outbreaks after the pandemic. Moreover, to provide the latest and up-to-date information about the number of polio cases websites of WHO and the Global Polio Eradication Initiative were searched for relevant data in March 2023.

Classification

The Picornaviridae family of enteroviruses includes the genus Poliovirus, which has three serotypes: PV1, PV2, and PV3. These serotypes have minimal heterotypic immunity, which means that immunity to one serotype does not provide enough immunity to other serotypes [2,11]. Vaccine-derived poliovirus (VDPV) is a variant of poliovirus that has mutated from the virus in Oral Polio Vaccine (OPV). OPV contains a live, weakened poliovirus that duplicates in the intestine; after that, immunity is manifested through the formation of antibodies [2]. Recently, there has been an increase in the spread of VDPV2 in different areas of the world, especially in Africa [12].

VDPVs are mainly divided into three types: circulating vaccine-derived polioviruses (cVDPs), ambiguous VDPVs (aVDPVs), and immunodeficiency-associated VDPVs (iVDPVs) [13]. Although cVDPVs are rare, low immunization rates in communities have caused an increase in their reported rates in their reported incidences [14]. cVDPV2 gained more attention due to its higher rate of detection, however, cVDPV1 and cVDPV3 emergences are also detected in several regions, especially in Africa and EMR [15,16]. The spread of cVDPVs has led to many outbreaks in poor vaccine coverage countries [2] and is a significant risk related to using OPV. There is evidence that the live sabin oral vaccine has the potential risk of causing mutation resulting in the virulent phenotype when replicated in the human intestine [17].

In regards to iVDPV, the possibility of prolonged vaccine strain replication in people with B cell immunodeficiencies has been reported many decades ago [18]. In recent years, several studies have evaluated the rate of long-term excretion of VDPV in immunodeficient subjects to better define the risk of iVDPVs [19]. In addition to the increased risk of developing paralysis, subjects with iVDPV might be potential initiators of VDPV outbreaks, even though no outbreaks have been detected to be caused by iVDPV yet [20].

Wild poliovirus (WPV) is the most widely recognized form of the virus [14]. There are three strains of WPV, which are immunologically-distinct: WPV type 1 (WPV1), WPV type 2 (WPV2), and WPV type 3 (WPV3). All three strains cause irreversible paralysis and might even lead to death. However, there are genetic and virologic differences, which make these strains three distinct viruses. Thus, each needed to be eradicated separately. WPV3 is the second strain being wiped out, following the announcement of the WPV2 eradication in 2015 [21]. A summary of classification is available in Table 1.

Table 1.

Poliovirus classification and summary of types.

Realm:	Riboviria
Kingdom:	Orthornavirae
Phylum:	Pisuviricota
Class:	Pisoniviricetes
Order:	Picornavirales
Family:	Picornaviridae
Genus:	Enterovirus
Species:	Enterovirus C
Virus:	Poliovirus
Poliovirus Serotypes:	PV1, PV2, PV3
Vaccine-Derived Poliovirus (VDPV):	Variant mutated from Oral Polio Vaccine (OPV)
VDPV Types:	Circulating VDPVs (cVDPVs), Immunodeficiency-associated VDPVs (iVDPV), Ambiguous VDPVs (aVDPVs)
Wild Poliovirus (WPV)	WPV type 1 (WPV1), WPV type 2 (WPV2), WPV type 3 (WPV3)

Epidemiology

Countries with higher risk

According to GPEI, there are currently two endemic countries, Pakistan and Afghanistan, where indigenous WPV transmission has never been stopped [22]. Thirty five countries are known as outbreak countries in five regions, Africa, North America, Eastern Mediterranean, Europe, and South-East Asia [23]. Furthermore, eight countries are identified as key at-risk countries, no longer infected by poliovirus but are still at high risk of outbreaks. Due to the lower levels of surveillance in those areas, there is a considerable risk of polio returning. These countries are China, Congo, Guinea, Iran, Liberia, Kenya, Mali, Sierra Leone, Sudan, South Sudan, and Tajikistan [24].

Endemic countries

As described, polio has been endemic in Afghanistan and Pakistan since the 1970s, and they are the last remaining countries, where the wild poliovirus is still prevalent [25]. These two countries, which have never experienced the stop of indigenous wild poliovirus transmission [22], have faced challenges in interrupting poliovirus transmission due to various factors, including low vaccination coverage, inadequate sanitation and hygiene practices, and vaccination hesitancy in some communities. Moreover, Additionally, Afghanistan has faced challenges in health service access for the general population due to the war. Ongoing efforts to eradicate polio in Afghanistan and Pakistan, including extensive vaccination campaigns, surveillance, and community engagement efforts to ensure that all children are vaccinated and protected against polio [4,26–28].

Despite these efforts, the polio vaccination efforts in Afghanistan and Pakistan faced significant challenges during the COVID-19 pandemic. Measures to limit the spread of COVID-19, such as lockdowns and travel limitations, disrupted routine immunization campaigns, including those for polio. This led to decreased access to vaccination services, decreased immunization coverage, and increased vulnerability to polio outbreaks. Additionally, misinformation and vaccine hesitancy related to COVID-19 and polio complicated vaccination efforts [28,29]. Despite all efforts to address these gaps during the pandemic, the risk of increasing polio cases after controlling the pandemic is undeniable; and to offer a comprehensive set of actions and facilitate commitment to eradicate polio, the GPEI introduced the Polio Eradication Strategy 2022–2026, a structured guide containing the goals that are aimed to be achieved by 2026 [30].

Areas at risk

An outbreak is defined as the occurrence of disease cases at a higher rate than expected within a specific geographical area or community in a specific period [31]. GPEI defined outbreak countries as countries with stopped indigenous wild poliovirus but experiencing polio re-infection either through the importation of WPV or the VDPV importation, circulation, and emergence [23]. Herein, we provide a summary of the current status of poliovirus worldwide, according to GPEI reports in March 2023, with caution that countries mentioned might be at risk of future outbreaks.

North America

The steps toward global polio eradication by the Pan American Health Organization (PAHO) were taken in 1985 through an intensive immunization campaign aiming to eradicate all wild polioviruses in the Americas. The favorable outcome led to the eradication of all indigenous wild types of polioviruses by 1991 [32].

In the United States, which stopped administering the oral polio vaccine over three decades ago following eradication, is now considered a country with circulating vaccine-derived poliovirus following the report of a confirmed cVDPV2 poliomyelitis in an unvaccinated adult in New York on 21 July 2022 [33]. This was one month after the Centers for Disease Control and Prevention (CDC) reported discovering the virus in a New York suburb’s wastewater in June, suggesting the presence of poliovirus in feces [34]. In 2022, there was a sustained period of community poliovirus transmission in five counties of New York. Unvaccinated and under-vaccinated people in these areas are highly susceptible to paralytic disease [35].

In contrast, Canada has had no confirmed case of cVDPV2; however, this type of virus was isolated from two environmental samples taken on 27 and 30 August 2022 in a retrospective analysis – the samples collected from a large wastewater treatment plant and a site in Montreal, Quebec. Canada is also known as a country affected by cVDPV2. Interestingly, the genetic sequencing confirmed a link to the cVDPV2 currently challenging New York, U.S.A. [36].

Europe

In June 2002, WHO declared the European region polio-free, including the three types of wild poliovirus [37]. The oral polio vaccine was replaced with an injectable inactivated vaccine in 2004, and since then, the live oral polio vaccine has not been administered in the UK. However, in May 2022, after the isolation of VDPV2 from environmental samples in London and further confirmation of circulation of this strain by virological and epidemiological analysis, the UK is also now an affected country by cVDPV2. No cases of AFP associated with this strain have been detected so far [38]. According to the UK Health Security Agency report, the source was probably someone recently vaccinated overseas with the live virus [39].

The WHO 2010 introduced Ukraine as a high-risk country for polio [40]. It became another European country affected by cVDPV2 after two polio cases were reported in 2021 [41]. The ongoing humanitarian crisis and conflict have addressed evidence of risk for future polio and some other viral outbreaks [42,43]. In November 2022 to January 2023, in the Netherlands, WPV3-shedding event was reported from an environmental surveillance sample of poliovirus facilities. Out of 51 workers being exposed, one shedding was reported. The case was asymptomatic and isolated with proper follow-up. The study highlighted the importance of environmental surveillance strategy [44].

Eastern Mediterranean

In this area, Egypt, Yemen, Djibouti, and Somalia are all affected by cVDPV2, and Yemen is affected by cVDPV1 as well [23]. The circulating poliovirus in Egypt is shown to be linked to and originated from an outbreak in Sudan [45].

Yemen, a country where humans are affected by protracted war and conflicts, reported 87 cases of cVDPV2, with the latest onset of paralysis on 14 December 2022, is one of the most affected countries with high numbers of type 2 polio cases [46]. In a retrospective analysis covering VDPV1 cases from January to December 2020, 75% of all AFP cases were under five years old, and 73% had no previous doses of OPV [47]. Suggested aggravating factors for the outbreak of VDPV1 could be delays in lab confirmation and the response to the outbreak. Furthermore, the low polio immunization profile of children in that area made the population vulnerable to outbreaks. This outbreak highlights the need to maintain regular biweekly shipments to referral polio labs in the short term and the exploration of other options in the longer term to enable the Yemen National Lab to process national samples itself fully [41,42].

In Israel, one case of cVDPV2 was reported with the onset of paralysis on 13 February 2023. One case of polio has also been reported in 2022. This case was caused by cVDPV3 with the onset of paralysis on 12 February 2022. Both cases were linked to environmental samples positive for cVDPV, which were previously detected in this country. With these reported polio cases, Israel is now considered a country affected by both cVDPV2 and cVDPV3 [41]. Similar to the poliovirus detected from sewage in February 2022, the virus detected in Israel was Sabin-like [48].

South East Asia

After isolating poliovirus from a seven-year-old boy, who was a case of AFP with paralysis onset on 9 October 2022, Indonesia is now a country with confirmed cVDPV2. The latest report of ‘polio this week’ by the GPEI was a case of cVDPV2, with the onset of paralysis on 20 February 2023. A risk assessment and urgent response have been planned [49]. From March 2022 to March 2023, four cases of cVDPV2 were reported by WHO in Indonesia [46].

Africa

Being challenged by not just cVDPV1 and cVDPV2 but also WPV1, Africa is the most affected area by poliovirus [46] The COVID-19 pandemic has also resulted in more complications and caused a discontinuity in previous polio eradication efforts and vaccination campaigns in Africa [50]. Therefore, the current status should be considered a real threat that can reverse efforts and steps toward eradication [51].

According to the data in WHO Headquarter on 21 March 2023, by excluding viruses detected from environmental surveillance and considering only confirmed affected cases, Mozambique is the country co-affected by WPV1, cVDPV1, and cVDPV2 [52]. Unfortunately, 8 cases of WPV1, 21 cases of VDPV1, and 1 case of VSPV2 have been reported since March 2022 [46]. While WHO declared the disruption of all indigenous wild polioviruses in Africa by 25 August 2020 [9].

On 18 December 2022, WHO was notified of the detection of cVDPV2 in a 48-month-old male with AFP from Western Sudan [53]. The case had the onset of paralysis on October 31 [53,54]. The sequencing results confirmed 38 nucleotide changes. The isolated virus was found to be closely similar to the strain that was circulating in Nigeria detected in 2021 [53].

Effect of COVID-19

The immunity gap

Despite the benefits of reducing SARS-CoV-2 transmission via non-pharmaceutical interventions (NPI) during the COVID-19 pandemic, including social distancing, hand washing, and mask usage, these strategies have led to some negative outcomes like the immunity gap [55]. An immunity gap occurs in situations where isolation from a pathogen (e.g. SARS-CoV-2), reduces the exposure to other community-acquired infections, eliminating the opportunity to develop immunity against those pathogens [56,57]. Additionally, lockdowns resulted in reduced healthcare delivery leading to fewer children receiving vaccinations, thus widening the immunity gap among vaccine-preventable pathogens such as polio, measles, and influenza [58–61].

In the post-pandemic period, this gap could lead to large outbreaks of viruses such as influenza, respiratory syncytial virus (RSV), and polio, especially among pediatric populations, due to increased susceptibility during lockdown periods [59,62]. It is also hypothesized that these outbreaks might occur among children of older ages, as their first exposure will be at an older age, out of their regular season, with atypical manifestations mandating more attention. Poliovirus is among the pathogens about which the immunity gap occurs through isolation and reduced vaccination [59].

As the pandemic progressed, staff and resources that once were dedicated to polio surveillance and control across the globe continued tackling COVID-19, which could contribute to creating a new immunity gap for polio. This issue is being addressed by the WHO and governments [63]. In response, important steps are being taken by some countries; for example, the UK released 70 million euros from its 2019–2023 pledge to aid the polio program, aiming to close immunity gaps and combat outbreaks [64].

Decreased rate of vaccination

Vaccination has a proven, undeniable impact on public health, saving millions of people’s lives each year. However, this important factor was neglected during the COVID-19 pandemic, and various studies have reported decreased rates of routine vaccination in many countries [65]. It is estimated that approximately 25 million infants below one year did not receive their routine vaccinations in 2021; this was the highest number of unvaccinated infants since 2009. A drop in global vaccination coverage from 86 % in 2019 to 81 % in 2021 was reported [66]. Moreover, it is reported that disrupted immunization programs were evident in more than 68 countries, encompassing more than 80 million children worldwide [67]. The poliovirus vaccine coverage also declined, with a 6% drop in the global vaccination coverage rate from 86 % in 2019 to 80 % in 2021 for the Polio vaccine 3^rd dose and a decrease from 83% to 79% in the same time interval for the inactivated polio-containing vaccine, 1^st dose [68].

One reason for this reduction could be the postponement of polio vaccination programs until the second half of 2020, as recommended by GPEI and WHO, to prioritize the laboratories, staff, and resources for COVID-19 [69,70]. Furthermore, the door-to-door polio vaccination campaigns could increase the risk of COVID-19 transmission to children receiving the polio vaccine, their families, and the vaccinators [70,71]. The postponement policy, the fear of COVID-19 infection, and the reduced healthcare facilities are defined as mobility restrictions. The first two factors resulted in immunization delays, and the last one caused disruptions in the distribution of vaccines and delivery of immunization services. Despite the benefit of decreased polio importation during the mobility restriction periods, the immunization delivery was disturbed as the period lengthened, and the risk of expired or damaged vaccines rose [72]. Additionally, with the introduction of the SARS-CoV-2 vaccine and increased demand all over the world, the storage capacity concerning cold chain, and temperature of other vaccines, including polio, were replaced [72,73].

Regarding the vaccine demand side, refusal and hesitancy have negatively impacted vaccination programs for decades [74,75]. It can be caused by misconception, lack of knowledge, myth, religious belief, or antivaccine movements [72,75,76]. The hesitancy of vaccination due to refusal, conflict, and insecurity has also been reported in the polio vaccination programs in several countries, including Pakistan, a country that struggled hard to eradicate the polio virus before the pandemic [1,77]. A survey in Indonesia also reported that 23 % of parents with children under two years of age refused the vaccination programs and 13 % of them were hesitant [72]. The COVID-19 pandemic faced a new vaccine hesitancy regarding the polio vaccine, which is due to the fear of the SARS-CoV-2 virus exposure at any vaccine injection center [78].

Resource diversion

During the pandemic, as mentioned, a significant increase in the demand for human resources occurred, leading to the diversion of staff toward pandemic response efforts. The reallocation of polio staff and resources to address COVID-19 pandemic requirements has been noted in several studies [79–82]. Moreover, technical assistance, infrastructure, and national and sub-national level budgets were prioritized and reallocated to manage the critical situation [72].

Another critical aspect to consider is occupational burnout and COVID-19 infection among healthcare workers during the pandemic, potentially exacerbating the resource shortage. Economic constraints and the need to prioritize diagnostic resources for COVID-19 also led governments to reallocate their budgets, significantly impacting polio vaccination programs and outbreak management [72,83].

Surveillance suspension

Polio surveillance is known to be one of the vital pillars of eradication, according to the GPEI. It has three types of poliovirus recognition, including the primary surveillance approach (surveillance for AFP cases), environmental surveillance, and polio surveillance in patients with primary immunodeficiency disorders (PIDs) [84].

A decrease in the reported number of AFP cases was observed from 81,439 in the first nine months of 2019 to 54,631 during the same period in 2020 [80]. It is also reported that there has been a simultaneous reduction in the rates of non-polio AFP, polio specimen sufficiency, and immunization coverage over the last 10 years, which reached their lowest rates in 2020, the middle of the COVID-19 pandemic [72].

During the COVID-19 pandemic, surveillance activities were limited due to the domestic lockdowns and the national and international borders closing and preventing the specimens from arriving at the testing laboratories, resulting in delayed detection of polio outbreaks [84,85]. The factors previously discussed regarding resource diversion also impacted AFP surveillance through the lack of AFP case detection due to AFP surveillance disruptions. Eventually, the combination of undetected AFP with the low quality of samples caused by delays in sample delivery during the mobility restriction period led to a decrease in the efficiency of AFP surveillance [72].

Polio eradication efforts: Focus on what has been done

In 1955, the announcement of an effective vaccine heralded a promising future in preventing polio [86]. The inactivated polio vaccine (IPV) is made from dead poliovirus and has been used internationally since the 1950s. It is also given through injection and is highly effective in preventing polio. The live attenuated oral polio vaccine (OPV) was introduced in 1960 to provide additional protection against the disease [87]. Based on current estimations, polio vaccines prevented 5 million cases of polio-related paralysis from 1960 to 1987 and 24 million cases from 1988 to 2021 worldwide [88].

The eradication campaign was established in 1988 when the wild poliovirus was endemic in 125 countries [89]. The Global Polio Eradication Initiative, a resolution adopted by the World Health Assembly since 1988, has led to the eradication of WPV2 and WPV3 and a 99.99% decrease in cases [90]. Furthermore, GPEI has prevented 2.5 to 6 million cases of paralytic polio since WHO launched it in 1988 [88].

The GPEI introduces four main points for polio eradication: 1. Routine immunization with OPV or/and IPV (Exception: In developing and endemic countries, four doses of OPV are administered in the first year of life for high immunization coverage) 2. Supplementary immunization: on occasions like national immunization days, administering booster doses of the OPV vaccine to all children under the age of five. 3. Surveillance: Vigilant monitoring for WPV through announcing and laboratory testing of all cases of intense AFP and loss of motion among children under fifteen. 4. Targeted campaigns: Campaigns aiming at limiting and eradicating the WPV in a specific area [91].

During the first nine months of 2020, a 33% decline in reported AFP cases compared with the same period in 2019 was indicated by polio surveillance. However, this reduction is believed to be due to a decline in the quality of case detection and increased interval from stool collection to laboratory analysis and not a decrease in a true number of polio patients [80].

Solution and suggestion for the post-pandemic era: Focus on what should be done

Vaccination and education

UNICEF vaccination program

UNICEF conducts large-scale, high-quality immunization campaigns to boost immunity and beneficially prevent the spread of the illness for any outbreaks. While responding to polio outbreaks, UNICEF has worked to ensure the commitment and support of governments, as well as helping nations manage their budgets and resources. Moreover, through the deployment of rapid response groups, they tried to supply and distribute polio vaccines and clarify vaccine misinformation. These rapid response groups are specialized teams composed of community health care workers who are mobilized quickly to areas experiencing or at risk of polio outbreaks to manage vaccine misinformation and distribute polio vaccines. Polio-affected regions are often the most underserved and marginalized communities globally, lacking access to essential services, including healthcare and water. WASH (Water, Sanitation, and Hygiene) interventions have a significant potential to prevent poliovirus infections by improving overall sanitary conditions and reducing the spread of the virus. Targeted polio vaccination is often the only service these areas receive regarding poliomyelitis management, which might result in mistrust and refusal of vaccination from parents, thus threatening worldwide polio eradication goals. As a solution, UNICEF has used social and behavioral change (SBC) strategies to increase demand and acceptance for the polio vaccine’s essential role among families and caregivers. Locally-based UNICEF polio volunteers and workers maintain connections with communities, visiting each family to effectively address questions, dispel anti-vaccine misconceptions, and encourage parents to vaccinate their children [92–94].

A successful collaboration between UNICEF and a non-governmental organization (NGO) in India focused on behavioral approaches at individual and community levels to eradicate polio. This initiative, part of the ‘CORE Group Polio Project,’ involved coordinators organizing meetings with mothers and influencers to educate and promote immunization and similar health-seeking behaviors. Influential figures, including politicians, religious leaders, medical doctors, artists, and athletes, supported the network and participated in educating the general population, helping to guide families’ decisions and actions toward vaccination [95].

Boosting vaccination effort

Countries like the US, UK, and Israel are intensifying their vaccination efforts to address the gaps created during the COVID-19 pandemic. In the UK, there is an ambitious plan to vaccinate all children from one to nine years old in London, an approach believed to be capable of halting a poliovirus outbreak before it develops. However, the campaigns in New York and London have utilized injectable vaccines, which may not prevent virus transmission. Consequently, exploring other options is considered crucial. For example, in 2020, the WHO listed a novel oral attenuated polio vaccine for emergency use. This vaccine, designed using knowledge of the virus genome and incorporating genetic changes to prevent the virus from regaining neurovirulence, has not undergone large-scale human testing and is yet to be approved by US or UK regulators. Nevertheless, it has been administered to approximately 100 million individuals without reports of vaccine-derived poliovirus emergence [96,97].

Gender equality and gender dynamics

In certain communities, male polio workers are not allowed to enter homes, increasing the need for female volunteers and impacting the dynamics of frontline operations, a phenomenon known as gender dynamics. Female health workers are crucial for the success of immunization campaigns in these settings, as they can access households and communicate with them more effectively. However, female healthcare workers often face challenges, such as limited opportunities for promotions to supervisory or managerial roles. It is reported that a lack of diversity in leadership and management negatively affects the quality of delivery, planning, and accountability. Gender power relations play a crucial role in the success of international health programs at all levels [93,98].

Gender-related barriers can also impact surveillance steps, causing delays in case detection. Issues may arise from the onset of paralysis to care-seeking, potentially due to a shortage of female healthcare providers, their limited decision-making power, or low awareness. Discriminatory attitudes toward female patients in some regions, such as prioritizing boys for care, can contribute to these challenges. Delays in notification may occur due to low awareness among women healthcare providers or unresponsiveness in medical hierarchies to female care worker reports. Furthermore, case investigation delays could be due to a shortage of female caregivers allowed to enter homes or security concerns for women in certain areas [84]. Increasing female health workers’ contributions can resolve many mentioned problems and also motivate more women to join the programs. Ensuring safe working conditions and addressing security concerns are vital to maintaining the participation of female workers, particularly in conflict or high-risk areas.

Role of religious leaders

Religious beliefs significantly influence health-seeking behaviors, including vaccination willingness [99]. Concerns among community members and religious leaders about the safety and efficacy of polio vaccinations have contributed to vaccine hesitancy in some societies [100]. This skepticism has directly affected parents’ vaccination decisions in countries like Ghana [100], Nigeria [101], and India [102], and is not confined to any specific religion. Misinformation, which often spreads quickly and influences emotions, reinforces these doubts. Engaging religious leaders is crucial to address emotional reactions and misinformation, thereby increasing vaccine demand in communities [100].

A successful example of this approach was in a UNICEF program in Uttar Pradesh, India. Here, religious leaders played a key role by delivering speeches during Friday prayers and making regular announcements in mosques. They focused on shaping social values, promoting health-seeking behaviors, encouraging voluntary work, and discouraging vaccination hesitancy. This approach was pivotal to the program’s success. Reports from the UNICEF program in India suggest that involving religious leaders from the beginning of any intervention is essential, as misconceptions become more challenging to address once they have spread [102].

Need for new vaccine

There is a pressing need for a new polio vaccine to more effectively prevent virus transmission. The novel oral polio vaccine type 2 (nOPV2), which received emergency approval from the WHO, is designed to be safer than its predecessor. Unlike the old vaccine, nOPV2 is ‘triple-locked’ through genetic engineering to prevent it from becoming harmful. Andino and Andrew Macadam, who began working on this vaccine in 2011, focused on altering ribonucleic acids in the vaccine. By strategically changing some bases, they made it difficult for the poliovirus to revert these alterations, significantly reducing the risk of the vaccine causing polio. No mutations were observed in animal models, cell cultures, or humans, and efforts were made to prevent virus recombination and slow evolution [103,104]. Since its introduction in March 2021, more than 525 million doses of nOPV2 have been administered in 25 countries, with the majority used in Nigeria, to address the increasing risk of cVDPV2 [105].

Interventions to break the transmission chain

Global polio eradication initiative surveillance

Detecting new outbreaks as soon as possible is essential for a prompt and effective outbreak response, as global efforts to eradicate poliovirus have reached significant milestones. Surveillance is based on reporting AFP in individuals younger than 15 years old, as well as laboratory confirmation through the isolation of the virus from stool samples. However, delayed testing and sample collection can hinder timely outbreak detection [106]. In fact, without surveillance, it is nearly impossible to ascertain the current status of polio circulation [107]. Polio surveillance identifies new cases and detects any circulation of poliovirus. In some regions, surveillance is enhanced by environmental surveillance, which involves periodic testing and collection of sewage samples to detect the presence of the virus. Surveillance faces challenges due to numerous asymptomatic cases and multiple causes of AFP, such as toxins, trauma, and enteroviruses. Therefore, testing stool samples in the laboratory is crucial to confirm the presence of poliovirus [108–110].

To maintain a sustained polio-free state in the world, a strategy with four defined goals has been introduced. The second goal of this post-certification strategy emphasizes the need for surveillance and outlines how to address prolonged poliovirus excretion among individuals with primary immunodeficiency disorders [111,112].

Importantly, global polio eradication programs should integrate and coexist with pandemic responses. National guidelines should be designed to resume services, as demonstrated by Indonesian health authorities who released technical guides for immunization services during the pandemic in 2020. Moreover, all vaccinators should be trained in COVID-19 control precautions, including using protective equipment, designated waiting areas, and safe handling of injection waste. Decisions should be made at the local and provincial levels, based on the specific context [72,113,114].

According to the strategic plans introduced by GPEI, two main types of surveillance are recommended. The first, AFP surveillance, involves finding children with AFP and transporting their fecal specimens for further analysis, isolating and defining the type of the virus, i.e. wild or vaccine-derived. The second type, known as environmental surveillance, involves testing sewage and other environmental samples to detect the presence of the virus. Detection of wild poliovirus infections in the absence of AFP cases is often confirmed through environmental surveillance [107]. Environmental surveillance has shown advantages in responding to ongoing cVDPV outbreaks and is suggested to be extensively conducted to complement AFP surveillance, as it has demonstrated significantly higher sensitivity [115]. The role of vaccination campaigns is critical for initiative surveillance and responding to polio outbreaks. The campaigns are helpful to monitor vaccination coverage and early detection of poliovirus cases.

In the polio eradication strategy 2022–2026, integrating polio surveillance with other vaccine-preventable disease surveillance systems is mentioned as a key suggestion for the post-COVID-19 pandemic world, in particular in countries with no active outbreaks [30]. Furthermore, the global polio surveillance action plan 2022–2024 aims to address subnational gaps in identifying AFP cases, improve the timeline for specimen transport, train skilled healthcare providers, implement focused monitoring and evaluation activities, and integrate AFP surveillance [84].

As mentioned, polio outbreaks could be initiated by iVDPVs. Thus, specific activities are outlined to address iVDPV surveillance in the GPSAP 2022–2024. The development of iVDPV surveillance is currently underway, particularly in countries at high risk of iVDPV, such as Iran, China, Egypt, and India. The plan’s activities mainly involve assessing pilot studies, developing long-term plans, and increasing access to antiviral therapies [84,116].

Wastewater surveillance

Wastewater surveillance is of special importance in regards to polio control, as demonstrated by the polio emergency in New York, where the virus was detected using this method [117]. In September 2020, the CDC established the National Wastewater Surveillance System (NWSS) in response to the COVID-19 pandemic. The virus can be detected in the feces of individuals infected with SARS-CoV-2, even if they are asymptomatic. Thus, the wastewater system is able to provide early warning of COVID-19 spreading in a community. Consequently, communities and health departments can react quickly to prevent the spread of COVID-19. This vital information on the prevalence of COVID-19 could improve public health strategies [118,119]. The crucial role of environmental surveillance is not limited to the US, as a very recent article showed the benefits of this system in Japan. The latest findings are indicative that the existing polio environmental surveillance system can be effectively used for SARS-CoV-2 sewage monitoring. Having a surveillance system that is beneficial in controlling both viral diseases, offers a cost-effective way to prevent future outbreaks in the post-COVID-19 era [120].

Conclusion

In this review, we provided a thorough overview of factors that weakened polio immunity status during the COVID-19 pandemic, which might cause several outbreaks after the pandemic. We also suggested solutions based on WHO and GPEI guidelines. Given the complexity of this public health issue, as mentioned, several factors such as lack of polio staff and resources diversion, vaccination cessation, and surveillance suspension have weakened the polio status globally.

Efforts should be intensified to polio outbreaks in various areas, as major polio eradication projects that ceased during the pandemic should be resumed. Cultural and gender-related challenges are also inevitable factors that must not be neglected when devising new programs for the post-COVID-19 pandemic era. Maintaining high vaccination coverage and developing alternative polio vaccines that eliminate the risk of vaccine-derived poliovirus transmission should be priories in planning. Enhancing the effectiveness of polio eradication efforts require a multifaceted approach. For registered dietitian nutritionists and other health professionals, the integration of immunization efforts with nutritional education is essential to bolster overall community health. Policymakers must invest in sustainable vaccination campaigns and ensure that resources are allocated effectively to reach all at-risk populations. In close cooperation with the health systems of impacted countries, we hope that lost ground for polio eradication during the COVID-19 pandemic can be regained, thereby preventing future outbreaks.

Funding Statement

The author(s) reported there is no funding associated with the work featured in this article.

Abbreviations

WHO

World Health Organization

GPEI

Global Polio Eradication Initiative

cVDPV2

Circulating vaccine-derived poliovirus 2

EMR

Africa and the Eastern Mediterranean Region

WPV

Wild Polio virus

VDPV

Vaccine-derived poliovirus

IPV

Inactivated polio vaccine

OPV

Oral polio vaccine

AFP

Acute flaccid paralysis

aVDPVs

Ambiguous VDPVs

iVDPVs

Immunodeficiency-associated VDPVs

VAPP

Vaccine-associated paralytic poliomyelitis

NPI

Non-pharmaceutical interventions

RSV

Respiratory syncytial virus

PIDs

Primary immunodeficiency disorders

NGO

Nongovernmental organization

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contributions

All listed authors have made a significant scientific contribution to the research in the manuscript approved its claims and agreed to be an author.

DA provided the study concept. DA, HK, NM, and KK drafted the manuscript. DA, NS, and MM critically revised the manuscript for important intellectual content and approved the final version to be published. MM and NS supervised the study. All approved the final version to be published.