Progress Toward Poliomyelitis Eradication - Pakistan, January 2020-July 2021

Mucosal Immunity to Poliovirus in Children 0-15 Years of Age: A Community-Based Study in Karachi, Pakistan in 2019

This study assesses poliovirus type 1 (PV1) immunity in children to inform the contribution of mucosal immunity in and prevention of poliovirus circulation. A community-based study was conducted in periurban Karachi, Pakistan. Randomly selected children (0-15 years of age) received oral poliovirus vaccine (OPV) challenge dose. Blood and stool samples were collected at several time points and evaluated for polio-neutralizing antibodies and serotype-specific poliovirus, respectively. Eighty-one of 589 (14%) children excreted PV1 7 days post-OPV challenge; 70 of 81 (86%) were seropositive at baseline. Twelve of 610 (2%) were asymptomatic wild poliovirus type 1 (WPV1) excretors. Most poliovirus excretors had humoral immunity, suggesting mucosal immunity in these children likely waned or never developed. Without mucosal immunity, they are susceptible to poliovirus infection, shedding, and transmission. Asymptomatic WPV1 excretion suggests undetected poliovirus circulation within the community.

Progress Toward Poliomyelitis Eradication - Pakistan, January 2022-June 2023

Since the establishment of the Global Polio Eradication Initiative in 1988, Pakistan remains one of only two countries (along with Afghanistan) with continued endemic transmission of wild poliovirus (WPV). This report describes Pakistan's progress toward polio eradication during January 2022-June 2023. During 2022, Pakistan reported 20 WPV type 1 (WPV1) cases, all of which occurred within a small geographic area encompassing three districts in south Khyber Pakhtunkhwa. As of June 23, only a single WPV1 case from Bannu district in Khyber Pakhtunkhwa province has been reported in 2023, compared with 13 cases during the same period in 2022. In addition, 11 WPV1 isolates have been reported from various environmental surveillance (ES) sewage sampling sites to date in 2023, including in Karachi, the capital of the southern province of Sindh. Substantial gaps remain in the quality of supplementary immunization activities (SIAs), especially in poliovirus reservoir areas. Despite the attenuation and apparently limited geographic scope of poliovirus circulation in Pakistan, the isolation of WPV1 from an ES site in Karachi is cause for concern about the actual geographic limits of transmission. Interrupting WPV1 transmission will require meticulous tracking and sustained innovative efforts to vaccinate children who are regularly missed during SIAs and rapidly responding to any new WPV1 isolations.

Update on Wild Poliovirus Type 1 Outbreak - Southeastern Africa, 2021-2022

Since the Global Polio Eradication Initiative (GPEI) began in 1988, the number of wild poliovirus (WPV) cases has declined by >99.99%. Five of the six World Health Organization (WHO) regions have been certified free of indigenous WPV, and WPV serotypes 2 and 3 have been declared eradicated globally (1). WPV type 1 (WPV1) remains endemic only in Afghanistan and Pakistan (2,3). Before the outbreak described in this report, WPV1 had not been detected in southeastern Africa since the 1990s, and on August 25, 2020, the WHO African Region was certified free of indigenous WPV (4). On February 16, 2022, WPV1 infection was confirmed in one child living in Malawi, with onset of paralysis on November 19, 2021. Genomic sequence analysis of the isolated poliovirus indicated that it originated in Pakistan (5). Cases were subsequently identified in Mozambique. This report summarizes progress in the outbreak response since the initial report (5). During November 2021-December 2022, nine children and adolescents with paralytic polio caused by WPV1 were identified in southeastern Africa: one in Malawi and eight in Mozambique. Malawi, Mozambique, and three neighboring countries at high risk for WPV1 importation (Tanzania, Zambia, and Zimbabwe) responded by increasing surveillance and organizing up to six rounds of national and subnational polio supplementary immunization activities (SIAs).* Although no cases of paralytic WPV1 infection have been reported in Malawi since November 2021 or in Mozambique since August 2022, undetected transmission might be ongoing because of poliovirus surveillance gaps and testing delays. Efforts to further enhance poliovirus surveillance sensitivity, improve SIA quality, and strengthen routine immunization are needed to ensure that WPV1 transmission has been interrupted within 12 months of the first case, thereby preserving the WHO African Region's WPV-free status.

Progress Toward Poliomyelitis Eradication - Afghanistan, January 2021-September 2022

Afghanistan and Pakistan are the two remaining countries with endemic wild poliovirus type 1 (WPV1) transmission (1). During 2019-2020, these countries reported their highest numbers of WPV1 cases since 2014 and experienced outbreaks of type 2 circulating vaccine-derived poliovirus (cVDPV2) (2-4).* In Afghanistan, the number of WPV1 cases nearly doubled, from 29 in 2019 to 56 in 2020; 308 cVDPV2 cases were reported during 2020. After years of active conflict, the Afghanistan government was fully replaced by the Taliban de facto government on August 15, 2021. This report describes activities and progress toward polio eradication in Afghanistan during January 2021-September 2022 and updates previous reports (3,4). During January-December 2021, four WPV1 and 43 cVDPV2 cases were detected, representing decreases of 93% from 56 cases and 86% from 308 cases, respectively, during 2020. During January-September 2022 (reported as of October 20), two WPV1 cases and zero cVDPV2 cases were detected. Although no supplementary immunization activities (SIAs)^† occurred during July-October 2021, SIAs resumed during November 2021 in all districts after the political transition, and 3.5-4.5 million previously unreachable persons have been vaccinated since. However, restrictions on how SIAs are conducted are still in place in the critical South Region provinces of Kandahar, Helmand, and Uruzgan. If efforts to vaccinate all children are enhanced and expanded, Afghanistan has an opportunity to interrupt WPV1 transmission during 2023.

Global polio eradication; can we replicate the smallpox success story?

Although the Global Polio Eradication Initiative has been largely successful in elimination of polio from various parts of the world, sporadic local outbreaks in non-endemic areas continue to pose a threat to global polio eradication efforts. In the two endemic countries, Pakistan and Afghanistan, a staggering 176 cases of wild poliovirus 1 (WPV1) were reported in 2019. In 2020 alone, 959 cases of Circulating Vaccine Derived Poliovirus 2 were reported globally from 27 countries. After staying polio-free for years, cases of WPV were detected in Malawi and Mozambique in 2022. The roots of the reported strains matched with the WPV strain from Pakistan. The emergence of WPV cases in Malawi and Mozambique underscores the fact that WPV still has the chance to spread beyond the Afghanistan-Pakistan region and sustained efforts are required for its complete eradication. In the case of smallpox, surveillance-containment was the key to eradication as many countries had already eradicated smallpox and the bigger concern was to track and contain any new cases emerging. Smallpox eradication followed a comprehensive plan which included elements like quality control and standardisation of vaccination protocols. Governments all over the world should prioritise immunisation drives, surveillance, and awareness campaigns to achieve the dream of a polio-free world.

The fight against poliomyelitis through the history: past, present and hopes for the future. Albert Sabin's missing Nobel and his "gift to all the world's children"

Polio, or poliomyelitis, is a disabling and life-threatening disease caused by three poliovirus (PV) serotypes. The virus spreads from person to person and can infect a person's spinal cord, causing paralysis. In 1988, when the WHO registered 350,000 cases of poliomyelitis in the world and 70,000 which occurred in Africa alone, global poliomyelitis eradication was proposed by the World Health Organization to its member States. On 25 August 2020, while the world was waging war against the Coronavirus pandemic, a historic milestone was reached: Africa was officially declared polio-free. It is an important result obtained thanks to an intensive large-scale vaccination campaign. The road was far from smooth, nevertheless, according to the WHO, a great effort needs to be made in order to facilitate access to vaccination and to promote its implementation in those countries where coverage is low and vaccine hesitancy is high because the risk of the spread of poliomyelitis is still relevant. Eradication of the virus in Africa provides us with an excellent opportunity to commemorate the many scientists who contributed to achieving this epoch-making goal: first of all, Jonas Salk, who developed a killed-virus vaccine in 1952, and, especially, Albert Sabin, who in 1961 launched programs of mass immunisation with his oral vaccine against poliomyelitis.

Progress Toward Poliomyelitis Eradication - Pakistan, January 2021-July 2022

After reporting a single wild poliovirus (WPV) type 1 (WPV1) case in 2021, Pakistan reported 14 cases during April 1-July 31, 2022. Pakistan and Afghanistan are the only countries where endemic WPV transmission has never been interrupted (1). In its current 5-year strategic plan, the Global Polio Eradication Initiative (GPEI) has set a goal of interrupting all WPV1 transmission by the end of 2023 (1-3). The reemergence of WPV cases in Pakistan after 14 months with no case detection has uncovered transmission in southern Khyber Pakhtunkhwa province, the most historically challenging area. This report describes Pakistan's progress toward polio eradication during January 2021-July 2022 and updates previous reports (4,5). As of August 20, 2022, all but one of the 14 WPV1 cases in Pakistan during 2022 have been reported from North Waziristan district in Khyber Pakhtunkhwa. In underimmunized populations, excretion of vaccine virus can, during a period of 12-18 months, lead to reversion to neurovirulence, resulting in circulating vaccine-derived polioviruses (cVDPVs), which can cause paralysis and outbreaks. An outbreak of cVDPV type 2 (cVDPV2), which began in Pakistan in 2019, has been successfully contained; the last case occurred in April 2021 (1,6). Despite program improvements, 400,000-500,000 children continue to be missed during nationwide polio supplementary immunization activities (SIAs),* and recent isolation of poliovirus from sewage samples collected in other provinces suggests wider WPV1 circulation during the ongoing high transmission season. Although vaccination efforts have been recently complicated by months of flooding during the summer of 2022, to successfully interrupt WPV1 transmission in the core reservoirs in southern Khyber Pakhtunkhwa and reach the GPEI goal, emphasis should be placed on further improving microplanning and supervision of SIAs and on systematic tracking and vaccination of persistently missed children in these reservoir areas of Pakistan.

Outbreaks of Circulating Vaccine-derived Poliovirus in the World Health Organization Western Pacific Region, 2000-2021

The World Health Organization Western Pacific Region (WPR) has maintained the polio-free status for more than two decades. At the global level, there were only 6 confirmed polio cases due to wild type 1 poliovirus in Pakistan, Afghanistan, and Malawi in 2021, therefore, the risk of the importation of wild poliovirus from the endemic countries to the WPR is considerably lower than ever before. On the other hand, the risk of polio outbreaks associated with circulating vaccine-derived polioviruses (cVDPVs) still cannot be ignored even in the WPR. Since late 2010s, cVDPV outbreaks in the WPR have appeared to be more extensive in frequency and magnitude. Moreover, the emergence of concomitant polio outbreaks of type 1 and type 2 cVDPVs in the Philippines and Malaysia during 2019-2020 has highlighted the remaining risk of cVDPV outbreaks in high-risk areas and/or communities in the WPR. The previous cVDPV outbreaks in the WPR have been rapidly and effectively controlled, however, the future risk of polio outbreaks associated with cVDPVs needs to be reconsidered and polio immunization and surveillance strategies should be updated accordingly.

Surveillance to Track Progress Toward Polio Eradication - Worldwide, 2020-2021

Since the Global Polio Eradication Initiative (GPEI) was established in 1988, the number of reported poliomyelitis cases worldwide has declined by approximately 99.99%. By the end of 2021, wild poliovirus (WPV) remained endemic in only two countries (Pakistan and Afghanistan). However, a WPV type 1 (WPV1) case with paralysis onset in 2021, was reported by Malawi a year after the World Health Organization (WHO) African Region (AFR) was certified as WPV-free and circulating vaccine-derived poliovirus (cVDPV) cases were reported from 31 countries during 2020-2021 (1,2). cVDPVs are oral poliovirus vaccine-derived viruses that can emerge after prolonged circulation in populations with low immunity and cause paralysis. The primary means of detecting poliovirus transmission is through surveillance for acute flaccid paralysis (AFP) among persons aged <15 years, with confirmation through stool specimen testing by WHO-accredited laboratories, supplemented by systematic sampling of sewage and testing for the presence of poliovirus (environmental surveillance). The COVID-19 pandemic caused disruptions in polio vaccination and surveillance activities across WHO regions in 2020; during January-September 2020, the number of reported cases of AFP declined and the interval between stool collection and receipt by laboratories increased compared with the same period in 2019 (3). This report summarizes surveillance performance indicators for 2020 and 2021 in 43 priority countries* and updates previous reports (4). In 2021, a total of 32 (74%) priority countries^† met two key surveillance performance indicator targets nationally, an improvement from 2020 when only 23 (53%) met both targets; however, substantial national and subnational gaps persist. High-performing poliovirus surveillance is critical to tracking poliovirus transmission. Frequent monitoring of surveillance indicators could help identify gaps, guide improvements, and enhance the overall sensitivity and timelines of poliovirus detection to successfully achieve polio eradication.