Progress Toward Poliomyelitis Eradication — Pakistan, January 2016–September 2017

Immunization Activities

Based on United Nations Children’s Fund (UNICEF) and World Health Organization (WHO) estimates, national vaccination coverage among infants with 3 doses of oral poliovirus vaccine (OPV [OPV3]) delivered through the routine immunization program was 72% in 2016, unchanged from 2014 and 2015 estimates (5). Administrative coverage with OPV3, calculated as the number of vaccine doses administered divided by the estimated target population, varied substantially by province.

Vaccination histories (based on immunization cards and parental recall) among children aged 6–23 months with acute flaccid paralysis (AFP) whose stool specimens tested negative for poliovirus (nonpolio AFP cases), are also used to estimate OPV coverage in target populations. The percentage of nonpolio AFP cases among children aged 6–23 months nationwide who had never received any OPV doses through routine immunization services or supplemental immunization activities (SIAs)* (i.e., zero-dose children) decreased from 2.1% in 2015 to 0.3% in 2016 and to 0.01% in 2017; the percentage who had received ≥4 OPV doses increased slightly from 96% in 2016 to 97% in 2017. The highest percentage of zero-dose children was recorded in the province of Balochistan during 2016 (3%) and 2017 (2%), and the lowest percentage was recorded in the province of Punjab (approximately 0% in both 2016 and 2017).

During January 2016–September 2017, 22 SIAs were conducted using bivalent OPV (bOPV; vaccine virus types 1 and 3), including eight full national immunization days and 14 subnational immunization days. To further boost the population immunity and enhance the prospects of interrupting of WPV transmission, injectable inactivated poliovirus vaccine (IPV) has been used in 10 SIAs conducted at fixed immunization posts since January 2016, reaching >9 million children in the provinces of Sindh, Federally Administered Tribal Areas (FATA), Balochistan, and Khyber Pakhtunkhwa.

During 2016, six SIAs using only IPV and targeting children aged <2 years were conducted in WPV1 core reservoir districts in Punjab, Khyber Pakhtunkhwa, FATA, Balochistan, and Sindh provinces. During the first two quarters of 2017, 14 SIAs (four national immunization days and 10 subnational immunization days), using both bOPV and IPV and targeting children aged <5 years, were conducted in the core polio reservoir districts in Khyber Pakhtunkhwa, FATA, Balochistan and Sindh. Administrative coverage with OPV3, calculated as the number of vaccine doses administered divided by the estimated target population, varied substantially by province.

In addition to SIAs, other initiatives have been established to help improve vaccination coverage in high-risk union councils (i.e., subdistricts) of the core polio reservoir districts. One initiative involves the use of community-based vaccinators to reach unvaccinated children in high-risk areas. Community-based vaccinators are recruited locally and focus on community engagement to vaccinate children on an ongoing basis, rather than solely during SIAs.

Surveillance Activities

AFP Surveillance. During 2016, a total of 7,847 AFP cases were reported in Pakistan; the highest number of cases was reported from the province of Punjab (3,939) and the lowest from the province of Gilgit-Baltistan (17) (Table). During 2016, the annual nonpolio AFP^† rate per 100,000 population aged <15 years was 12.6 nationally, ranging from 2.5 to 30.7 among the seven provinces of Pakistan (Table). The percentage of AFP cases with adequate stool specimens^§ was 87.4%, ranging from 71% (Gilgit-Baltistan) to 94% (Islamabad). For 2017, the annualized national AFP rate was 13.9. The percentage of AFP cases with adequate stool specimens was 88% (provincial range = 85%–93%), and the minimum target of 80% stool specimen adequacy was met in all provinces.

Environmental Surveillance. Periodic testing of sewage samples for poliovirus at designated sites in the provinces of Punjab, Islamabad, Sindh, Khyber Pakhtunkhwa, and Balochistan supplements AFP surveillance. The number of environmental samples collected during January–September 2017 increased 34% compared with the same period in 2016. During January–September 2017, WPV was detected in 72 (15%) of 468 environmental samples from 53 sampling sites within 34 districts, compared with 36 (9%) of 348 environmental samples from 41 sampling sites during the same period in 2016.

In 2016, four environmental surveillance samples tested positive for circulating vaccine-derived poliovirus (cVDPV) type 2 (cVDPV2)^¶ in the province of Balochistan (one in the Pishin district in April 2016, and three in the Quetta district during March, April, and May 2016). No cVDPVs have been isolated from samples collected in 2017 to date.

WPV and VDPV Epidemiology

In 2016, Pakistan reported 20 WPV1 cases; as of September 25, 2017, five cases have been reported for 2017, representing a 69% decrease from the 16 cases reported during the same period in 2016. The WPV1 cases reported in 2017 occurred in Punjab, Gilgit-Baltistan, Sindh, Khyber Pakhtunkhwa, and Balochistan provinces. (Figure 2). During 2016, WPV1 cases were reported from 14 districts, compared with only five districts to date in 2017.

All five WPV1 cases reported in 2017 occurred among children aged <36 months. Only one of these five children had never received a dose of OPV, compared with one of 14 WPV1 cases reported during January 2016–August 2016, and 12 (35%) of 34 WPV1 cases reported during the same period in 2015. A second WPV1 case in 2017 occurred in a child who had received no OPV through routine immunization services, but had received three OPV doses through SIAs.

Concomitant with the decrease in the number of WPV1 cases, transmission of several genetic lineages detected in 2015 was apparently interrupted during the reporting period, particularly during the second half of 2016 and first half of 2017 (1). WPV1 isolates from at least two main genetic clusters (groups of polioviruses sharing ≥95% sequence identity in the viral capsid protein VP1) have been detected during the 2016–2017 low transmission season by AFP surveillance, indicating continued circulation in the core reservoirs in the Sindh province and Quetta district. One case of paralysis associated with cVDPV2 was detected in the Quetta in 2016; no cVPDV2 cases have been detected in 2017 to date.

Discussion

During January 2016–September 2017, a total of 25 WPV1 cases were detected in Pakistan, representing a 64% decline, compared with the 69 cases reported the same period during 2015–2016 (1); WPV1-positive environmental surveillance samples increased 65%, associated with a 34% increase in sampling. Despite the sharp decline in WPV cases in 2017, at least three areas of continued transmission exist, as indicated by continuing isolation of WPV from environmental samples. The detection of WPV circulation through environmental surveillance in the absence of positive cases of AFP is concerning, because it suggests that some population groups might not be covered by existing AFP surveillance. Genomic sequence analysis provides additional evidence of remaining surveillance gaps in the country in 2017.

The intensified SIA schedule throughout Pakistan and the focus on identifying and immunizing previously unvaccinated children has been followed by a sharp decline in WPV cases in the country. In addition, the introduction and expansion of the community-based vaccinators initiative, which uses local permanent vaccinators who possess the ability to build community trust, has helped to track and vaccinate children who are repeatedly missed during SIAs in high-risk areas, including those from underserved communities, such as seasonal laborers, nomadic families, and populations in transit, and including new birth cohorts.

During 2017, seven WPV1 cases have been reported through September in neighboring Afghanistan, one of the three remaining countries with endemic poliovirus transmission. Genetic sequencing of WPV1 isolates from these cases and from environmental samples indicate close genetic links to the WPV1 circulating in Pakistan. The epidemiology and genetic sequencing of WPV1 isolated during the reporting period indicate that the polio reservoirs continue to span the Pakistan-Afghanistan border and persist in at least three remaining areas in Pakistan. Ongoing challenges in the border areas include the large-scale movement of highly mobile population subgroups in two main poliovirus corridors (6). One of the two main geographic corridors extends from FATA and surrounding areas in Pakistan to the eastern region of Afghanistan, and the other from the Quetta block (Pishin, Killa Abdullah, and Quetta districts) of Balochistan province in Pakistan up to Kandhar and Helmand provinces in southern Afghanistan. Despite these persistent challenges, polio eradication crossborder efforts between Afghanistan and Pakistan continue to improve, through regular meetings and information exchange between teams. Efforts to reach more of the unvaccinated children in the mobile population, coupled with an intense SIA schedule, must be sustained to interrupt WPV transmission in Pakistan.