Methods and Applications: Risk Assessments for Type 1 Wild Poliovirus and Type 2 Vaccine-Derived Poliovirus Importation and Transmission — China, 2023

The WPV1 risk assessment tool implemented in China, originally developed by Zhang Ying et al. in 2013 (8), is detailed inTable 1. This tool was subsequently enhanced to assess the risks associated with VDPV2 importation and transmission, utilizing a modified Delphi technique (9). During this process, two rounds of questionnaire-driven consultations were conducted with 20 experts in polio, AFP surveillance, enterovirus research, and vaccine-preventable diseases. These experts evaluated various indicators, selecting those that most effectively represented the risks of VDPV2 importation and transmission, as shown inTable 2. The weighting values assigned to each significant indicator reflect its relative importance. InTables 1and2, the experts rated the weightings for each leading indicator, where each expert distributed weights across three primary indicators up to a possible total of 10 (8–9).

The two tools, WPV1 and VDPV2 risk, utilize three primary indicators: population immunity, surveillance system quality, and importation risk. Population immunity refers to the community-level resistance against WPV1 or VDPV2 poliovirus. The surveillance system quality is assessed by evaluating the performance of AFP and environmental surveillance systems. Importation risk evaluates the susceptibility to virus importation and subsequent transmission. The VDPV2 tool differs from the WPV1 tool by incorporating additional indicators: IPV coverage, implementation of IPV supplementary vaccination activities, and the presence of direct travel connections between any PLAD in China and a country experiencing cVDPV2 transmission.

The sum of weighted scores (S) was calculated as S=w_A×f(A)+w_B×f(B)+w_C×f(C), where w represents the weight of the secondary indicators and f represents the scores of the secondary indicators. The S scores were ranked from high to low and categorized into high, medium and low-risk groups. The 10% of PLADs with the highest scores were considered high risk, and the remaining PLADs were divided into half with medium risk and half at low risk.

This study sourced data on vaccine coverage from the China Immunization Information System (CIIS), AFP data from the Acute Flaccid Paralysis Surveillance Information Report Management System (AFPSIRMS), and measles incidence data from the National Notifiable Disease Reporting System (NNDRS). Demographic data were obtained from the China Statistical Yearbook, covering the period from 2018 to 2022. These data were organized, computed, and analyzed descriptively using Microsoft Excel (version 2019, Microsoft, Redmond, USA).

Table 3presents the risk scores for WPV1 and VDPV2 at the provincial level. For WPV1, S-values ranged from 0 to 34. Scores above 27 denote a high risk, scores between 13 and 27 indicate a medium risk, and scores below 13 represent a low risk. For VDPV2, S-values varied from 17 to 77, with scores above 60 signaling a high risk, scores between 40 and 60 suggesting a medium risk, and scores below 40 signifying a low risk.

For WPV1 risk assessment, the PLADs categorized as high-risk included Xizang, Qinghai, and Xinjiang. Those identified as medium-risk were Chongqing, Ningxia, Jilin, Hubei, Tianjin, Heilongjiang, Anhui, Jiangxi, Guangdong, Gansu, Liaoning, Shanxi, and Guizhou. PLADs such as Beijing, Inner Mongolia, Jiangsu, Zhejiang, Fujian, Shandong, Hainan, Shaanxi, Hunan, Shanghai, Henan, Hebei, Sichuan, Guangxi, and Yunnan were considered low-risk.

For the risk of VDPV2, Xizang, Hainan, and Shaanxi were identified as high-risk PLADs; Chongqing, Tianjin, Qinghai, Shanxi, Xinjiang, Beijing, Guizhou, Hebei, Inner Mongolia, Hubei, Hunan, Anhui, Guangdong, Gansu, and Henan were classified as medium-risk; and Heilongjiang, Fujian, Shandong, Ningxia, Liaoning, Jiangxi, Jilin, Jiangsu, Yunnan, Zhejiang, Shanghai, Guangxi, and Sichuan were considered low-risk PLADs. Detailed scores for provincial second-level indicators are presented in Supplementary Tables S1 and S2 .

Our study on polio risk assessment employed two independent tools, utilizing provincial-level input data, to ascertain PLADs most vulnerable to the importation and transmission of WPV1 and VDPV2 polioviruses. The findings indicate that Xizang, Qinghai, and Xinjiang are the PLADs most susceptible to WPV1, while Xizang, Shaanxi, and Hainan face the highest risk for VDPV2. Despite the absence of imported polioviruses during 2022–2023, the potential for poliovirus importation from neighboring and distant countries remains a significant concern, posing a continued risk of transmission within China should an importation occur.

The World Health Organization (WHO) recommends that all polio-free countries perform routine annual risk assessments to facilitate the implementation of preventive measures in high-risk areas (10). Such assessments provide a scientific framework for governments to make informed programmatic decisions. Our study utilized both qualitative and quantitative methods to evaluate the poliovirus risk. Research in Pakistan and Afghanistan employed mathematical models to examine transmission dynamics, revealing that these countries form an epidemiological block characterized by sustained transmission among under-vaccinated populations (11-12). In a similar vein, Australian researchers developed a semi-quantitative risk assessment tool that incorporates two primary indicators: likelihood factors (reintroduction hazard and population susceptibility) and impact factors (detection and response capabilities), which were weighted and integrated (13).

Although certain PLADs were identified as higher-risk areas, the overall risk of WPV1 or VDPV2 importation and transmission in China for 2023 remains relatively low. This is attributed to China’s consistent achievement of a high polio vaccine coverage rate and the maintenance of high-quality AFP surveillance (14-15). Moreover, the WHO Western Pacific Region has classified China as a low-polio-risk country.

China’s WPV polio risk assessment was last reported in 2019 (16). The recent WPV1 risk assessment results align with those from the past several years, identifying the same three PLADs as high-risk. We recommend that these high-risk PLADs assess their risk factors to refine targeted preventive measures and reduce the risk of importation and transmission.

This study utilized a novel tool designed to assess the risk of importation and transmission of type 2 poliovirus (9). This tool represents an extension of the established WPV risk assessment tool and marks the first deployment of the new VDPV2 risk assessment tool in China. Our findings indicate that numerous PLADs identified as medium to high risk possess correspondingly low levels of population immunity. The study derived dose-number-based IPV coverage rates from individual vaccination records, revealing that IPV coverage among children under 5 years has not reached 90% in some regions. Furthermore, 24 PLADs had more than 20% of age-eligible children receiving one IPV dose, 6 PLADs exceeded 10% of age-eligible children without any IPV doses, and 26 PLADs had less than 90% of children under 5 years receiving two or more IPV doses (SupplementaryTable 2). The results suggest that during the implementation of the updated polio vaccination strategy — shifting from 1 dose of IPV and 3 doses of bOPV (bivalent oral poliovirus vaccine) to 2 doses of IPV and 2 doses of bOPV (17) — many children born during the transition received only one or no IPV doses without any supplementary immunization. This concerning trend was notable in Hainan and Shaanxi, where the WPV1 risk was low, yet VDPV2 risk was high (Table 3). In these areas, many children received inadequate IPV doses, insufficient for robust type 2 PV protection as antibody levels against VDPV2 might be relatively low. The efficacy of two IPV doses is superior to a single dose (18). Despite high overall vaccination rates in these PLADs, the inclusion of bOPV and tOPV doses might mask the transmission risk of VDPV2 unless indicators A5, A6, and A7 are considered within the VDPV2 risk assessment. PLADs at high risk should urgently implement catch-up IPV vaccinations to enhance population immunity against type 2 PV. IPV supplementary immunization and catch-up campaigns are actively being carried out in several PLADs, including the border regions of Yunnan and Xinjiang.

The sensitivity and timeliness of AFP surveillance are paramount. Our assessment of the WPV1 risk revealed that the NPAFP reporting rate fell below the target of 1/100,000 in five PLADs, indicating a need to enhance active case monitoring and reporting in the pediatric, infectious diseases, and neurology departments of hospitals. Additionally, the isolation rate of NPEV from 2020 to 2022 was below 3% across 17 PLADs, underscoring a potential reduction in AFP surveillance sensitivity. This decline could reflect improved sanitary conditions locally. In terms of environmental surveillance, 14 PLADs maintained ongoing wastewater monitoring in 2022, and by 2023, three more PLADs had initiated this practice. Environmental surveillance is critical for monitoring poliovirus distribution in the population via detection in wastewater, as polioviruses primarily spread through fecal contamination. Given that China’s immunization strategy commences with the IPV, which prevents paralysis but not infection, supplementing AFP surveillance with environmental monitoring is crucial to ensure effective poliovirus detection.

This study evaluated the potential for virus importation following the conclusion of the COVID-19 pandemic. Our analysis included key factors such as ongoing poliovirus (PV) circulation in neighboring countries. Unlike the risk assessment for WPV1, the tool for VDPV2 incorporated additional quantifiable indicators. These indicators include direct air transport connections with countries experiencing VDPV2 outbreaks and the implementation of supplementary or catch-up vaccination programs to mitigate importation risks. For instance, despite their robust population immunity and active surveillance for AFP, jurisdictions like Beijing and Guangdong face higher importation risks due to globalization and frequent international exchanges. Accordingly, these areas should enhance their capabilities to detect polioviruses and prevent transmission.

This study was subject to several limitations. First, the data were collected from various PLADs, where the quality of AFP surveillance and polio immunization programs may differ significantly. Second, the accuracy, completeness, and quality of data for each indicator influenced the final risk assessment results, potentially leading to an underestimation of risk due to incomplete case data or insufficient field investigations. Additionally, the usage of dynamic data for each secondary indicator may have contributed to instability in the findings throughout the study period. It is important to note that the risk results presented are relative risks and should be used primarily for guidance and reference. Lastly, the selection of indicators by experts introduces a degree of subjectivity.

This study revealed that in the 2023 WPV1 importation and transmission risk assessment for China, Xizang, Qinghai, and Xinjiang were identified as the regions with the highest risk. Similarly, the VDPV2 risk assessment indicated that Xizang, Shaanxi, and Hainan bear the greatest risk of VDPV2 importation and dissemination. Such risk assessments are crucial for pinpointing areas where public health programs require enhancement. Despite the absence of local poliovirus transmission in China, it is advisable for these high-risk PLADs to augment their poliovirus vaccination rates, particularly the 2-dose IPV, to maintain robust population immunity. Furthermore, enhancements in the timeliness, sensitivity, and detection capabilities of the AFP surveillance system are recommended. PLADs with medium or low risk should also adopt preventative strategies tailored to their specific risk profiles to bolster and preserve a polio-free status across the nation.