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Bulletin of the World Health Organization

Print version ISSN 0042-9686

Bull World Health Organ vol.78 n.3 Genebra Mar. 2000

http://dx.doi.org/10.1590/S0042-96862000000300006 

THEME PAPERS

 

Excluding polio in areas of inadequate surveillance in the final stages of eradication in China

 

Analyse typologique des cas de paralysie flasque aiguë compatibles avec la poliomyélite et comportant des facteurs de risque importants, Chine, 1997

 

Análisis por conglomerados de casos de parálisis fláccida aguda poliocompatibles con factores de alto riesgo, en China, 1997

 

 

E.J. HoekstraI; Chai FengII; Wang Xiao-junII; Zhang Xing-luIII; Yu Jing-jinIV;J. BilousV

IMedical Officer, World Health Organization, 9–2–151 Ta Yuan, 1 Xindonglu, Dongzhimen Wai, Beijing 100600, China
IIMedical Officer, Vaccine Preventable Disease Surveillance Division, Chinese Academy of Preventive Medicine, Beijing, China
IIIDirector, Vaccine Preventable Disease Division, Chinese Academy of Preventive Medicine, Beijing, China
IVDirector, Immunization Program, Ministry of Health, Beijing, China
VRegional Adviser EPI, World Health Organization, Western Pacific Regional Office, Manila, Philippines

Correspondence

 

 


ABSTRACT

In 1996, China adopted a virological classification of acute flaccid paralysis (AFP) cases for its surveillance system. Only AFP cases with wild poliovirus in stool specimens are confirmed as polio. Cases with adequate stool specimens that are negative for wild poliovirus are not counted. This paper describes a methodology to rule out poliomyelitis in AFP cases with inadequate stool specimens. National surveillance data were analysed using dot maps to detect clusters of AFP cases with high-risk factors for poliomyelitis. The surveillance system and vaccine coverage were assessed during field investigations. Four clusters of AFP cases were identified, but no poliomyelitis cases. Programmatic failures in the identified high-risk areas included low vaccination rates, poor stool specimen collection and inadequate AFP surveillance. Programme strategies were implemented to correct the identified failures. Use of this methodology provides strong evidence consistent with the absence of wild poliovirus in China.

Keywords: poliomyelitis, diagnosis; paralysis, diagnosis; epidemiological surveillance; disease notification, methods; China.


RÉSUMÉ

En Chine, les efforts d’éradication de la poliomyélite ont été couronnés de succès, le dernier poliovirus sauvage ayant été isolé en septembre 1994. Depuis 1995, la surveillance de la paralysie flasque aiguë (PFA) a très bien fonctionné, avec des taux de détection de plus de 1,0 pour 100 000 enfants de moins de 15 ans. En 1996, le Ministère chinois de la Santé a adopté le système de classification virologique des cas de PFA recommandé par l’OMS, venu remplacer le système de classification clinique en usage depuis 1991. Avec ces nouveaux critères virologiques, la confirmation définitive d’un cas de poliomyélite à partir des cas de PFA notifiés repose sur l’isolement du poliovirus sauvage au laboratoire. Seuls les cas de PFA pour lesquels on retrouve dans les échantillons de selles la présence de poliovirus sauvage sont des cas de poliomyélite confirmés; pour ceux dont les échantillons de selles ont été recueillis dans de bonnes conditions et ne montrent pas la présence de poliovirus sauvage, le diagnostic de poliomyélite est écarté. Les cas de PFA pour lesquels les échantillons de selles n’ont pas été recueillis dans de bonnes conditions et ne montrent pas la présence de poliovirus sauvage, qui se caractérisent par une paralysie résiduelle, le décès des patients ou le fait que ces derniers ont été perdus de vue, sont classés comme étant compatibles avec la poliomyélite.

Dans cet article, on trouvera une méthodologie permettant d’écarter un diagnostic de poliomyélite applicable à l’analyse des 18% de cas de PFA rangés dans la catégorie des cas compatibles avec la poliomyélite en 1997 en Chine. Les cas de PFA compatibles avec la poliomyélite doivent être considérés comme des échecs de la surveillance; les études menées avec cette méthodologie peuvent être accompagnées de mesures visant à améliorer la surveillance.

Les données nationales de la surveillance ont été analysées au moyen de cartes de points servant à déceler les groupes de cas de PFA compatibles avec la poliomyélite comportant des facteurs de risque importants. Le système de surveillance et la couverture vaccinale ont été évalués au cours d’études sur le terrain.

Sur les 4771 cas de PFA notifiés en Chine en 1997, 57 (1,2 %) ont été rangés dans la catégorie des cas de PFA compatibles avec la poliomyélite et comportant des facteurs de risque importants d’après les critères retenus dans cette étude. Parmi eux, on a recensé quatre groupes de cas situés dans cinq districts de trois provinces, mais aucun cas de poliomyélite n’a été confirmé. Des recherches actives dans les dossiers médicaux des médecins de village et des hôpitaux municipaux ou de district ont permis de découvrir sept cas de PFA supplémentaires qui n’avaient pas été notifiés. Le taux de PFA non poliomyélitique pour 100 000 enfants de moins de 15 ans dans les districts touchés était supérieur au taux national, à savoir 2,9 contre 1,6. Les données de la surveillance de la PFA dans ces districts étaient incomplètes ou inexactes. Des échantillons de selles prélevés dans de bonnes conditions ont été obtenus pour 68% des cas de PFA. La couverture médiane par au moins trois doses de VPO avant l’âge d’un an dans les régions étudiées était de 49% (intervalle de confiance à 95 %, 35-96) et inférieure à 80% dans six des neuf zones étudiées.

Dans les régions à haut risque ainsi recensées, les défaillances des programmes tels les faibles taux de vaccination, le recueil des échantillons de selles dans de mauvaises conditions et la surveillance insuffisante de la PFA ont été corrigées en mettant en œuvre des stratégies appropriées.

L’application de cette méthodologie laisse donc fortement à penser que le poliovirus sauvage est absent en Chine.


RESUMEN

Los esfuerzos desplegados en China para erradicar la poliomielitis han tenido mucho éxito, habiéndose notificado en septiembre de 1994 el último caso de poliovirus salvaje. Desde 1995, la vigilancia de la parálisis fláccida aguda (PFA) ha sido muy eficaz, y se han logrado tasas de detección de la PFA de más de 1,0 por 100 000 niños menores de 15 años. En 1996, el Ministerio de Salud de China adoptó para los casos de PFA el sistema de clasificación virológica recomendado por la OMS, en sustitución del sistema de clasificación clínica utilizado desde 1991. Con los nuevos criterios virológicos, la confirmación definitiva de un caso de poliomielitis a partir de los casos de PFA notificados depende de que se aísle poliovirus salvaje en el laboratorio. Sólo se confirman como casos de poliomielitis los casos de PFA con presencia de poliovirus en las muestras fecales; aquellos otros en los que no se detecta poliovirus salvaje en muestras fecales de características adecuadas se descartan y no se consideran casos de poliomielitis. Los casos de PFA cuyas muestras fecales son inadecuadas y negativas para el poliovirus salvaje, que tienen parálisis residual, que fallecen o con los que se pierde el contacto durante el seguimiento, se clasifican como «poliocompatibles».

En el presente artículo se expone un método de análisis del 18% de los casos de PFA chinos clasificados como poliocompatibles en 1997, a fin de descartar la poliomielitis. Debe considerarse que los casos de PFA poliocompatibles se deben a fallos de la vigilancia; las investigaciones basadas en esta metodología pueden acompañarse de medidas para mejorar la vigilancia.

Se analizaron los datos nacionales de vigilancia utilizando mapas de puntos para identificar los conglomerados de casos de PFA poliocompatibles con factores de alto riesgo de poliomielitis. Durante las investigaciones de campo se evaluaron el sistema de vigilancia y la cobertura de vacunación.

De los 4771 casos de PFA notificados en China en 1997, 57 (1,2%) se clasificaron como casos de PFA poliocompatibles con factores de alto riesgo según los criterios utilizados en este estudio. Entre ellos se identificaron cuatro conglomerados en cinco distritos de tres provincias, pero no hubo casos de poliomielitis confirmada. Las búsquedas activas realizadas en los historiales de los dispensarios de médicos de aldea y en hospitales comarcales y cantonales revelaron siete casos de PFA que no habían sido notificados. En los distritos afectados, la tasa de PFA no poliomielítica por 100 000 menores de 15 años era superior a la tasa nacional (2,9 y 1,6, respectivamente). Los datos de la vigilancia de la PFA en esos distritos estaban incompletos o eran inexactos. Se obtuvieron muestras fecales adecuadas en el 68% de los casos de PFA. En las zonas investigadas, la cobertura sistemática mediana con tres o más dosis de vacuna oral contra la poliomielitis antes de cumplir un año de edad fue del 49% (intervalo de confianza del 95%: 35%-96%), con una cobertura inferior al 80% en seis de las nueve zonas.

Los fallos del programa registrados en las zonas identificadas como de alto riesgo (tasas de vacunación bajas, obtención defectuosa de muestras fecales, vigilancia inadecuada de la PFA), fueron corregidos aplicando las estrategias programáticas adecuadas.

La utilización de esta metodología aporta pruebas firmes que son coherentes con la ausencia de poliovirus salvaje en China.


 

 

Introduction

Polio eradication efforts in China have been highly successful, with the last case of indigenous wild poliovirus having been reported in September 1994 (16). Since 1995, acute flaccid paralysis (AFP) surveillance has reached high levels of performance, with AFP rates of more than 1.0 per 100 000 children under 15 years of age being detected (710). In 1996, the Chinese Ministry of Health adopted a WHO-recommended virological classification scheme for AFP cases (Fig. 1) to replace the clinical classification scheme in use since 1991 (11, 12). With the new virological criteria, final confirmation of polio depends on the isolation of wild poliovirus in the laboratory (13). Only AFP cases with wild poliovirus in stool specimens are confirmed as polio, while those with adequate stool specimens that are negative for wild poliovirus are considered as non-polio cases. Adequate stool specimens are defined as two stool specimens (8–10 g) collected within 2 weeks after onset of illness, with an interval of more than 24 hours between collections, transportation on ice, and arrival at the laboratory in good condition (no desiccation, no leakage). The question remains as to how to interpret the AFP cases whose stool specimens are both inadequate and negative for wild poliovirus.

 

 

This article describes a methodology to rule out polio in the 17.9% of Chinese AFP cases with inadequate stool specimens, so that polio can be excluded as a possible cause of paralysis. Because AFP cases with inadequate stools are regarded as failures of surveillance, the methodology should be accompanied by measures to improve surveillance.

 

Methods

AFP cases reported in 1997 to the national AFP surveillance programme at the Chinese Academy of Preventive Medicine (CAPM) were analysed to detect clusters of AFP cases with a high risk for polio. We used Epi Info software, version 6 (14) to select the high-risk AFP cases.

Selection of high-risk polio-compatible AFP cases

High-risk AFP cases were defined as those that satisfied the following criteria: no wild virus has been isolated; stool specimens were inadequate; residual paralysis was present after 60 days, or death occurred within 60 days, or the case was lost to follow-up; fever was present at the onset of paralysis; and vaccination was incomplete (fewer than three doses of Sabin live attenuated oral poliovirus (OPV) vaccine had been administered). For the analysis, the OPV vaccination status of the children was adjusted for age. Only those polio-compatible AFP cases that met all the high-risk criteria were further analysed for clusters.

Clusters

We defined clusters as two or more high-risk polio-compatible AFP cases in any one county or in adjacent counties with the date of onset of paralysis within 2 months of each other. MapInfo software (15) was used to create dot-maps of the cases found to identify clusters.

Field investigations

Field investigations were conducted in each of the counties with one or more high-risk polio-compatible AFP cluster cases.1 The investigations included the following assessments:

• Analysis of all AFP cases reported in 1997 in the affected counties to assess the accuracy, timeliness and completeness of AFP surveillance.

• Active medical record search (12) to identify unreported AFP cases in 1997 in the doctors’ clinics of affected villages and in the hospitals of the townships and counties concerned.

• Analysis of the stool specimen collection and handling methods for all AFP cases reported in these counties in 1997 to assess the quality, quantity and timeliness of stool collection.

• Physical examination of each child classified as a high-risk polio-compatible AFP case to assess residual flaccid paralysis.

• OPV vaccine coverage surveys in the affected villages to estimate routine OPV coverage and coverage for the subnational immunization days (SNIDs)2 conducted in December 1997 and January 1998. Children aged 1–4 years of age were randomly selected in house-to-house area surveys. Vaccination dates and doses reported by the parents were verified with the village doctors’ records.

 

Results

No wild poliovirus was identified in any of the stool specimens collected from 4771 AFP cases reported in 1997 in China. A total of 4526 (94.9%) of the nationally reported cases could be discounted as polio, i.e. all the cases with adequate stool specimens and cases with inadequate stools without residual paralysis at 60 days after onset of the illness (Fig. 1 and Fig. 2).

 

 

A total of 57 (1.2%) of the 4771 AFP cases reported in China in 1997 were classified as high-risk AFP cases according to the criteria used in this study (Fig. 2). The distribution of these 57 cases by province is shown in Fig. 3. Four clusters comprising nine of the high-risk AFP cases were identified in five counties of three provinces (Fig. 4). Two clusters were found in Guizhou Province: the first included two cases in the two adjacent Hezhang and Nayong counties; the second included two cases in Zunyi county. The third cluster comprised three cases in Zen Xiong county in the north-east of Yunnan Province. The fourth cluster, with two cases, was found in Guigang county in Guangxi Province.

 

 

 

 

Table 1 summarizes the findings of the field investigations. In 1997, the five counties concerned reported a total of 47 AFP cases, nine of which were classified as high-risk AFP cases, occurring in four clusters. Active medical record searches in village doctors’ clinics and township and county hospitals revealed seven additional unreported AFP cases, none of which was high risk. The non-polio AFP rate per 100 000 children under 15 years of age in the affected counties was higher than the national rate, being 2.9 and 1.6, respectively. The AFP surveillance data obtained by examining case investigation forms in each county were incomplete or inaccurate on case variables such as fever, doses of OPV received, stool collection and follow-up visits. Adequate stool specimens were taken by the county Epidemic Prevention Station (EPS) from 32 (68%) of the 47 AFP cases, compared with 82% taken nationally. Follow-up visits were carried out on 42 (89%) of the reported cases to evaluate for residual paralysis 60 days after onset of AFP, compared with 91% of such visits conducted nationally. One child diagnosed with leptospirosis in Zunyi county, Guizhou Province, died within 60 days of onset of paralysis. Two of the remaining eight high-risk AFP children examined had no residual flaccid paralysis at the time of the cluster investigation: a 1-year-old boy with a hip dislocation and a 2-year-old boy with encephalitis. Six of the eight high-risk polio-compatible AFP children examined had residual paralysis at the investigation date. These six cases were diagnosed as follows: Guillain–Barré syndrome (3 cases), traumatic neuritis (2), and transverse myelitis (1). The OPV coverage was assessed for a random sample of 407 (27%) of the 1481 children aged 1–4 years living in nine affected areas. In 1997 the national reported coverage of three or more doses of OPV by 1 year of age was 97%, while that reported by national survey among children born in 1997 was 92% (unpublished data, Chinese Academy of Preventive Medicine, Beijing). The median routine coverage of three or more doses of OPV by 1 year of age in the areas where clusters were identified was 49% (range, 35–96%), with coverage in six of the nine areas being below 80%; 11% (range, 1–27%) of the children had received no doses of OPV at 12 months of age. The median coverage of at least one dose of OPV during SNIDs in 1997–98 was 86% (range, 68–91%).

 

 

Dicussion

With increasing progress towards polio eradication, identification and resolution of AFP cases whose stool specimens are both inadequate and negative for wild poliovirus becomes more and more important because they may indicate weaknesses in the surveillance system. This article describes a method of identifying clusters of AFP with high-risk factors for polio. While, by definition, all high-risk AFP cases have an incomplete vaccination status and inadequate stool collection, further assessment of the high-risk indicators in areas identified by the clusters can also reveal inadequacies in OPV coverage and AFP surveillance. Actions should be taken to improve the coverage and the quality of surveillance when inadequacies are identified.

Investigation of clusters of high-risk AFP cases in China found no polio cases in 1997. However, all affected counties had inadequate stool specimen collection methods and inaccurate and incomplete AFP surveillance data, while routine OPV coverage was less than 80% in six of the nine affected areas. In all counties action has subsequently been taken to correct and improve the surveillance, stool collection and immunization coverage. Counties with low routine OPV immunization have improved their coverage through additional funding, conducting SNIDs and undertaking at least six rounds of immunization per year, increasing access to health care, and training. The quality of AFP surveillance, active surveillance and stool specimen collection has been improved by training EPS staff. If indicated, iceboxes for the transport of specimens have been provided. The outcomes of these interventions are being monitored and evaluated at province level.

The areas identified by this method tallied with those where wild poliovirus was last reported and where poorly implemented poliomyelitis eradication strategies were known to exist (9). Analysis of the high-risk polio-compatible AFP cases from 1996 found clusters in the same areas (16). At that time, no corrective programmatic measures were taken. In 1998, however, in response to the 1997 corrections that were implemented in the affected areas, only two high-risk AFP case clusters were found, one in the west of Guizhou Province and the other in the south of Quinghai Province. These findings support the validity and usefulness of the methodology.

The method described in this article is particularly useful in very populous countries such as China and India, but it cannot replace the WHO AFP surveillance indicators (12) for detecting surveillance deficiencies. In smaller countries the incidence of clustering of high-risk AFP cases will be very low unless the AFP surveillance is very poor.

China is currently in the process of being certified as polio-free. The Regional Committee for Certification of Poliomyelitis Eradication is closely evaluating the situation. However, it remains difficult to prove the absence of polio in China, particularly with 852 (17.9%) of the AFP patients reported in 1997 having inadequate stool specimens, of which 245 (5.1%) had residual paralysis, had died or had been lost to follow-up after 60 days of onset of paralysis. The method described here can be used to evaluate the most critical cases and pinpoint their location. Reporting the results of cluster analysis in the national documentation for certification of polio eradication may provide important supporting evidence consistent with the absence of wild poliovirus in China.

 

Acknowledgements

The following individuals deserve recognition for their contribution to various aspects of this study: Zhang Xiaohui, Ding Zhengrong, Dong Baiqing, Li Zhao Yi and S. Guastaferri.

 

References

1. Wang K et al. Status of the eradication of indigenous wild poliomyelitis in the People’s Republic of China. Journal of Infectious Diseases, 1997, 175 (Suppl. 1): S105–S112.         [ Links ]

2. EPI information system: global summary. Geneva, World Health Organization, 1997 (unpublished document WHO/EPI/ GEN/ 97.02).         [ Links ]

3. Li Q et al. An epidemiological analysis of acute flaccid paralysis (AFP) in 1996, China. Chinese Journal of Vaccines and Immunization, 1997, 3: 251–256 (in Chinese).         [ Links ]

4. Li Q et al. Evaluation of the 4 times of OPV NIDs conducted in China. Chinese Journal of Vaccines and Immunization, 1997, 3: 263–267 (in Chinese).         [ Links ]

5. Centers for Disease Control and Prevention. Progress toward poliomyelitis eradication — Western Pacific Region, January 1, 1996 – September 27, 1997. Morbidity and Mortality Weekly Report, 1997, 46: 1113–1117.         [ Links ]

6. Hull HF et al. Paralytic poliomyelitis: seasoned strategies, disappearing disease. Lancet, 1994, 343: 1331–1337.         [ Links ]

7. Cao L et al. The analysis of the working status of AFP surveillance system in China. Chinese Journal of Vaccines and Immunization, 1997, 3: 257–262 (in Chinese).         [ Links ]

8. National Epidemiological Surveillance Center of Poliomyelitis. Evaluation on working status of AFP surveillance systems distributed nationwide in 1995. Chinese Journal of Vaccines and Immunization, 1997, 2: 202–208 (in Chinese).        [ Links ]

9. Centers for Disease Control and Prevention. Progress toward poliomyelitis eradication — People’s Republic of China, 1990–1994. Morbidity and Mortality Weekly Report, 1994, 43: 857–859.         [ Links ]

10. Zhang J et al. Surveillance for polio eradication in the People’s Republic of China. Journal of Infectious Diseases, 1997, 175 (Suppl. 1): S122–S134.         [ Links ]

11. National action plan on poliomyelitis eradication 1996-2000 and virological classification of AFP cases. Beijing, Ministry of Health, 1996 (publication MOH/DCD;1996: 22).         [ Links ]

12. Field guide for supplementary activities aimed at achieving polio eradication. Geneva, World Health Organization, 1996 (unpublished document WHO/EPI/GEN/95.01 Rev.1).         [ Links ]

13. Manual for the virological investigation of polio. Geneva, World Health Organization, 1997 (unpublished document WHO/EPI/ GEN/97.01).         [ Links ]

14. Dean A et al. Epi Info, version 6: A word-processing, database, and statistics system for public health on IBM-compatible microcomputers. Atlanta, GA, Centers for Disease Control and Prevention, 1995.         [ Links ]

15. MapInfo. Reference. Troy, NY, MapInfo Corporation, 1994.         [ Links ]

16. Hoekstra EJ. Cluster analysis of higher risk AFP polio-compatible cases. World Health Organization Regional Office for the Western Pacific, Manila, Philippines, 1997 (unpublished document WP/ICP/VID/RS/96/0532).        [ Links ]

 

 

Correspondence
E.J. Hoekstra
World Health Organization
9–2–151 Ta Yuan, 1 Xindonglu, Dongzhimen Wai
Beijing 100600, China
E-mail: hoekstrae@who.org.cn

 

 

1 Immunization survey data were collected by the staff of the Chinese Academy of Preventive Medicine from the Epidemic Prevention Stations (EPS) in each province. EPS collated the immunization dates and doses from information recorded in immunization records held by patients and physicians’ medical records. Informed consent was obtained from the parents of all children examined.

2 SNIDs are focal mass campaigns in high-risk areas conducted over a short period (days) in which two doses of OPV are administered to all children in the target age group (0–3 years) regardless of previous vaccination history, with an interval of 4 weeks between doses.