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Final Stages of Poliomyelitis Eradication -- Western Pacific Region, 1997-1998

In 1988, the World Health Assembly resolved to eradicate poliomyelitis globally by 2000 (1). A plan of action for polio eradication in the Western Pacific Region (WPR) by 1995 was adopted in 1990. The plan was based on routine and supplemental vaccination activities with oral poliovirus vaccine (OPV) and acute flaccid paralysis (AFP) surveillance in the eight countries where polio was endemic (Cambodia, China, Laos, Malaysia, Mongolia, Papua New Guinea, Philippines, and Vietnam) (2). Regionwide, the number of reported polio cases decreased from approximately 6000 in 1990 to zero in 1998. This report describes the extensive efforts to eliminate the last chains of poliovirus transmission in the Mekong River area.

AFP surveillance was introduced in Cambodia, Laos, and Vietnam in 1992, and has improved steadily (during 1994-1997, the proportion of AFP cases with two adequate stool samples increased from 7% to 71% in Cambodia, 0 to 70% in Laos, and 49% to 84% in Vietnam). From 1992 to 1997, the number of confirmed polio cases decreased from 557 to one in Vietnam; from 146 to eight in Cambodia; and from seven to zero in Laos. In addition, analysis of 1996 data suggested that poliovirus transmission was limited to focal areas in Cambodia, Laos, and Vietnam. During 1996, 21 confirmed cases of polio were reported in WPR (17 cases from the Mekong River area of Cambodia, one from nearby southern Laos, and three imported into China from Myanmar) (3).

National Immunization Days (NIDs) * were conducted from 1993 through 1998 in Vietnam and Laos, and from 1995 through 1998 in Cambodia. A total of 12.5 million children were targeted in the three countries during each round of NIDs, and the reported coverage was generally high (Table_1). During January-March 1997, nine polio cases were reported from the Mekong River area. Analysis of data from supervisory teams and AFP surveillance indicated that a substantial proportion of undervaccinated children were residing on the extensive waterways of the Mekong River and many had been missed previously by both routine and supplemental vaccination activities. Therefore, to interrupt the transmission of wild poliovirus in 1997, additional rounds of supplemental vaccination focused on these unreached areas and populations. Cambodia, Laos, and Vietnam conducted two synchronized rounds of "mopping-up" ** supplementary vaccination in high-risk areas in May and June 1997; the second two rounds in Cambodia and Vietnam occurred during February-April 1998 (Figure_1).

A combination of strategies, including fixed and mobile vaccination sites and mobile teams, were used to ensure that every child aged less than 5 years in the selected areas would receive two doses of OPV, regardless of vaccination history. In many areas, hundreds of mobile teams, using a ratio of one team for each 120 children, visited from house to house and boat to boat to reach the target population.

Because an accurate target denominator was unknown, coverage data were considered unreliable for monitoring the quality of mopping-up. Therefore, the total number of children reached by mobile teams and the proportion of children for whom no vaccination dose ("zero-dose" children) was recorded previously served as an alternative indicator of the quality of the mopping-up rounds. In Cambodia, the proportion of "zero-dose" children in selected areas decreased from 22% during the first round in May 1997 to 1% in 1998 (Table_1).

AFP surveillance data indicated that the last person reported with polio in WPR had onset of illness in Cambodia on March 19, 1997. No other cases of polio have been detected in WPR despite the reporting and investigation of greater than 9000 AFP cases in 1998 (data as of December 10, 1998); two stool samples had been taken from 85% of persons with cases within 14 days of onset of paralysis. Vietnam reported 463 AFP cases (nonpolio AFP rate of 1.5 per 100,000 population aged less than 15 years) in 1997 and 492 cases (nonpolio AFP rate of 1.7) in 1998 (data as of December 10). In 1997, stool specimens were available from 83% of persons with AFP, and in 1998, from 95%.

Cambodia reported 178 AFP cases in 1997 and 142 cases in 1998, for nonpolio AFP rates of 3.2 and 2.8, respectively. Despite problems with transport and communication, adequate stool sampling rates of 71% in 1997 and 80% in 1998 were achieved in Cambodia. Laos reported 76 AFP cases in 1997, for a nonpolio AFP rate of 3.5. The number of AFP cases reported in 1998 was 75, and the nonpolio AFP rate was 3.7. Adequate stool collection rates for 1997 and 1998 were 75% and 77%, respectively.

Reported by: Expanded Program on Immunization, World Health Organization, Office for the Western Pacific Region, Manila, Philippines; Dept of Vaccines and Other Biologics, World Health Organization, Geneva, Switzerland. National Institute of Infectious Diseases, Tokyo, Japan. Respiratory and Enteric Viruses Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; Vaccine Preventable Disease Eradication Div, National Immunization Program, CDC.

Editorial Note

Editorial Note: Countries in WPR have conducted polio eradication efforts since 1991, and WPR is now apparently polio-free. To complete eradication, Cambodia and Vietnam conducted eight supplementary vaccination rounds: four rounds of mopping-up and four rounds of NIDs in the high-risk areas along the waterways of the Mekong River and its tributaries from November 1996 through April 1998. These waterways support large populations, many of whom are highly mobile and not regularly reached by routine vaccination services. The supplemental vaccination efforts appear to have been successful in eliminating the last remaining reservoirs of wild poliovirus in WPR.

To reach a high proportion of the target groups for mopping-up vaccination, substantial efforts were devoted to planning, monitoring and supervision, and evaluation. The planning process used high-quality AFP and laboratory surveillance data to identify target areas. The timely availability of laboratory results from stool specimens enabled the precise location of wild polioviruses within 60 days of onset of paralysis for 85% of cases. The criteria for selecting high-risk districts included those with wild poliovirus during the preceding 24 months, clusters of clinically confirmed polio cases, poor surveillance performance, boat-dwelling populations, and borders with other countries where polio is endemic. In timing mopping-up, it was considered more important to choose the better access afforded by the hot, dry summer months rather than the low transmission for enteroviruses during the winter season.

Each district and subdistrict prepared logistic plans showing population, vaccine, staff and other requirements, and maps to locate the position of vaccination posts and routes to be taken by mobile teams. In certain areas, aerial photography was used over the waterways to locate boat-dwelling populations.

In Cambodia and Vietnam, 1 million children aged less than 5 years were included in both the 1997 and 1998 mopping-up, and Laos, which conducted mopping-up in 1997 only, targeted 50,000 children aged less than 5 years. In Cambodia, the mopping-up rounds were staggered over 12 days to allow time for supervisory teams to visit all areas; 14 days were used in Laos, and 3 days in Vietnam. Given the large amount of cross-border traffic between Cambodia and Vietnam, special efforts were made to coordinate the mopping-up by synchronizing the dates, and deploying mobile teams and fixed posts at border crossing points.

Despite initial concerns regarding a potential negative effect of polio eradication on routine vaccination, routine coverage with three doses of OPV among 1-year-old children increased substantially during 1993-1997 (Cambodia from 36% to 70%, Laos from 26% to 69%, and Vietnam from 91% to 95%).

The efforts needed to interrupt the final chains of poliovirus transmission in the last few remaining areas were far more intense than in the early stages when polio was widely endemic. Critical conditions for the success of the mopping-up were 1) availability of high-quality AFP and virological surveillance to identify high-risk areas; 2) timely analysis of surveillance data to identify areas not reached by previous supplementary vaccination rounds; 3) timely availability of laboratory results to identify areas where wild poliovirus was circulating; 4) detailed local planning including the use of maps at the sub-district level; and 5) use of new vaccination approaches, including mobile teams to reach all target children.


  1. World Health Assembly. Global eradication of poliomyelitis by the year 2000. Geneva, Switzerland: World Health Organization, 1988; WHA resolution no. WHA 41.28.

  2. Hull HF, Ward NA, Milstien JB, de Quadros C. Paralytic poliomyelitis: seasoned strategies, disappearing disease. Lancet 1994;343:1331-7.

  3. CDC. Progress toward poliomyelitis eradication -- Western Pacific Region, January 1, 1996-September 27, 1997. MMWR 1997;46:1113-7.

* Mass campaigns over a short period (days to weeks) in which two doses of OPV are administered to all children in the target group (usually aged 0-4 years) regardless of previous vaccination history, with an interval of 4-6 weeks between doses. 

** Focal mass campaign in high-risk areas over a short period (days to weeks) in which two doses of OPV are administered during house-to-house and boat-to-boat visits to all children in the target age group, regardless of previous vaccination history, with an interval of 4-6 weeks between doses.
Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 1. Coverage achieved during National Immunization Days (NIDs)*
and proportion of "zero-dose" children+ during mopping-up vaccination, by year --
Cambodia, Laos, and Vietnam, 1996-1998
                                     Cambodia                     Laos                      Vietnam
                              ------------------------   -----------------------    ------------------------
Year                            NIDs      "Zero-dose"      NIDs      "Zero-dose"      NIDs      "Zero-dose"
          Activity   Round    coverage     children      coverage     children&     coverage     children
1996        NIDs       1         89%                        85%                        99%          23%
                       2         91%                        84%                       100%
1997        NIDs       1         94%                        87%                        99%
                       2         96%                        87%                        99%
         (Cambodia)    3         98%
         (Cambodia)    4         96%
1997     Mopping-up    1         98%          22%          105%                        97%          11%
                       2        100%                        93%                        98%
1998    NIDs/SNIDs@    1         96%**                      93%                        97%**
                       2                                    89%
1998     Mopping-up    1         94%           1%           ++                        100%          10%
                       2         96%                                                  100%
*  Mass campaigns over a short period (days to weeks) in which two doses of oral poliovirus
   vaccine are administered to all children in the target group (usually aged 0-4 years)
   regardless of previous vaccination history, with an interval of 4-6 weeks between doses.
+  The proportion of children for whom no vaccination dose was recorded previously.
&  Information is not available for Laos.
@  Subnational Immunization Days.
** Preliminary reports.
++ Mopping-up campaigns were not conducted.

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