When thousands of cases of polio were being reported across the Region, the goal was to reduce the number of cases with large-scale, widespread immunization campaigns. The more difficult task emerged when only a few cases remained. At the end of 1996, there were 21 cases of wild poliovirus in the Region; 18 of them were reported around the Mekong River Delta area, the only area where wild poliovirus was still in circulation. This vast network of waterways, which includes Cambodia’s Tonle Sap, or Great Lake, is populated by millions of people who make their living from fishing.

The children in this area were often missed during routine immunization because of their family's mobile lifestyle. As such, the poliovirus was still holding out along these waterways and riverbanks. It took hundreds of mobile immunization teams moving from boat to boat and house to house to find and vaccinate every child under five years of age.

In just over 12 months, from the end of 1996, two million children were immunized during eight rounds of campaign in these high-risk areas. The effort proved successful. No new cases were reported after 19 March 1997. Similar “mopping-up” or supplementary immunization campaigns were carried out in 1998 and 1999. These final efforts set the Region on its three-year course towards polio-free certification in 2000.