Programme evaluation: identifying gaps and providing clear epidemiological picture
As control activities to achieve eradication become more sophisticated by targeting specific poliovirus serotypes with type-specific monovalent oral polio vaccines (OPV) or with the new bivalent OPV containing type 1 and 3 serotypes (particularly where co-circulation of both serotypes exists), there is a concurrent need to better assess the coverage achieved during supplemental immunization activities (SIAs) and to better measure the impact of the SIAs on actual population immunity against polioviruses.
As a supplement to existing monitoring procedures, and to help assess vaccination coverage achieved during SIAs, the Global Polio Eradication Initiative (GPEI) adopted the existing methodology of Lot Quality Assurance Sampling (LQAS) to the specific needs of the eradication programme. In November 2009, the new cluster LQAS methods were piloted in Nigeria, and applied subsequently over the next six months in Nigeria and other countries in the WHO African Region.
To better measure population immunity, a number of seroprevalence surveys have been conducted or are planned for the next six months, both in Asia (ie India and Pakistan) and Africa (ie Nigeria). These seroprevalence surveys usually focus on very young infants, ages 6-11 months, to provide the most conservative population immunity profiles (immunity in these young infants is lowest since maternally-derived antibodies have waned and they have received relatively fewer doses of polio vaccines than older age groups). At end-2009, the GPEI had already documented a marked increase in type 1 immunity in northern India, from ~80% in 2007 to >99% in 2009, confirming and validating the corresponding decrease in type 1 poliomyelitis cases in this area.
While both methods provide important data to validate programmatic efforts to refocus the vaccine mix if necessary, or to provide confidence to programme managers that activities are on the right path, these methods are time- and resource intensive, and should only be used to answer specific important questions. Thus, while both methods are unlikely to become standard tools for the GPEI, these methods will be applied in areas where there are questions about programme performance or where case reporting data may conflict with SIAs coverage data.