ABSTRACT
The phenomenon of herd immunity has been invoked as a strategy for protecting young infants during the window of vulnerability age interval. It is expected that as immunization coverage increases in a community, the risk of measles exposure before the age of nine months will diminish. However, this strategy has not always been effective. In urban districts of Guinea-Bissau, an increase in vaccine coverage from 61% to 80% did not reduce measles incidence among infants <9 months of age, presumably because of the virus’ extreme contagiousness (19). If crowding is present, even if coverage is very high, MV manages to infect the susceptibles (20). Furthermore, although the recent experience of the measles elimination program in the Americas shows that transmission can be interrupted, sustained control of endemic measles requires that very high immunization coverage be maintained. This has necessitated a first dose at 9 to 12 months or shortly thereafter, plus a second dose provided through either routine services and/or repeated supplemental campaigns (21). The occurrence of multi-country outbreaks involving tens of thousands of cases in Latin America (following an importation) illustrates the daunting task of sustaining measles elimination despite the implementation of supplemental campaigns (22). Currently, all countries in the Americas, and selected countries in Europe, the Middle East, sub-Saharan Africa, Oceania, and Asia have adopted immunization strategies aimed at measles elimination and have made substantial progress toward this goal (10,23). These successes have led some experts to believe that measles elimination in all regions of the world is feasible on the basis of existing measles vaccine and strategies (24). However, other authorities, taking into account the extreme transmissibility of measles and the limitations of the current vaccines, are more skeptical (25). Still others take the view that measles elimination in some regions such as sub-Saharan Africa might be possible if an improved vaccine were available that could reliably immunize and protect very young infants (26). The following characteristics would be desirable for an improved, new measles vaccine: (i) is safe for young infants; (ii) is not neutralized by maternal antibody; (iii) induces protective immunity equivalent to the current vaccines; (iv) is effective earlier in life than the current vaccines; (v) is amenable to large-scale economical manufacture; and (vi) is easy to administer. Optimally, the vaccine could be administered using a needle-free delivery system.
