DECAY OF MATERNALLY DERIVED MEASLES ANTIBODY IN A HIGHLY VACCINATED POPULATION IN SOUTHERN ISRAEL

The introduction of live attenuated measles vaccine in Israel during 1 967 dramatically decreased the incidence of measles. However, cases st ill occur in periodic outbreaks and epidemics, with an increasing prop ortion of infants and children younger than 2 years of age. We examine d the decay of maternally derived measles antibody during the first ye ar of life in the Jewish population of Israel which represents a highl y vaccinated population with immunization rates exceeding 90%. We used sera of healthy full term infants born in 1988 and 1989. Fifty specim ens for each of the following age groups were used: 0 (cord blood), 2, 4, 6, 7 and 12 months. Three assays for each specimen were used: enzy me-linked immunosorbent assay (ELISA); hemagglutination-inhibition tes t (HI); and neutralization test (NT). Good correlation among all 3 tes ts was found. All cord blood specimens were positive by at least 2 ass ays. Seropositivity rates declined rapidly with age. Fifty percent of all 4-month-old infants and <30% of all 6-month-old infants were posit ive by 1 test or more; at 12 months of age none of the tested specimen s was positive by HI or NT and only 1 of 50 infants was positive by EL ISA. In infants younger than 6 months of age, 5 (22%) of 23 specimens negative both by ELISA and by RI were positive by NT, but in 6-month-o lds, only 2 (7%) of 28 negative by ELISA and HI were positive by NT, a nd in 12-month-olds none was positive. The results from southern Israe l are similar to those obtained in North America and provide evidence that infants older than 6 months of age in a well-immunized population may be poorly protected against measles. On the basis of this informa tion and epidemiologic data, the Israel Ministry of Health has recomme nded lowering the immunization age for measles, mumps and rubella from 15 months to 12 months.