MEASLES IMMUNITY AND RESPONSE TO REVACCINATION OF A YOUNG-ADULT POPULATION IN ISRAEL

In order to evaluate the true immune status and the effect of revaccin ation on a young adult population, we collected serum samples from 289 military recruits who were vaccinated during an outbreak in 1991. Mos t vaccinees, age 18-25 years, had apparently been immunized once befor e as infants. Sera collected just prior to the vaccination and 14 and 28 days afterwards were tested for measles antibodies by hemagglutinat ion inhibition (HI) and enzyme-linked immunosorbent assay (ELISA)-IgM. Before vaccination, 46 (15.9%) of the subjects had no HI antibodies, (< 1 : 4) and 48 (16.6%) had borderline (1 : 4) HI titer. Following va ccination, only ten (3.5%) remained negative and 19 (6.6%) had borderl ine titer. The increase in HI antibody titer was inversely proportiona l to the prevaccination titer, and 159 subjects (55.0%) showed no incr ease at all. The geometric mean titer (GMT) rose from 9.14 to 21.47. A mong the prevaccination-negative subjects (HI < 1 : 4) 28 (60.9%) reac hed a postvaccination titer of greater than or equal to 1 : 8, and eig ht (17.4%) reached a titer of 1 : 4. Twelve (26.1%) of the negative su bjects seroconverted and developed IgM, 16 (35%) seroconverted without IgM, and 18 (39%) remained negative and did not develop IgM. A group of eight vaccinees with prevaccination titer of greater than or equal to 1 : 4 developed IgM. Some were probably infected by the circulating wild-type virus prior to the vaccination. Thus, a total number of 20 of the 289 subjects studied (6.9%) had true negative preimmune status as judged by the IgM test. However, the vaccination campaign prevented further measles cases, apparently by increasing the population's immu nity, particularly in individuals with very low titers or without meas les antibodies. (C) 1996 Wiley-Liss, Inc.