Wiping out measles: When to vaccinate? CMAJ 1997;156:979 The measles outbreak reported in the article "Outbreak of measles in a highly vaccinated secondary school population" (CMAJ 1996;155:1407-13 [abstract / résumé]), by Drs. Penny A. Sutcliffe and Elizabeth Rea, is one of many such outbreaks during the last several years in North America. These outbreaks prompted our southern neighbour to switch to a 2-dose measles-vaccination strategy a long time ago. The article and the accompanying editorial "Elimination of measles in the Americas" (CMAJ 1996;155:1423-6 [full text / résumé]), by Dr. John Furesz, support a 2-dose strategy to eliminate measles. However, the timing of the 2 doses is an issue that remains to be settled. In all Canadian provinces, the first dose of measles­mumps­rubella (MMR) vaccine is administered at 12 months of age, except in PEI, where it is given at 15 months. In the new 2-dose strategy, a second dose is given at 18 months in Newfoundland, Quebec, Saskatchewan, BC, Yukon and the Northwest Territories, and at 4 to 6 years in PEI, Nova Scotia, Ontario, Manitoba and Alberta. Both schedules are consistent with the recommendations of the National Advisory Committee on Immunization. Our studies of measles-vaccine response, vaccine failure and waning immunity shed some light on the timing of the 2 doses. Our data show that up to 16% of children who receive the first dose of MMR vaccine at 12 months do not respond adequately and remain without protective immunity after the first dose.[1,2] This lack of immunity cannot be attributed entirely to maternal measles antibody interfering with the live-virus vaccine, since two-thirds of the 16% of children without immunity do not have maternal antibody at the time of vaccination.[1] In addition to the immunogenicity of the vaccines used, the suboptimal response has to do with the maturity of the immune system and its ability to respond at the time of vaccination. Pools of susceptible children therefore remain after the first dose,[1­3] and this could have led to outbreaks in vaccinated children, like the one in Ontario, during the past decade. This implies that, if the first dose is given at 12 months, a second dose should be considered sooner than later. In this context, giving a second dose at 18 months appears appropriate. However, administering a second dose at 4 to 6 years of age, around the age of school entry, addresses the immediate public health concern about school-based outbreaks and is also convenient and economical. Our data indicate that 28% of children 5 to 17 years of age who received a single dose of MMR vaccine at 1 year of age have inadequate protective immunity against measles.[4] A second dose given at 4 to 6 years of age can act as a booster for those with waning immunity. Nevertheless, delaying a second dose to 4 to 6 years may not be a sound decision if the first dose was given at 12 months. A substantial proportion of preschool children would remain without adequate protection because of primary failure or suboptimal response. The question is whether these infants would form a large enough pool to allow outbreaks or simply to help sustain the transmission of measles. The alternative is to delay the first dose to 15 to 18 months to ensure a better initial response, in which case giving the second dose around school entry becomes a suitable strategy. As mentioned in the editorial, this strategy, among other factors, has been successful in eliminating not only measles but also rubella and mumps in Finland. Interestingly, the smallest Canadian province has chosen this strategy; outcomes in PEI could provide important information for the rest of the country. Our study data also indicate that, in contrast to measles, vaccine-induced rubella immunity declines significantly only after 8 years of age.[5] In this regard, a second dose of MMR vaccine may also help prevent secondary failure of vaccination against rubella. Also, from the standpoint of sustained immunity to rubella during childbearing years, administration of the second dose around school entry or even later is likely to be more beneficial.[6] Canada is poised to achieve the goal of elimination of measles and rubella during pregnancy. Since the provinces have adopted different delivery schedules to achieve this goal, we should be able to find some answers to the question of timing in order to develop an optimal strategy for Canada. Sam Ratnam, PhD, MPH Roy West, PhD Veeresh Gadag, PhD St. John's, Nfld. References Ratnam S, West R, Gadag V, Burris J. Measles immunization strategy: measles antibody response following MMR II vaccination of children at one year of age. Can J Public Health 1996;87:97-100. Ratnam S, Chandra R, Gadag V. Maternal measles and rubella antibody levels and serologic response in infants immunized with MMR II vaccine at 12 months of age. J Infect Dis 1993;168:1596-8. Ratnam S, West R, Gadag V, Williams B, Oates E. The Newfoundland measles cohort study: measles immunity after one and two doses of measles­mumps­rubella (MMRII) vaccination [abstract]. Immunizing for Health -- Achieving our National Goals conference; 1996 Dec 8­11; Toronto. Ratnam S, West R, Gadag V, Williams B, Oates E. Immunity against measles in school-aged children: implications for measles revaccination strategies. Can J Public Health 1996;87:407-10. Ratnam S, West R, Gadag V, Williams B, Oates E. Rubella antibody levels in school-aged children in Newfoundland: implications for a two dose rubella vaccination strategy. Can J Infect Dis. In press. Johnson CE, Kumar ML, Whitwell JK, et al. Antibody persistence after primary measles­mumps­rubella vaccine and response to a second dose given at four to six vs eleven to thirteen years. Pediatr Infect Dis J 1966;15:687-92. 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