Measles Vaccine: First Dose at 12 Months May Erode Potency Later

Diedtra Henderson

October 21, 2013

Quebec schoolchildren were 6.2 times more likely to contract measles during a 2011 outbreak if they had received the first of a 2-dose vaccination at 12 to 13 months of age than if they received it at 15 months of age, according to a case-control study.

Fannie Defay, MSc, from the Unité de Recherche en Santé Publique, Centre hospitalier universitaire de Quééec, Canada, and colleagues published the results of their study online October 21 in Pediatrics.

An initial investigation of the 2011measles outbreak, centered at a Quebec high school, revealed a sobering vulnerability among schoolchildren who had received a second dose of the measles vaccine. Rather than boosting their protection, vaccine effectiveness at that high school dipped among twice-vaccinated students (94.2% vaccine efficacy) vs once-vaccinated students (95.9%).

Defay and coauthors sought to shed light on how the age of students' first vaccination affected their measles risk. Although the recommended age for the first measles shot for American infants is 12 to 15 months, in Canada, the recommended age for the first dose has been 12 months since 1970. The research team looked at all measles cases reported in Quebec in 2011 among twice-vaccinated school-age patients attending 17 schools, matching 102 students with confirmed measles with 510 control students. All of the students had received their first vaccine dose at age 12 months or older.

"In participants outside the outbreak school, when the first dose was administered at 12 to 13 versus ≥15 months of age, measles risk was 6.2 times higher (95% [confidence interval] CI, 1.33–29.3; P = .02), and in pooled analysis it was 5.2 times higher (95% CI, 1.91–14.26; P = .0013)," Defay and coauthors write.

"A pooled fivefold greater risk of measles among those whose first [measles, mumps, and rubella] dose was administered at 12 to 13 vs ≥15 months is concerning, especially in the context of measles elimination efforts that require high levels of immunity."

The vaccine efficacy results come on the heels of a study published last week in JAMA Pediatrics that found compelling safety reasons for not delaying the first measles dose beyond 12 to 15 months.

The lead author of a commentary published in concert with that study told Medscape Medical News that both articles underscore the importance of optimally timing measles vaccination.

"The most important thing is to ensure protection when children are most at risk, and to do so safely," Kristen A. Feemster, MD, MPH, MSHPR, from the Department of Pediatrics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, and Division of Infectious Diseases and the Vaccine Education Center, The Children's Hospital of Philadelphia, Pennsylvania, told Medscape Medical News. "The paper [by Defay and colleagues] raises some issues about different efficacy when vaccinated at 12 vs 15 months, but for a pediatrician, I think it's still important to focus upon the currently recommended window," Dr. Feemster said.

The authors of the Quebec study caution that the erosion of vaccine protection in certain children should trigger additional studies.

"Although unvaccinated people should remain the prime target for measles vaccination, the unexpected vulnerability we have identified in twice-vaccinated people could ultimately lead to failed measles elimination efforts," Defay and colleagues conclude. "If the effect of early vaccination permanently alters the ability to respond to subsequent doses, even adding a third or fourth dose may not provide long-lasting protection."

Support for the study was provided by the Ministère de la Santé et des Services Sociaux du Québec. One author disclosed receiving research funds from GlaxoSmithKline (GSK) and Sanofi Pasteur and travel reimbursement from GSK. A second author disclosed receiving research funds from GSK. Another study author disclosed employment by Medicago Inc; receiving research funds from GSK, Sanofi Pasteur, and Pfizer; and being an advisory board member for GSK, Merck, Novartis, Pfizer, and Sanofi Pasteur. The remaining authors and the commentator have disclosed no relevant financial relationships.

Pediatrics. Published online October 21, 2013. Abstract

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