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Most U.S. infants lose their mothers' measles antibodies by 6 months of age — well before the first MMR vaccine at 12–15 months — leaving a dangerous window of vulnerability. A new study presented at IDWeek found that fewer than 1 in 5 infants were still measles-seropositive by 6 months. Researchers say it may be time to rethink vaccination timing, especially as the U.S. faces its worst measles surge since 1991.
Most U.S. infants are losing their maternal measles antibody protection by 6 months of age — months before they're scheduled to receive their first MMR vaccine dose at 12–15 months. A retrospective analysis presented at IDWeek 2026 by Dr. C. Buddy Creech of Vanderbilt University found that by age 6 months, fewer than 20% of infants remained seropositive for any of the three measles strains tested, raising serious concerns about an unprotected window during a record-breaking measles outbreak year.
The study analyzed leftover serum samples from 92 mother-infant pairs. While infants started with higher antibody levels than their mothers at birth, those levels dropped sharply — falling below the protective threshold of 120 mIU/mL for all three measles strains by 6 months. A key driver: today's vaccinated mothers pass on fewer antibodies than mothers who had natural measles infection, shrinking the window of infant protection from ~6 months to ~4 months.
By the Numbers:
Why it matters: With the U.S. potentially losing its measles elimination status and vaccine skepticism on the rise, this study highlights a critical gap in infant protection. Researchers are calling for clinical studies on earlier vaccination — potentially at 9–10 months — to close the window before more vulnerable infants are exposed.
