— But moving up the initial dose could come with its own trade-offs
October 23, 2026 • 3 min read
WASHINGTON -- At 6 months of age, most U.S. infants no longer had detectable levels of maternal neutralizing antibodies for measles, potentially leaving them at risk of an infection months before their first dose of the measles, mumps, and rubella (MMR) vaccine at 12 months of age, according to a retrospective analysis.
At birth, infants' median concentrations of neutralizing antibodies for three major circulating measles strains exceeded their mothers' levels. But those infant levels began consistently dropping below their moms' levels by 2 months of age. By 6 months, the infants' median neutralizing antibodies had fallen below 120 mIU/mL -- the study's correlate of measles protection -- for all three measles strains.
At 6 months, the proportions of infants who were seropositive for the three measles strains were 8.7% for the Edmonston strain (A), 19.6% for Maryland.USA/18.18 (B3), and 17.4% for Missouri.USA/15.18 (D8), reported Buddy Creech, MD, MPH, of Vanderbilt University in Nashville, Tennessee.
"What this means is that infants are going to be susceptible to measles virus infection sooner than maybe we previously thought," Creech said at the IDWeekopens in a new tab or window annual meeting. "As measles circulation increases, infants are going to be an important vulnerable population to consider in our response."
The findings could force a rethink about measles vaccination timing, he cautioned. "The immunologic impact of earlier vaccination, such as 9 or 10 months of age, should be evaluated in a well-powered clinical study," Creech noted. "At the very least, we're going to have to think about what it means to vaccinate in early toddler time -- 12 to 15 months -- and maybe accelerate the second dose."
The American Academy of Pediatrics (AAP) recommends two doses of either the MMR vaccine or the combination MMR and varicella vaccine, with the first dose at age 12-15 monthsopens in a new tab or window and the second dose at 4-6 years. CDC recommendationsopens in a new tab or window mirror those of AAP, though the agency controversially moved to recommend separate MMR and varicella vaccinesopens in a new tab or window for the first dose. Further complicating things, the Trump administration has proposed separating all componentsopens in a new tab or window of the MMR vaccine into individual shots, and spacing them out whenever possible.
The findings come as the U.S. grapples with its worst measles case total since 1991. As of Oct. 22, the CDC reported 4,588 casesopens in a new tab or window, already doubling last year's total of 2,289. Nearly all (96%) of this year's cases have happened in people who were unvaccinated or whose vaccination status was unknown. While the CDC reports three measles deaths this year, Pennsylvania alone has reported five measles-associated deathsopens in a new tab or window.
Despite the spike, Health Secretary Robert F. Kennedy Jr. and other Trump administration officials continue to fuel vaccine skepticism with disproven claims of vaccine-driven dangersopens in a new tab or window, including from the MMR vaccine.
Starting Sooner Comes With Costs
Given the range of risks for infants, including subacute sclerosing panencephalitisopens in a new tab or window, why not move up the first measles vaccine dose to 6 months? That's already happening when an infant is going to travel internationally or when they might be exposed to a local outbreak cluster, Creech noted.
Measles vaccine effectiveness is very different, however, when the first dose is at 6 months rather than 9 months: 51% versus 83%, according to a 2019 meta-analysisopens in a new tab or window, he cautioned.
Vaccinating infants with a first dose at age 9 months might be warranted given higher measles virus circulation in the U.S., he noted, particularly if the U.S. loses its measles elimination statusopens in a new tab or window. "I think our data would imply that if we continue to wait until 12-15 months to vaccinate, we're going to have a subsection of our population at risk for measles and for its complications," Creech said.
Creech's team analyzed measles binding and neutralizing antibodies in serum samples from maternal-infant pairs and adults in the MOMI-Vaxopens in a new tab or window and the Mix & Matchopens in a new tab or window studies, which explored COVID-19 vaccination.
Most of the infants' 6-month neutralizing-antibody levels were at assays' lower detection limits, a finding that indicates their antibody titers were probably much lower than that, Creech noted. The same downward trend seen in neutralizing antibodies happened with the infants' binding antibody levels. By age 6 months, median levels were below the correlate protection levels.
A secondary analysis showed that the immunologic protection given by moms to babies has waned over time as vaccination rates have risen. Among mothers who were born before the 1967 launch of the U.S. measles eradication program, infants' antibody levels remained high enough to likely confer protection for an estimated 6 months. Among mothers born after 1967 -- who didn't face measles infection or residual circulating measles -- infant protection fell to an estimated 4 months.