Written by Vanessa McMains, PhD
After public health experts declared measles eliminated in the U.S. in 2000, the U.S. Center for Disease Control and Prevention (CDC) established seven indicators of measles elimination status to ensure that the country remained on track. Now, analyzing these same indicators, Boston Children's Hospital researchers find that the U.S. missed four of the seven criteria, with the others at risk. These findings are published in The Lancet .
The researchers who performed the analysis included Maimuna Majumder, PhD, MPH, the Inaugural Peter Szolovits Distinguished Scholar in the Computational Health Informatics Program at Boston Children's, and their postdoctoral research fellow Anne Bischops, MD, a pediatrician and German National Academy of Sciences Leopoldina Fellow. The team evaluated the number of U.S. measles cases, outbreaks, their origination, and the levels of transmission. Their results suggest that measles is making a comeback in the US, spreading continuously for more than a year.
The latest string of U.S. outbreaks began in Texas in January 2025. Since then, outbreaks have spread to 45 states. When the U.S. was last recertified for measles elimination status in 2011, the country achieved all the measles elimination indicators established by the CDC's National Immunization Program. But this year, according to this new research, most of the indicators are in the red.
Missed indicators of measles elimination status:
- Low number of cases with a cutoff of less than one case per 10 million people. As of early 2026, the U.S. had about 93 cases per 10 million people, exceeding this limit.
- Most of the measles cases should come from abroad rather than from internal spread. Since the start of 2025, only 6-7 percent of the measles cases came from abroad, meaning most cases derive from within the U.S.
- A limited number of outbreaks (a cutoff of approximately four) with no more than about six cases each. Last year in the U.S., 48 outbreaks resulted in more than 2,000 cases. And in early 2026, at least 19 outbreaks already resulted in more than 1,600 cases.
- A level of transmission less than one, meaning one infected person only spreads measles to fewer than one other person on average. The U.S. exceeded this rate more than 75 percent of the time since early 2025.
Indicators at risk:
- The country achieves four weeks with all infections deriving from outside the U.S. Since the first infection in January 2025, the U.S. hasn't gone four weeks without infections with 90% of cases acquired here.
- Herd immunity through vaccination. An estimated 95 percent of people need to receive two-doses of the measles vaccine to achieve herd immunity, typically given as part of the MMR (measles, mumps, and rubella) vaccine. However, the U.S. average vaccination rate of kindergartners in the 2024-2025 school year was 92 percent, meaning this indicator is at high risk. In Texas for example, vaccine rates range as low as 79 to 90 percent, according to an earlier study from the team .
- Measles cases don't share a common viral strain. From ongoing genetic analyses, preliminary data show that the majority of cases share the same viral strain and are likely part of the same transmission chain.
"Declining vaccination rates have already been a warning sign that measles could return," said Bischops. "However, losing status would be a clear and very concerning indicator."
The researchers believe these findings make a strong case for vaccinating children to protect them from a young age.
"Viral infections aren't all benign and a measles infection even when cleared can result in lifelong problems," said Majumder. "Babies less than a year old are among those at greatest risk for severe complications, and the full impact on children exposed during the current outbreak may only show up years later."
This data aims to provide insights/an early warning framework ahead of a meeting of the Pan American Health Organization's expert panel in November 2026 to reevaluate the U.S.'s measles elimination status.
Research letters published in the Correspondence section include research findings and are externally peer-reviewed. Unlike Articles containing original data, research letters are shorter and the research they contain is usually preliminary, exploratory, or reporting on early findings.
