CDC EIS research finds U.S. measles cases likely undercounted, outbreak larger than reported

CDC’s measles dashboard showed 1,893 confirmed U.S. cases as of May 14, in data updated May 15. Research presented at the Centers for Disease Control and Prevention’s Epidemic Intelligence Service conference in late April suggested the real number of infections is higher than that official tally. (cdc.gov) The gap matters because the CDC count includes confirmed cases reported by jurisdictions, while public health investigators say some infections are missed when people are not tested or do not seek care. School exposure notices in Utah this week offered a local example of how transmission is still surfacing in routine settings. (mediarelations.gwu.edu) ### If the CDC says 1,893 cases, why do researchers think the outbreak is bigger? CDC said its national page counts confirmed measles cases that jurisdictions have notified to the agency, and it noted it is aware of probable cases that are not included in that total. (kutv.com) As of May 14, the agency said 1,893 confirmed cases had been reported in 40 jurisdictions, with 27 outbreaks in 2026 and 93% of cases linked to outbreaks. George Washington University, in an April 27 media summary of research presented at the CDC conference, said new evidence suggests measles may be spreading “far more widely” than official counts show. That summary said advanced genomic sequencing from an outbreak spanning Arizona and Utah indicated the outbreak may be at least 6.5 times larger than reported, and that transmission may have been ongoing for more than a year. (cdc.gov) ### What is the evidence for “silent spread”? Investigators studying the Arizona-Utah outbreak used genomic evidence to estimate when transmission began and how large it became, according to reporting on the conference findings. (cdc.gov) CNN, in a report carried by AccuWeather, said health officials believed the true number of patients was larger than known and that the outbreak along the Arizona-Utah line had grown to more than 600 reported cases. Oregon added a second signal. The George Washington University summary said wastewater surveillance detected measles virus 100 times across 23 counties despite only six reported cases there, which researchers cited as evidence of underdetection. (mediarelations.gwu.edu) ### Why would measles cases go uncounted? Doctors and patients are part of the reporting gap, according to the same outbreak reporting. CNN said outbreaks are often undercounted because clinicians may miss diagnoses of diseases they rarely see and some patients may not seek medical care or testing. In the Arizona-Utah outbreak, the report said families described illnesses consistent with measles before the first official case was logged on Aug. 8, 2025. (accuweather.com) The CDC’s own case page also points to the limits of the official number. The agency said its national total excludes probable cases and depends on reports sent in by states and other jurisdictions. (mediarelations.gwu.edu) ### What do the latest school exposures show? Heber City, Utah, reported new school-linked exposures this week. KUTV said the Wasatch County Health Department notified parents that positive measles cases were identified at Old Mill Elementary and Timpanogos Middle, with possible exposure at the elementary school on May 15 and at the middle school on May 14 and May 15. KPCW reported one student at each school had measles and said unvaccinated students and staff were being asked to stay home for 21 days after the last known exposure date. (accuweather.com) The station said Wasatch County had recorded 11 confirmed cases since November 2025. (cdc.gov) ### How large is the U.S. outbreak now? The CDC said 1,893 confirmed cases had been reported by May 14. Earlier weekly updates showed how quickly the count was rising: the American Academy of Pediatrics reported 1,792 cases on April 24 and said about 72% of cases were in children and teens, while about 92% were in unvaccinated people or those with unknown vaccination status. (kutv.com) The next public markers are likely to come from the CDC’s weekly case updates and from state and local health departments posting new exposure notices. AAP News also reported that a Pan American Health Organization committee is scheduled to meet in November to determine whether the United States and Mexico have lost measles elimination status. (publications.aap.org) (cdc.gov) (kpcw.org)