US builds case to retain measles elimination status as infections mount

CHICAGO – US officials are building a case that ongoing measles outbreaks in South Carolina and other states are unrelated to the major outbreak in Texas in 2025, as it tries to retain its status of having eliminated the disease after recording the highest number of confirmed infections in three decades.

To be considered measles-free by the World Health Organization (WHO), a country must have no locally transmitted cases of the same strain for 12 months or longer.

Scientists studying the issue say the elimination assessment may not be ironclad, as it is difficult to be certain that all cases potentially linked to the Texas outbreak have been reported.

A large outbreak of measles began in Texas in January 2025 and spread to several states, followed by large outbreaks along the Arizona-Utah border and in South Carolina, which continue to cause infections.

In November, the Pan American Health Organization (PAHO) – part of the WHO – determined that Canada had lost its measles elimination status after nearly three decades, due to its failure to curb a year-long outbreak of the vaccine-preventable disease.

In 2026, PAHO will evaluate US measles data, which involves 2,065 confirmed cases, to see if it can retain the measles elimination status it has had since 2000.

Former US public health officials have described measles elimination status as a key indicator of a nation’s health.

Must prove cases are unrelated

The US will have to prove the ongoing measles cases are not related to the Texas outbreak and were instead brought into the country by infected travellers, said Dr William Moss, an epidemiology professor at the Johns Hopkins Bloomberg School of Public Health.

State public health officials have used traditional methods of interviewing infected individuals to identify chains of transmission, as well as comparing the viral genotype – a section of the virus’ DNA – to see if cases are related. Experts said this may still not be sufficient to link cases in a large outbreak.

Dr Kelly Oakeson, who leads next-generation sequencing at the Utah Public Health Laboratory, said none of the patients interviewed in Utah mentioned travel to Texas or contact with people from Texas, but many gave incomplete information.

Based on the state’s detailed genetic analysis, however, he said the Utah strain is “different enough” from the Texas outbreak to suggest they are unrelated.

“We do not think there is a direct link,” he said, adding that the state has been working with the Centers for Disease Control and Prevention (CDC), Texas and Arizona to characterise transmission patterns.

In addition, the CDC has been analysing the entire genetic code of virus samples to determine if the outbreaks are related, a newer technique that has not traditionally been used to assess measles transmission.

Early evidence from the CDC analysis, which has not been made public, suggested the outbreaks are not related, according to two sources who have seen the data.

In a Dec 5 post on social media platform X, CDC acting director Jim O’Neill said there was no epidemiological evidence linking the Texas outbreak with the ongoing cases in South Carolina.

A US Health and Human Services spokesman confirmed that the CDC has found no epidemiological evidence linking ongoing outbreaks to Texas, but said many recent US cases share the same genotype and have no known source of infection, “which could indicate ongoing domestic transmission”.

Possible gap in cases

Dr Noel Brewer, an infectious disease doctor at the University of North Carolina who has seen the CDC data, said the preliminary evidence suggests that new cases in Texas stopped in July, and there was a three-week gap before the Utah-Arizona outbreak began.

“No cases linked the two outbreaks in any clear way,” said Dr Brewer, who chairs an independent committee that will analyse the US data and make a report to PAHO on whether it agrees with the US assessment.

The CDC’s inability to link the outbreaks, he said, may simply reflect that the virus is circulating broadly in the US, and that it is “not possible at this time to track all of the cases”.

There are many gaps in epidemiological knowledge in the US outbreaks and not all cases are identified or reported, said Johns Hopkins’ Dr Moss, so you can miss cases linking two outbreaks.

In addition, many of the larger US outbreaks are occurring in communities that do not trust the public health system and may be less likely to report cases or participate in an investigation.

That could lead scientists to incorrectly conclude that two viruses are different when it was just a sampling problem, Dr Moss said.

Dr Demetre Daskalakis, former CDC director of the National Center for Immunization and Respiratory Diseases, who quit in August over concerns about US vaccine policy under Health Secretary Robert F. Kennedy Jr, said PAHO will make the call based on the evidence it receives.

“If they do not believe in the strength of one part of the data, and there are other compelling factors that say that the US has lost elimination, it means that the US has lost elimination.” REUTERS