US measles cases are undercounted, experts say, but real numbers are proving hard to pin down

CNN

15-04-2025

During a Cabinet meeting last week, US Health and Human Services Secretary Robert F. Kennedy Jr. downplayed what is now the second-worst measles outbreak in the US since the disease was declared eliminated in 2000.
Kennedy has claimed repeatedly that measles cases have 'plateaued,' despite contradictory data from his own federal agencies.
On Friday, the health department in Texas – the center of the outbreak that now spans multiple states – reported 232 new cases over the past three weeks, 42% more than in the three weeks before that. New outbreaks have also been reported in Indiana and Ohio.
Overall, there have been at least 735 cases in 24 states this year, according to CNN's tally.
However, many experts believe that the actual case count stands in the thousands.
'I don't think any of us have full situational awareness of what's going on with this outbreak,' said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security. 'You can't say something is flattening if you don't actually know the denominator of cases or [have] an understanding you're getting [the] full capture of the cases.'
Some experts think the number of deaths alone indicates that cases are deeply undercounted. The measles fatality rate is typically up to 3 deaths per 1,000 cases. But three deaths have already been reported in the ongoing outbreak in — two children in Texas and one adult in New Mexico — even though the official outbreak case count stands at not quite 650 between Texas, New Mexico, Oklahoma and possibly Kansas.
Underreporting could also be happening for a variety of reasons, and that makes it difficult to predict exactly how bad this outbreak will get and how long it will last, said Dr. Nina Masters, a senior research scientist at Truveta, a health-care data and analytics company, and a former epidemiologist at the US Centers for Disease Control and Prevention.
Better data would be helpful, Masters said, to help public health officials set expectations about how big an outbreak might get, to help public health departments figure out where to allocate resources and to better understand what might be affecting cases.
The outbreak has mainly infected people who were not vaccinated or whose vaccine status is unknown. If a local agency knows who is unvaccinated, it could do more targeted outreach to help persuade them to get protected against the virus.
The measles-mumps-rubella (MMR) vaccine works rapidly and has been shown to provide the best protection against measles, particularly when it's given to people in affected areas quickly.
During a measles outbreak at a migrant shelter in Chicago last year, Masters – then with the CDC – created a model to predict the size of the outbreak and another to determine what effect interventions would have.
'In both of those models, the results were very non-subtle that mass vaccination is really the way to bring these outbreaks under control, and the days matter. If you delay by a week, that increases the size of the outbreak,' she said.
Without mass vaccinations in the affected community in Chicago, the model predicted, there would be 250 cases. But because vaccines were administered quickly, there were only 57.
In 2019, Dr. Mark S. Roberts was part of an effort to create a model that took into account Texas' expected number of unvaccinated to predict what a measles outbreak there would look like.
Until recently, the prediction was eerily similar to the case count in the current outbreak, said Roberts, a distinguished professor in the Department of Health Policy and Management and director of the Public Health Dynamics Laboratory at the University of Pittsburgh School of Public Health.
Now, though, cases in some counties have even outpaced the prediction.
Gaines County, Texas, the epicenter of the outbreak, had 355 cases as of April 11, and the state health department says that number is expected to continue to rise.
If there had been an outbreak in Gaines County in 2018, Roberts' model predicted 270 cases, 100 of whom would be 'bystanders' — not children whose parents refused vaccination but those who couldn't get vaccinated or for whom the vaccine hadn't fully worked.
Roberts and many other public health experts are frustrated by the deaths that have already taken place.
'It's freaking 2025, and a child has died of measles,' Roberts said. 'It's just not right. This is a disease that, 12 years ago, we basically had eradicated.'
Models are helpful in an outbreak, but even with all the data in the world, Roberts knows, not everyone is going to listen to the message that the unvaccinated need protection.
In 2019, he presented his model to the Texas Legislature. Lawmakers asked good questions and understood what he showed them, he said, but not a single law was passed to improve vaccination rates in Texas.
'The thing that's really sad about this is that all of this was predictable and totally avoidable,' Roberts said. 'It's just really sad.'
Cases may be undercounted in the ongoing outbreak, Masters said, because people aren't getting tested or because they may be staying away from hospitals. In the Mennonite community that has been the center of the outbreak in Texas, there is a great sense of self-reliance and a preference for home remedies over doctors.
'I think it's a little difficult to tease apart which element of the underreporting Is leaning in which direction,' Masters said.
Another reason it's been hard to get an accurate case count is because of massive federal funding cuts for public health. The CDC pulled back over $11 billion in grants allocated during the Covid-19 pandemic that state and local health departments had been using to respond to a variety of public health threats, including measles.
In New Mexico, where at least 58 cases have been reported, a pullback of federal funds meant the public health department had to terminate contracts for 20 temporary workers who were helping with the measles response by checking vaccination records.
Dr. Phil Huang, Health and Human Services director in Dallas County, Texas, said that in addition to canceling dozens of vaccine clinics in school districts with low vaccination rates, he had to lay off 11 permanent staff and 10 temporary workers, including three who work in the labs used to detect measles.
'Those are our numbers so far,' he said. 'We are looking at perhaps a few further cuts in the near future.'
These cuts mean real numbers may become even more difficult to pin down.
Measles, a highly contagious virus, behaves in an entirely predictable fashion, Masters said.
'It behaves almost the same way in almost every setting,' she said. 'If someone has measles, they go into a room with a bunch of people who are susceptible to measles, a lot of those people are going to get sick.'
Kennedy said last week that the US is a 'model for the rest of the world' when it comes to managing measles, but the country is no longer as effective at preventing outbreaks because of decreasing vaccination rates.
'Saying we're better or no worse off than other countries, that's misleading at best and kind of a whitewash of the significance of this measles outbreak,' said Dr. Bill Moss, a professor in epidemiology and executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health.
'It is true that, historically, the United States has done better than many other countries in the world,' he said. 'That's because we've had a strong immunization program.'
Measles had been declared eliminated in the US in 2000 because the majority of people were vaccinated, but that number has been falling since the Covid-19 pandemic – even before a lawyer with a history of vaccine skepticism took over HHS.
In 2024, just 68.5% of kids in the US had their first dose of the MMR vaccine by 15 months of age, according to a new report from Truveta. This is a significant decline from 2020, when more than 77% of kids got their first shot by this age. And it's far short of the federal goal of 95% of children in kindergarten having had their second dose, the threshold needed to prevent measles outbreaks.
Coverage of the MMR vaccine is particularly low in Gaines County, where nearly 1 in 5 incoming kindergartners in the 2023-24 school year had not gotten the shot. Some other counties in the affected states also fall below the 95% benchmark, according to CDC data.
In Kansas, for example, only 90% of kindergartners have gotten their MMR vaccines. The Sublette Unified School District in the southwestern part of the state, where most of the state's cases are concentrated, has a vaccination rate of just 44%, according to data from the state health department.
Working on Truveta's report 'set off real alarm bells for me,' Masters said.
Current measles vaccination rates mean the country is no longer closing immunity gaps like it used to, 'which is very concerning for the future of this outbreak,' she said.
Kennedy has said that people should get the MMR vaccine, but he has also said that doing so should be a personal choice.
With so much hesitancy in areas that have seen so many cases, experts say that here needs to be a clearer message from the top and that, as with a 2019 measles outbreak, many more people will need to be convinced that getting a vaccine is not just in their best interest, it is also in the best interest of the community.
'It's important for people to know, not vaccinating your child doesn't only put your child at risk, it puts other children at risk,' Roberts said.
CNN's Deidre McPhillips contributed to this report.