The back of a woman with measles. Credit: Wellcome Collection via Wikimedia Commons.

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US measles outbreak: is it the next pandemic’s pathogen of choice?

Apr 17 2025

Not stopping the US’ measles outbreak in its tracks could have dire global repercussions

By mid-April 2025, the measles outbreak in the United States continued to spread without appearing to slow down, according to data collated by the US Centers for Disease Control and Prevention (CDC), the country’s federal public health agency. Worldwide, other organisations tracking public health are reporting cases are on the rise across the globe.

Measles is highly contagious respiratory-borne virus with a reproduction number – the so-called ‘R-number’ – of up to 18, resulting in very many new infections from contact with a single disease-carrying individual. Indeed, when an infected individual coughs or sneezes the virus can stay suspended in the air for more than two hours. When seen in the context of outbreaks across multiple countries at the same time, public health concerns are heightened about measles spreading globally and becoming increasingly difficult to contain.

Outbreak spreads

Officials in the US have confirmed more than 500 cases of measles including localised hot spots in Washington, DC and Maryland. There have been two deaths reported so far: a child in Texas and an unvaccinated adult in New Mexico. The New Mexico State Health Department said in a statement that the deceased had not sought any medical care. These are the first deaths from measles in the US since 2015.

Reports also show that almost 95% of infections are among the unvaccinated, or those who have a vaccination status that could not be confirmed. Roughly 80% of cases are in children and adolescents, up to 19 years old.

Vaccination programmes introduced in the US in 1963 had led to measles being declared eliminated in the country in the year 2000. Vaccination rates had become sufficiently high to bring about so-called ‘herd immunity’ which can contain any onward transmission from an infected subject by severely limiting contact with a new host to infect. At that time, it was considered unthinkable that the US would see a measles outbreak of the current magnitude in the country again; in one of the wealthiest countries in the world.

It should be noted that elimination only means: ‘reduction to zero incidence of infection caused by a specific agent in a defined geographic area’. The only disease to ever be eradicated, through widespread and worldwide vaccination efforts, has been smallpox.

Professor Amira Roess, of George Mason University, College of Public Health, who is an epidemiologist and infectious disease expert, has highlighted that the US is at risk of losing its status as a country that had eliminated the measles virus. Further developments in the outbreak across borders could add additional pressure to global public health systems.

Additionally. there remains the danger of viral mutation and therefore greater risk of future vaccine escape. According to the WHO the source of this outbreak is yet to be identified, and there is no available evidence of any decreased vaccine effectiveness or changes in the virus that would result in increased severity of disease. Indeed, in 2024 an average of 93% of school-aged children received the measles vaccine in the US, with some regional variation.

‘World’s most infectious’

According to Dr Alexis Robert, an assistant professor at the London School of Hygiene and Tropical Medicine, which is part of the University of London:

“The measles virus is one of the world’s most infectious respiratory pathogens and can cause life-long disability or death. In the absence of vaccination, measles can spread very rapidly … it used to cause hundreds of thousands of cases per year. Outbreaks [today] are much smaller, because most people are protected against measles thanks to the two doses of mumps/measles/rubella [MMR] vaccine they receive in childhood.”

Since 1998, the MMR vaccine has been the subject of an implied and unwarranted link to autism thanks to a now wholly discredited research paper by Dr Andrew Wakefield, and others, which was published in the British medical journal The Lancet. Twelve years later, in 2010, the same journal retracted the paper saying that: “several elements [of the 1998 paper] are incorrect, contrary to the findings of an earlier investigation”.

Also, in January of 2010, the UK’s General Medical Council (GMC) – the professional body which regulates and licences doctors to practise in the UK – ruled that the children Wakefield had studied had, in fact, been carefully selected. This was contrary to his assertion that they merely presenting in clinic. The GMC also ruled that some of Wakefield’s research had been funded by lawyers who had been acting for families pursuing lawsuits against vaccine manufacturers.

Following on from this scandal, the UK did lose its measles-free status because of the fall it saw in rates of MMR immunisation uptake. Coverage of 95% of the population is considered necessary to prevent outbreaks. But among children aged 24 months in England, vaccination dropped from 91.2% in 2017-18 as far as 90.3% in 2018-19, which was the lowest rate since 2010. The COVID-19 pandemic further hindered effective deployment of infant vaccine programmes.

Indeed, every year UK universities conduct immunisation drives with each new intake of undergraduate students, as the babies born at the height of the MMR scare, in the mid- to late-2000s, work their way through the educational system. Despite these efforts there is still an annual uptick in measles cases each autumn in the UK.

Cuts and consequences

The current US outbreak has emerged in a rural area of Texas but has quickly spread to neighbouring states, into Mexico and then on to other countries. Authorities ability to deploy sufficient response to this current disease flare-up has been hampered by the Trump Administration’s cutting back on funding to both the federally organised public health surveillance infrastructure and to infectious disease research.

All at a time when the US health system is already under additional strain following a large tuberculosis outbreak in Kansas, H5N1 avian flu being endemic in the wild across the country but also now affecting dairy herds, and the highest number of seasonal influenza cases the country has seen in 15 years.

As the COVID-19 pandemic so brutally highlighted, infectious disease outbreaks can rapidly see clinics fill up – and then as staff also get sick – begin to operate on fewer staffing numbers, with those remaining health care professionals overstretched and exhausted.

Individual impact

Measles can be serious enough to hospitalise up to 20% of those infected. Complications of the disease can include pneumonia – 5% of children, especially the very young – encephalitis, where the brain swells that can lead to convulsions which are associated with deafness and learning impairment.

And in almost 3 in 1,000 children who become infected with the measles virus will die from respiratory and neurologic complications. And yet hospitalisations and deaths from measles are preventable with just the two doses of MMR vaccine.

This current US outbreak started in 2024 and has already shown to be difficult for clinics and hospitals to prevent and slow the spread of measles. Mexico has had similar concerns with the outbreak being unchecked by the (political map) border.

Given the pathogen’s very high R-number, if the outbreak continues to grow, both federal and state public health authorities will have to consider setting up separate measles units, to attempt to quarantine the sick and limit hospital-borne infections, especially when there may be immunocompromised patients.

Is the worst yet to come?

A 2019 study by Mina et al, published in the journal Science linked measles infections to the destruction of an infected person’s immune system and decimating its ability to respond to both new pathogens but – critically – pathogens that it had encountered before, in the what the authors described as:

“…the establishment of an immune amnesia state after measles…"

Although it is not yet clear whether measles infection can cause long-term damage to an individual’s immune memory, the research found through blood tests of children that their ‘antibody repertoire’ was ‘greatly diminished’. By contrast, no adverse effect on the immune system protection was seen in children that had been vaccinated with MMR.

Dr. Michael Mina, an infectious-disease immunologist at the Harvard T. H. Chan School of Public Health in Boston, Massachusetts, (at the time of the study) and colleagues analysed blood samples from 77 unvaccinated children from three schools in the Netherlands, taken before and after a measles outbreak in 2013. The unvaccinated children were part of an orthodox protestant community that avoided ‘modern medicine’. The team also collected blood samples from 33 children before and after their first vaccination with MMR.

Two months after the unvaccinated children had recovered from measles, the team found a range of between 11% and 73% of their antibodies against other bacteria and viruses had been erased. By contrast, the children who received the MMR vaccine showed no reduction in antibodies and retained more than 90% of their other antibody repertoire over the same period of time.

In a secondary experiment, Mina et al also infected macaques with measles and monitored the animals’ antibodies against other pathogens for five-months. Over that period the monkeys also lost 40% to 60% of their antibodies against previously encountered pathogens, suggesting the measles virus can destroy otherwise-long-lived bone marrow plasma cells that otherwise can produce pathogen-specific antibodies for decades.

And a further study, led by Dr Velislava Petrova, of the Wellcome Sanger Institute and Cambridge University, saw evidence that measles erodes other lines of defence in the immune system. When the body encounters a previously unseen pathogen, the immune system pumps out thousands of immune cells – each with slightly different receptors on their surfaces – but with a collective ability to combat, at least in part, almost any pathogen. However, after a measles infection, the children in the study showed a far more restricted range of diversity in immune response.

In conclusion, while the measles outbreak alone has the potential to cause serious illness and death, if it spreads unchecked, particularly among children it could also create the conditions for future severe disease either through the diminution of immune response following infection or creating a sufficiently large pool of onward infection that could drive viral mutation.

And finally, anyone in the US living in an area where they might be at increased risk of contact with measles – and is aged 62 and older – should consider having vaccinations themselves, given that the US only started to institute immunisation for measles in 1963.

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