Immune amnesia: Why even mild measles infections can lead to serious disease later

Los Angeles Times

03-04-2025

Dr. Adam Ratner has heard a lot of myths and misunderstandings about measles in his decades as a New York City pediatric infectious disease specialist.
A troubling untruth he's seen circulating on social media during the current outbreak is that being infected with the virus instead of getting vaccinated confers benefits on the immune system — a strength-training program of sorts for the cells.
The truth, Ratner said, 'is exactly the opposite.'
Measles is a highly contagious virus that presents as a rash and cold-like symptoms for many patients, and can lead to serious or fatal complications for others. An outbreak that began in west Texas in January has since infected nearly 500 people across 19 states, including eight people in California.
An insidious but lesser-known consequence of even a mild measles infection is that it kills the very cells that remember which pathogens the patient has previously fought and how those battles were won. As a result, recurring bugs that might have caused only minor symptoms make patients as sick as if they'd never encountered them before.
Measles destroys lymphocytes that defend against other bugs to make way for ones that defend against measles, an immunity won at the cost of other protections.
This 'immune amnesia,' physicians say, leaves patients vulnerable to reccurrences of diseases their immune cells were previously able to resist.
If a child gets sick with measles, 'for the next two or three years, you kind of have to be looking over your kid's shoulder, wondering if some otherwise routine virus or bacteria that they should be very well protected against is potentially going to land them in the hospital,' said Dr. Michael Mina, an epidemiologist who was previously an assistant professor of immunology and infectious diseases at Harvard Medical School.
'Even if your measles virus infection seemed mild and you kind of blew through it, it doesn't mean that it was mild on your immune system,' Mina added.
Take rotavirus, Ratner said, which causes severe diarrhea that can be life-threatening for children if untreated. A child who has rotavirus once will have antibodies that offer protection against future infections.
But a measles infection, said Ratner, author of the recent book 'Booster Shots: The Urgent Lessons of Measles and the Uncertain Future of Children's Health,' 'could wipe out that immunity and they could be just as vulnerable to rotavirus as if they had never seen it before.'
Immune amnesia results from the measles virus' plan of attack. Viral particles travel via airborne droplets of saliva, mucus and cells that make their way into a new body when their unsuspecting host breathes them in.
From there, they sneak past the protective barrier lining the respiratory system and head to the lymph nodes in search of cells that express a particular protein called signaling lymphocytic activation molecule, or SLAM.
The virus then rides around the bloodstream on these hijacked SLAM-expressing cells, further infecting and destroying other SLAM expressers it meets on the way.
Among the SLAM-expressing cells that measles wrecks are memory B and T cells, two crucial players in a functioning immune system. Memory B cells manufacture the right antibodies quickly when a familiar microbe appears. Memory T cells recognize and kill viruses that your cells have encountered in the past.
A measles infection feeds on these memory cells. Vaccines, in contrast, stimulate the production of memory B and T cells without consuming others in the process.
This was not yet understood in the decades before the measles, mumps and rubella vaccine's approval in 1963, when measles was a common childhood disease that killed some 400 children in the U.S. each year.
'For 100 years or more, we've known that measles does cause an acute susceptibility to other infections,' Mina said.
A measles infection temporarily suppresses the immune system, Mina said, and it was long assumed that opportunistic infections around the time of the illness were the result of that short-term suppression.
In 2015, Mina and colleagues published a paper that looked at mortality data in the U.S., the United Kingdom and Denmark before and after measles vaccines were introduced. They found that whenever there were measles outbreaks, childhood deaths from all other infectious diseases remained significantly higher for two to three years in outbreak locations, an increase that accounted for up to half of all childhood deaths from infectious disease.
Once those countries rolled out the MMR vaccine, measles cases fell, as expected. But so did childhood deaths from other infectious diseases, by about half.
Three years later, Mina and his collaborators took blood samples from 77 unvaccinated children in a community in the Netherlands before and then two to six months after the children contracted measles. They found that the virus wiped out 11% to 73% of the children's preexisting antibodies to a host of pathogens.
Just as children in preschool fall ill constantly with common diseases they're encountering for the first time, unvaccinated children who contract measles are at higher risk in the ensuing years for common early childhood sicknesses such as respiratory infections, earaches and viruses that cause diarrhea, said Shelly Bolotin, a scientist at Public Health Ontario in Canada and director of the Center for Vaccine Preventable Diseases at the University of Toronto.
'In order to correct that depletion [of B and T cells], you need to be reexposed to everything you were immune to before, and this can take years,' she said.
As of late March, 97% of the people sickened in the current outbreak were unvaccinated or didn't disclose their vaccine status. The measles virus is attenuated in the MMR vaccine, meaning that it has been altered to produce the appropriate immune response without triggering the disease itself. In the case of measles, that means no mass destruction of the cells that hold the immune system's memory.
'It doesn't have this very, very damaging effect, which is why we recommend vaccination, because we get all of the immunity with none of the adverse consequences,' Bolotin said.