As the US measles cases soar following 2025’s record-shattering outbreaks, experts warn that healthcare practitioners and parents may be unprepared for the virus’s hidden threat: immune amnesia.

The condition is pretty much what it sounds like. The virus destroys immune memory, wiping out memory B cells and T cells and forcing the immune system to rebuild its defenses from scratch.

“Nearly every unvaccinated child who gets measles can be impacted,” said Patricia Stinchfield, CPNP, past president of the National Foundation for Infectious Diseases. Rebuilding immune memory leaves patients vulnerable to new exposures, and infections, hospitalizations, and missed work and school can linger for up to 5 years, casting a long shadow after recovery.

Immune amnesia was officially recognized in 2015, but clues emerged before that. Scottish scientists in the 1700s described waves of infections and deaths in measles’ wake. When measles vaccines arrived in the 1960s, ’70s, and ’80s, researchers noticed unexpected drops in seemingly unrelated health problems. And looking back at prevaccination records, they documented higher rates of infections and deaths in kids for several years after they had this highly-contagious childhood illness.

Lots of deaths. “Half of all childhood infectious-disease deaths were related to immune amnesia caused by measles,” said Michael J. Mina, MD, PhD, an infectious disease expert and former assistant professor of epidemiology and immunology, Harvard Medical School, Boston. Mina’s landmark research, published from 2015 to 2019, helped illuminate the measles-immune amnesia link. “Before vaccines, it was difficult to see the connection because virtually everyone got measles. There was no one to compare them to.”

Experts long believed measles suppressed immunity for only a few weeks. But after the turn of the century, evidence began mounting that its effects lasted much longer.

Many theories circulated, yet nobody had an answer — until a series of scientific advances brought a shocking picture into view.

Forgotten Immunity

The first breakthrough came in 2000, when Japanese researchers identified a unique interaction with immune cells. “The measles virus binds to a receptor called CD150,” Mina said. “That’s an immune memory cell marker.” The virus then travels from lungs to lymph tissue, using those cells to replicate.

Mina’s 2015 study, published in Science, demonstrated the long-term impact. Graphs in the study resembled an Olympic-level downhill ski slope, with dramatic drops in nonmeasles deaths in the decades after the vaccine was introduced in the US, Denmark, England, and Wales. “The results were so striking,” Mina said. “This wasn’t just statistical magic.” From the mid-1960s through the ’80s, measles vaccination was associated with a 50% reduction in deaths from other infectious diseases.

Breakthrough number two followed shortly after, when Harvard researchers unveiled VirScan, a biological scanner that detects a wide range of antibodies from a small blood sample. Researchers now had a tool to track changes in infection-fighting antibodies — they just needed before-and-after samples from children who’d had measles.

Amazingly, those samples existed — sitting in a Dutch lab freezer. Virologist Rik de Swart, PhD, an associate professor at Erasmus Medical Center in Rotterdam, Netherlands, had collected blood samples from infected and uninfected children during a 2013 measles outbreak in Amsterdam. “Rik had the foresight to obtain the samples and store them in the hopes they could be used,” Mina said. “I spent several years working on the antibody scanner. Then I called Rik and said, ‘Can we look at your samples?’”

Their 2019 Science study was the first to measure the immune memory loss following infection. Using VirScan to compare antibodies in 77 infected children before and up to 4 months after infection — and against 115 uninfected kids and adults — they found dramatic losses in some.

“The kids who had measles lost 11%-73% of their whole immunological memory pool,” Mina said. “Overall, half of their total memory cells were gone.”

It was the biological explanation for immune amnesia.

Infection Risk

Thanks to modern medicine (and likely less exposure to dangerous infections), immune amnesia after measles is likely not deadly these days. But kids who get measles can expect more illnesses for the next couple of years.

In an analysis of health records from 1990 to 2014, Mina, de Swart, and colleagues found that the month following measles infection is the riskiest. Compared with 19,900 vaccinated peers, 2200 infected children were 43% more likely to catch another infectious illness, 2.7 times more likely to be hospitalized, and three times more likely to receive a prescription anti-infective. The risk remained 22% higher for the rest of the year and 10%-15% higher for 2-5 years.

De Swart says immune amnesia’s health risks are likely greater in “low-income countries with poor hygiene and weak healthcare systems…leading to higher morbidity and mortality.”

Studies in the Democratic Republic of the Congo found that children who received tetanus vaccines and later had measles created fewer tetanus antibodies than those who didn’t get measles. Measles was also linked to more postinfection fevers in young children. “There’s this perception that measles is no big deal,” said James D. Cherry, MD, Distinguished Professor in Pediatric Infectious Diseases at the David Geffen School of Medicine at UCLA, who co-authored the studies. “But measles is bad for a lot of reasons no matter where you live.”

New Research

Johns Hopkins researchers are exploring ways to measure immune amnesia while testing antiviral treatments that might interrupt the process. In a 2025 lab study, they found that the antiviral drug remdesivir reduced antibody loss, “suggesting that early therapeutic intervention could decrease the morbidity and mortality from the measles-induced susceptibility to other infections,” said lead author Andy K.P. Chan, a PhD candidate in immunology at Johns Hopkins.

In Netherlands, researchers at Erasmus Medical Center are launching a study of 100 children and teens — half vaccinated, half unvaccinated. They’ll track participants’ health and changes in antibodies and immune memory cells over 3 years. “We plan to look at the loss of immunity to other pathogens after measles,” said lead researcher Corine Geurts van Kessel, MD, PhD. “Of course, we would also like to examine what kinds of infections these children experience, but the study is likely too small. [Immune amnesia] is definitely a health risk.”

Yet Another Reason to Vaccinate

Measles already carries well-known risks and complications, including ear infections, diarrhea, a 20% risk for hospitalization, a 5% risk for pneumonia, a 1 in 1000 chance of brain swelling, up to 3 in 1000 odds of death, and about a 1 in 10,000 risk for subacute sclerosing panencephalitis (an often-fatal neurologic disorder).

“Parents should find comfort in the knowledge that vaccine safety science is robust and the best way to keep your child healthy and safe is to vaccinate,” Stinchfield said.

Cherry agreed. “Most patients thankfully still trust their pediatrician and primary care doctor,” he said. Educate your patients about immune amnesia: “Physicians need to talk about the risks of not being vaccinated.”

The effects of immune amnesia after recent US outbreaks have not been formally documented, experts said. One unpublished survey of a 2018-2019 New York state outbreak found twice as many infections in unvaccinated, measles-infected children in the year after they were diagnosed compared with uninfected kids, most of whom were vaccinated.

In the US and other developed countries, unvaccinated children and teens who get measles are most likely to experience a few more infections for the next 2-5 years compared with vaccinated peers — and those illnesses may be more severe. “It’s the stuff kids are already susceptible to such as colds, flu, respiratory syncytial virus, COVID, and adenoviruses that can cause respiratory and digestive-system symptoms,” Mina said. The added risk lasts until the immune system rebuilds.

“Discussing the ramifications of a measles infection with parents is really important,” he said. “This isn’t just the measles. There’s more to it. Our healthcare system gets kids through acute infections really well. But when your child is getting 3 or 4 or 5 extra infections a year, it adds up.”