As measles cases surge worldwide, experts are raising alarms about a lesser-known danger: immune amnesia.
The highly contagious virus doesn’t just cause a rash and fever — it can also wipe out the immune system’s memory, leaving survivors vulnerable to infections they’ve fought off before, like the flu, a cold, or even diseases they’ve been vaccinated against.
Health Canada warns that measles can lead to serious immune suppression, known as immune amnesia. This can increase the risk of other illnesses and even raise the chances of death for months or even years after the infection, the health agency states on its website.
“Immune amnesia basically destroys your immune system. And with respect to the measles virus, it can infect the cells that process the memories or prior immune or viral infection and other pathogens, it can infect those cells and kill the cells, so it erases the actual physical memory,” explained Stephen J. Elledge, a professor of genetics and medicine at Harvard Medical School.
“And it depletes that to the point where you no longer can respond to proper pathogens as well as you would have been able to, had you not had measles.”
Elledge, who has published research on immune amnesia, said the side effect is especially concerning as measles outbreaks continue in Canada and the United States, potentially leaving many people vulnerable to other infections after contracting the virus.
A 2019 study published in Science, co-authored by Elledge, found that measles can erase up to 73 per cent of a person’s protective immune memory.
Because your immune system is left in a near-blank state, similar to that of a baby’s, studies show it can take years to slowly rebuild and relearn how to defend against infections.
Measles, known for its characteristic red rash, is one of the most contagious viruses on the planet, with an R number of 12 to 18 — meaning one infected person can spread it to up to 18 others in an unvaccinated population. To put that in perspective, COVID-19’s original strain had an R number of about two to three, and even highly transmissible variants like Omicron rarely exceeded 10.
Once declared eradicated in Canada in 1998, measles is now making a comeback, spreading rapidly in recent months, especially in places like Ontario, due to declining vaccination rates.
The virus can cause serious complications, including severe illness and even death. And another side effect is immune amnesia.
Our immune system has a built-in memory, thanks to special cells called B and T lymphocytes, Elledge said. These cells help us fight off infections, not just in the moment but also in the future.
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When we get sick, our immune system remembers the virus or bacteria, so if we run into it again, it can recognize and attack it much faster.
A lot of viruses can weaken the immune system — like the flu, which damages the airways and makes it easier to get pneumonia. But measles is different. It doesn’t just weaken immunity; it actually erases the body’s disease memory, wiping out the immune cells that remember past infections.
“Every virus has a recognition protein that it likes to bind to to gain entry to the cells, and the measles virus binds to a protein called SLAM, and that protein is on T cells, B cells and plasma cells,” Elledge said.
Plasma cells, he said, which pump out antibodies to protect you, are a target for the measles virus. It finds them, infects them and wipes them out, essentially destroying the immune system’s memory.
Although immune amnesia can erase your immunity to diseases you’ve already fought off — like chickenpox, mumps, or meningitis — oddly enough, it doesn’t forget the measles itself. That’s the one infection your immune system still remembers.
“That is because your immune system has to win that battle or you die. Your immune system wins, but is left with the carnage of other cells,” Elledge said.
Virtually everyone who gets measles experiences some level of immune amnesia, but the extent can vary, Elledge said.
“It is very common; we saw it in most of the children who were infected in our study,” he said. “But it’s not the same for everybody,” he said, adding that some people may be healthier when they get sick, or there could be genetic differences that affect how quickly they can fight off the virus.
Elledge explained that immune amnesia is especially dangerous for babies, since they aren’t vaccinated until around 12 months and are highly vulnerable during that time.
Older adults are also at risk, as their immune systems naturally weaken with age.
A 2015 study published in Science found that it takes about two to three years after a measles infection for the immune system to fully rebuild its protective memory.
During that time, your immune system can forget past infections and even the protection from vaccines you’ve had.
In more serious cases of measles, Elledge recommends checking in with your health-care provider to see if you might need to get revaccinated.
“This is not a normal type of virus; it is very serious,” he said.
The best way to prevent immune amnesia is to avoid getting measles in the first place — and that means getting vaccinated, Elledge said.
The MMR (measles, mumps and rubella) vaccine is highly effective at protecting against measles and the immune system damage it can cause. Experts emphasize that vaccination doesn’t just prevent the short-term symptoms of the virus — it also preserves the immune memory you’ve built up over your lifetime.
The shot is estimated to be 85 to 95 per cent effective after a single dose given at 12 or 15 months of age. With a second dose, efficacy rises to nearly 100 per cent, according to the Public Health Agency of Canada.
