As measles cases soar worldwide, scientists have discovered yet another danger of the disease: the measles virus can wipe out the immune system, making people more susceptible to other illnesses later. The research sheds new light on a virus that’s infected humans for centuries.

The phenomenon is called “immune amnesia,” and a new study — published in Science — documented how it works. The measles virus appears to erase the body’s immune memory, destroying an average of 40 of the antibodies against other viruses and bacteria subjects in the study built up before the measles virus hit. This means people who get measles are more susceptible to other illnesses — pneumonia, flu, and skin infections — after an encounter with the virus, and that immune suppression can last for years.

This new side effect is an addition to all of the other well-known symptoms of measles, such as rash, cough, fever, and malaise. Measles can also lead to serious complications — pneumonia, brain swelling — and even death. In 2017, measles killed 110,000 people around the world and infected 6.7 million. About 1 in 5 unvaccinated people in the US who get measles need to be hospitalized, according to the Centers for Disease Control and Prevention. Immune amnesia helps explain why the majority of deaths and complications are caused by infections people acquire after measles.

“[Measles] not only destroys overall immune function for a few weeks as children recover from the virus, it also prevents children’s ability to defend against pathogens they should have been equipped to deal with over the long term,” Michael Mina, an assistant professor of epidemiology at the Harvard T.H. Chan School of Public Health and lead author on the paper, told Vox.

The research, which built on previous studies about measles immune system suppression, also has massive public health implications.

Measles vaccine rates are dropping worldwide, with measles cases tripling between 2018 and 2019, mostly because of problems with vaccine access or vaccine refusal. Several countries — including the United Kington, Greece, and Brazil — have recently lost their measles elimination status, meaning they had outbreaks lasting for more than a year at a time. And this year, the US nearly did too. So we’re sliding back on measles vaccination just as researchers are discovering how damaging the disease is to the immune system.

Why it’s even more important to get vaccinated against measles

The measles virus is one of the most infectious diseases known to humans. Let’s say a person with measles coughs in a room and leaves; hours later, if you’re unvaccinated, you could catch the virus from tiny droplets in the air the infected person left behind. No other virus can do that.

For anyone born before 1960, there’s a good chance they suffered through a measles infection. They may have lived to tell about it, but they probably had friends who didn’t. In the US, before a vaccine was introduced in 1963, there were 4 million measles cases each year, resulting in 48,000 hospitalizations and 500 deaths. Measles was also a leading killer of children globally.

The beauty of the vaccine is that most people who get the proper doses will never get sick with measles, even if they’re exposed. And by 2000, because of widespread vaccination, the virus was declared eliminated in the US.

Researchers have long known vaccinating against measles works. And the side effects are rare and mostly very mild. But what they’re now coming to realize is that getting immunized is important for more reasons than they’d appreciated.

For the Science study, the researchers tracked 77 unimmunized children in the Netherlands, part of a vaccine-refusing Orthodox Protestant community there. They drew blood samples during a 2013 measles outbreak (before and two months after the kids got the infection) and looked for changes in the children’s immune systems using VirScan, a tool that can measure antibody levels in the blood for hundreds of pathogens. Separately, they tracked five unimmunized children who didn’t get the disease.

When the researchers compared the blood samples before and after measles, they found the virus had erased between 11 and 73 percent of a child’s antibodies, with an average loss of about 40 percent. They didn’t find the same effect in the five kids who never got the disease. And in a control group of children immunized with the measles, mumps, rubella (MMR) vaccine, they also didn’t find any immune system loss.

In another simultaneously published study, in the journal Science Immunology, researchers studied the same cohort of kids but focused instead on their B cells, a type of white blood cell that includes memory cells that produce antibodies in the body. The researchers came to a similar conclusion: Measles primes the body for other diseases. In this case, the virus reduced the body’s immune memory B cells and reverted the immune system to an immature, baby-like state.

“The studies show that measles is more dangerous than we think and can have consequences on our immune system long after the symptoms of measles are gone,” said the lead author of the second paper, Velislava Petrova, a postdoctoral fellow at the Wellcome Sanger Institute. The public health message, she added, is that getting the MMR vaccination “is important not only to prevent you from measles disease, but also from other secondary infections you can have if you get measles and have your immune system damaged.”

Maybe this new research — coming at a time of measles resurgence around the world — will encourage people on the fence about the vaccine to get immunized. “In an era where individuals and even policy makers have a sort of amnesia about just how bad measles was,” said Harvard’s Mina, “this makes glaringly clear that a measles infection is not just a simple benign childhood infection.”

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