Measles infections eliminate the immune system's memory of infectious diseases, resetting the human immune system to a baby-like state and increasing vulnerability to other dangerous infectious diseases like diphtheria, tuberculosis, and the flu, according to two new studies published today.
The findings have implications for public health and highlight the importance of getting vaccinations. Vaccinations against measles prevented more than 21 million deaths between 2000 and 2017, according to the World Health Organization (WHO). However, a surge in foregoing vaccinations has spurred a spike in global measles cases, which are up 300 percent from last year.
"For the first time, we see that measles resets the immune system and it becomes more baby-like, limiting how well it can respond to new infections. In some children the effect is so strong it is similar to being given powerful immunosuppressive drugs,” said researcher Colin Russell. It seems the measles virus essentially reduces its host’s antibodies to other diseases, which makes a person more susceptible to infection. Some children saw immune suppression for up to five years following measles infection.
Measles is a highly contagious disease and is responsible for more than 100,000 deaths per year in unvaccinated communities, according to the WHO. A study published in May of this year found that measles weakened the immune system, but the exact mechanism was not understood.
To understand why some children catch other infectious diseases after contracting the measles, researchers analyzed blood samples of non-vaccinated groups of people in the Netherlands both before and after a 2013 measles outbreak. They then sequenced antibody genes from 26 children before and for a month following measles infection. Publishing their work in Science Immunology, the researchers found that immune memory cells that had built up against other diseases were present before measles infection but disappeared following.
"This study is a direct demonstration in humans of 'immunological amnesia', where the immune system forgets how to respond to infections encountered before. We show that measles directly causes the loss of protection to other infectious diseases,” said lead author Velislava Petrova in a statement.
Researchers then tested immunological amnesia in ferrets by infecting the animals with a measles-like virus and then the flu virus, which they had been vaccinated against, and found that the animals experienced worse flu symptoms after having been infected with measles.
"We showed that measles-like viruses can delete pre-existing flu immune memory from ferrets," said researcher Paul Kellam. "Even after the ferrets had been successfully vaccinated against flu, the measles-like virus reduced levels of flu antibodies resulting in the animals becoming susceptible to flu infection again and experiencing more severe flu-like symptoms. This shows that measles could reverse the effects of vaccination against other infectious diseases."
A second study published in Science analyzed blood samples from 77 children before and after measles infection and gained similar results. Using a virus-detection technology called VirScan, researchers found that measles infection eliminated between 11 and 73 percent of protective antibodies in children, making them not quite as defenseless as a newborn baby, but close. This test was repeated in macaque monkeys before and up to five months after infection. These monkeys similarly lost an average of 40-60 percent of antibodies that had protected them against pathogens.
During a measles infection, people have fewer white blood cells, which are responsible for protecting the body against diseases. After a few weeks, a patient’s white blood cell count will increase but they may still be susceptible to disease. An outbreak in April killed more than 1,200 people in Madagascar and cases around the world are on the rise. This year, the UK lost its measles-free status, which was established in 2017. The US has also seen a resurgence in measles cases despite having eliminated the disease in 2000; in just the first three months of this year, measles cases surpassed all of those reported in 2018. The Centers for Disease Control and Prevention have reported 1,250 cases as of October 3.