Scientists have shown how measles causes long-term damage to the immune system, leaving people vulnerable to other infections.

The international team, which includes the Wellcome Sanger Institute, the University of Amsterdam and Imperial College London, revealed that the measles virus deletes part of the immune system’s memory, removing previously existing immunity to other infections, in both humans and ferrets.

The work shows for the first time that the virus resets the human immune system back to an immature state with only limited ability to respond to new infections.

According to the researchers, the findings explain why children often succumb to other infectious diseases after they have had measles and highlights the importance of getting vaccinated against measles.

The research, published in Science Immunology, also has implications for public health, as falling vaccination rates and resulting in rising cases of measles, which could also cause an increase in cases of other dangerous infections such as flu, diphtheria or tuberculosis, even in people who were previously immune. 

Highly contagious

The measles virus is highly contagious and causes coughing, rashes and fever, and can lead to potentially fatal complications including pneumonia and encephalitis, leading to more than 100,000 deaths per year worldwide in unvaccinated communities.

A highly effective vaccine was introduced into the UK in the late 1960s, and the disease had been completely eliminated from the UK in 2017. However, measles cases are rising again. The UK vaccination rate has also dropped below the required level of 95 per cent of the population, leading to the UK losing its World Health Organisation (WHO) measles elimination status recently.

Measles could reverse the effects of vaccination against other infectious diseases. Professor Paul Kellam Department of Infectious Disease

It is known that measles weakens the immune system, even after the initial infection has cleared, but how the virus has this lasting effect on the body was unclear. During infection, people have fewer white blood cells, which protect the body against disease, and this is seen in the clinic as a low white blood cell count.

However, after a few weeks, the patient’s white blood cell count goes back up to previous levels and they have recovered from the measles, yet they are still much more susceptible to other infectious diseases.

To find out what measles does to the immune system, researchers looked at a group of non-vaccinated people in the Netherlands – from a non-vaccinating Orthodox Protestant community.

Blood samples were first taken from healthy volunteers from this community, who were followed-up for repeat sampling after a measles outbreak in 2013.

The researchers sequenced antibody genes from 26 children, before and 40-50 days after their measles infection. The team discovered that specific immune memory cells that had been built up against other diseases, and were present before the measles virus infection, had disappeared from the children’s blood. This would leave them vulnerable against infectious diseases they had previously been immune to.

Dr Velislava Petrova, lead author from the Wellcome Sanger Institute and Cambridge University, said: “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.” 

Immunological amnesia

Researchers then tested this ‘immunological amnesia’ directly in ferrets, showing that infection with a measles-like virus reduced the level of flu antibodies in ferrets that had been previously vaccinated against flu. These ferrets also had worse flu symptoms when infected with flu virus after the measles-like infection.

“We showed that measles-like viruses can delete pre-existing flu immune memory from ferrets,” explains Professor Paul Kellam, from the Department of Infectious Disease at Imperial and previously from the Wellcome Sanger Institute.

“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.”

The researchers also discovered that the measles virus resets the immune system to an immature state that can only make a limited repertoire of antibodies against disease. This means that measles makes it difficult for the immune system to respond to any new infections, increasing the risk of secondary diseases.

'Resetting' the immune system

Professor Colin Russell, senior author from the University of Amsterdam, Netherlands, said: “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. Our study has huge implications for vaccination and public health as we show that not only does measles vaccination protect people from measles, but also protects from other infectious diseases.”

The study reveals that it is extremely important that everyone who can be vaccinated, is vaccinated, to prevent the resurgence of measles and other diseases that we have developed immunity to in childhood or for which we vaccinate.

After measles, some children still show signs of immune suppression for up to five years although they appear healthy when their white blood cell counts are measured. This study shows how genetic techniques can reveal new mechanisms of disease that are undetectable using routine clinical tests, and that further research is needed to understand the full effects of measles.

Dr Charlie Weller, Head of Vaccines at Wellcome, said: “These findings further strengthen the vital role the MMR vaccine plays in public health and protecting us from deadly disease. It is yet another reminder of how important vaccines are as a vital resource in eliminating infectious disease.”

This article is based on materials provided by the Wellcome Sanger Institute.

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‘Incomplete genetic reconstitution of B cell pools contributes to prolonged immune suppression after measles’ is published in Science Immunology.