Scientists have discovered that the measles virus deletes part of the immune system's memory, leaving those affected more vulnerable to other diseases.

It is already known that measles weakens the immune system, with people often catching other infectious diseases after a bout of the illness. However until now, it was not known why.

Scientists in the Netherlands have found that measles causes long-term damage to the immune system by deleting part of the system's memory. This means that previously existing immunity to other infections is removed.

In other words, the immune system is reset back to an immature baby-like state, with only limited ability to respond to new infections.

The scientists took blood samples from healthy volunteers who had not been vaccinated against measles. These were then followed up after a measles outbreak in 2013.

The team analysed the samples of 26 children both before they had measles and 40-50 days after they had been infected with the virus. They found that specific immune memory cells that had been built up against other diseases, and were present before the measles infection, had disappeared from the children's blood.

These children were now vulnerable to infectious diseases they had previously been immune to.

"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," explained the study's lead author, Dr Velislava Petrova, of the Wellcome Sanger Institute in the University of Amsterdam.

Further research carried out in animals (ferrets) showed that infection with a measles-like virus reduced the level of flu antibodies in ferrets that had previously been vaccinated against flu. These animals also had worse flu symptoms when infected with flu virus after the measles-like infection.

Meanwhile, the scientists also discovered that the measles virus resets the immune system to an immature state, which is only capable of making a limited number of antibodies against diseases. This means that the measles virus makes it difficult for the immune system to respond to any new infections that the person is exposed to.

"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. 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," commented the study's senior author, Prof Colin Russell, of the University of Amsterdam.

The scientists insisted that the findings show that everyone who can be vaccinated against measles should be vaccinated, as some children show signs of immune suppression up to five years later despite appearing healthy.

"Measles is highly contagious and its potentially devastating consequences are well known. This study finds that measles also has the potential to weaken our body's existing immune response to other diseases, leaving us vulnerable to infections. These findings further strengthen the vital role the MMR (measles, mumps rubella) 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," said Dr Charlie Weller, head of vaccines at the Wellcome Sanger Institute.

Details of these findings are published in the journal, Science Immunology.

Measles starts with a fever, cough, runny nose and red eyes. It is followed a few days later by a red rash that starts on the head and spreads downwards over the face, neck and body.

Measles can cause a range of health issues, including ear infections, diarrhoea and pneumonia. It can also lead to swelling of the brain and brain damage. For every 1,000 people who get measles, one to two people will die as a result.

The measles virus is highly contagious and spreads through the air via coughing and sneezing.

The MMR vaccine offers protection against the virus. It is offered to children in Ireland free of charge as part of the Childhood Immunisation Schedule. Children receive two doses - one at 12 months of age and one at four/five years of age. For more information on measles and the MMR vaccine, click here.