London - Children must get the MMR jab, as a bout of measles may devastate their immune system and leave them open to other serious diseases, say scientists.
Most youngsters who die from measles are killed by complications such as pneumonia or swelling of the brain.
While white blood cells needed to fight infections are destroyed by measles, the cells do come back in force once the rash disappears.
This has left scientists struggling to understand why those who have had measles then suffer with other health problems.
However researchers have now discovered measles "resets" the immune system, wiping out the immunity children have developed to other illnesses, effectively leaving them with the same scant protection they had as newborns.
A study of 26 unvaccinated children, after they had measles, found they lost large numbers of a specific type of immune cell they had built up before the disease.
This gave almost two-thirds of the participants the immune system of a young baby which has never encountered pneumonia bacteria or other dangerous bugs, so has no protection. Up to 15 percent of children still have weaker immune systems five years after recovering from measles.
The authors of the study, who include a team from the Wellcome Sanger Institute in Cambridge, say children denied the MMR jab could even lose their immunity to illnesses they have been vaccinated against, such as whooping cough, diphtheria and tuberculosis.
Researchers took blood from 26 unvaccinated children before they fell ill with measles and around 40 days afterwards. All children have immune cells which have learned to recognise viruses and bacteria, from pneumonia to TB, whether they have come across them or been vaccinated with traces of them.
These "memory" cells clone themselves, to create a vast army which can fight infections if they strike.
But scientists, who read genetic "barcodes" for these cells so they could recognise them in the 26 children, found they were hugely depleted after measles.
Some of the children, aged four to 17, appeared to have had parts of their immune memory completely wiped out.
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 disease."
Pneumonia affects one in 20 children with measles, while others suffer dangerous stomach bugs and one in ten get ear infections which can lead to permanent hearing loss.