The measles virus erases the immune system's memory, leaving patients vulnerable to other infectious diseases.
Measles is often painted as a trivial disease by the anti-vaccination movement. It is not – it kills or causes brain damage in two or three out of every 1000 cases, even in wealthy countries. Here's another reason it isn't trivial: having measles destroys your immunity to other diseases – and some of those are far more deadly.
The upshot? Getting your child vaccinated will protect them from much more than just measles.
Some 650 children a year used to die from measles in the US. When mass vaccination came in after 1960, measles deaths plummeted. But oddly, so did childhood deaths from infectious disease generally, in every country where the measles vaccine was introduced. This has been a major mystery in public health: the vaccine was supposed to protect you from measles, and nothing else.
The measles virus is known to kill the white blood cells that have a "memory" of past infections and so give you immunity to them. It had been thought that those cells quickly bounce back because new ones appear a week or two after someone gets over measles.
However, recent work in monkeys that have recovered from measles shows that these new memory cells only remember measles itself; the monkeys lose cells that recognize other infections. If humans get similar "immune amnesia" after measles, childhood deaths from infectious diseases should rise and fall depending on how many had measles recently, and how long the effect lasts, reasoned Michael Mina of Emory University in Atlanta, Georgia.
It looks as if they do. Mina and his colleagues used a complex statistical model to analyze child mortality records from the US, UK, and Denmark in the decades before and after measles vaccination began. They found that infectious disease deaths did rise and fall depending on measles cases. In all three places, the timing of this surge exactly matched what would be expected if immune amnesia after measles lasted on average 27 months. The biggest killer was pneumonia, followed by diarrhoeal diseases and meningitis.
The effect was so large that in any given year, the number of children who died of infectious disease was clearly linked to how many measles cases there had been two to three years previously. When measles was common, the team calculates that it was implicated in around half of all childhood deaths from infectious disease.