For many years, we’ve known that measles virus infection messes with your immune system.  During infection, you are at greater risk of infections caused by other pathogens because measles thwarts the proper immune attack.  At the same time, your body makes a long-lasting response to the measles virus. This is often known as the “measles paradox”.

A study a few years ago by de Vries and colleagues showed that one way measles bamboozles the immune system is by depleting memory T cells–the cells that patrol the body and protect you from infections you’ve either had before or been vaccinated against.  This depletion is known as “immunological attrition” and happens during a variety of infections. One reason why this may happen is simply so that the immune system does not grow and grow and grow upon encounter with the same pathogen, or even family of pathogen.  Another reason is so that memory cells do not outcompete the progression of new (first timer) immune responses.  When pathogens mutate and change as fast as they do, this helps us continuously evolve our immune response to them. But importantly, during immunological attrition, it is quite rare that all memory cells are lost.  So the body retains awareness of prior infections and vaccinations, instead it goes through short periods of immune suppression, but does not reset the entire immune system after each infection.

Measles infection seems to be a special circumstance because it depletes many more of these protective memory cells.  What this means is that your immune system’s ability to remember previous vaccinations and infections (and protect you if you see the bug again) is lost after infection with measles, leaving you at risk of a variety of diseases.  Measles causes “immune amnesia”.

In a study published in Science last month, Michael J. Mina and colleagues tested this “immune amnesia” hypothesis using epidemiological data from pre-vaccine and post-vaccine eras in populations with high vaccination rates.  They first compared measles incidence, measles-related deaths, and all infectious disease-related deaths from children in England, Wales, United States, and Denmark.  Then, using mathematical modeling, they extrapolated a measles prevalence (the frequency of patients with measles at any given time) that also took into account the duration of immune amnesia (that is, the time it would require to rebuild all your immune memory if it was lost).  What they found was striking:

First, the rates of childhood deaths from infectious diseases plummeted in populations where MMR vaccination is (was) common and measles incidence correlated with all infectious disease-related deaths.

Second, immune amnesia after measles infection lasts much longer (up to a few years!) than previously described immunological attrition.

Finally and perhaps most importantly, there was a strong and significant positive correlation between measles infection and all infectious disease-related deaths.

Therefore, measles vaccination not only protects kids from measles, but protects their memory cells from “immune amnesia” and reduces all infectious disease-related deaths.