Measles-Induced Immune Amnesia

The measles vaccine has been around, in one form or another, for over 50 years. Ever since its introduction, we’ve seen a striking reduction in childhood morbidity and mortality – by as much as 90 percent in impoverished populations (1), but the revolutionary effects aren’t limited to developing countries. Even in the United States, it’s estimated that the first 20 years of measles vaccination prevented 52 million cases and over 5,000 deaths, saving the healthcare system about US$5 billion (2). But the benefits of the vaccine aren’t fully explained by its prevention of measles infections – so what accounts for the profound effect of measles vaccination on children’s health?

The nonspecific effects of childhood vaccinations have been noted since the late 1980s, but the reasons behind them have remained a mystery – researchers have proposed various beneficial immunological mechanisms, but none can be confirmed as the cause of the reduction in disease mortality. Recent work, however, suggests a different method: that measles infection may erase a patient’s immune memory, replacing lymphocytes designed to defend against non-measles pathogens with those specific to the measles virus (3). Because the previous memory cells are replaced with measles-specific ones, the overall blood counts return to their original levels within a few weeks, masking the patient’s “immune amnesia” to other infections.

One group of researchers hypothesized that this effect, if indeed present, should be easy to spot by tracing the relationship between measles cases and deaths from other infectious diseases during times when measles was common. Not only would this kind of analysis confirm the existence of the effect, but also its duration – by asking how long the correlation between measles and deaths from other infections lasts, the researchers would also find out how long thew immunosuppression might last. And it worked; the mathematical analysis of data from England, Wales, Denmark and the United States consistently revealed a correlation that lasts for two to three years (4). That may be the amount of time needed for a measles-depleted immune system to rebuild.

Of course, the idea has yet to be mechanistically proven – there’s no guarantee that the children who contract measles are the same ones who later die of other childhood infections, nor have there yet been any human studies of immune memory after measles infection. But the evidence from the mathematical study is compelling and, if nothing else, should provide one more reason for doctors to encourage childhood vaccination.