Since mass measles vaccination was introduced in the 1960’s, scientists began to notice a strange phenomenon: child mortality from all infectious diseases,not just the measles MV virus, has plummeted. In a recent publication in Science, Mina and colleagues found strong evidence that suggests that measles vaccinations prevent other infectious diseases by preventing “immune amnesia.” In other words, because a measles infection can reset any previously acquired immunity for up to five years, protecting against the MV virus prevents this immune amnesia effect and in turn maintains immunological memory.
The authors cite immune amnesia as a concept presented in numerous other recent studies in animal models. In an interview with NPR, Mina elucidates the concept with an example: if one gets chickenpox, their immune system develops antibodies and immunological memory in order to ensure that it can successfully fight off the virus should it be encountered again. If, however, that person gets infected with the measles virus afterwards, their entire immunological memory could be lost (also known as immunomodulation) for two or three years− including the immune system’s knowledge of how to fight the chicken pox.
To confirm this “immunological hypothesis,” Mina and colleagues first obtained decades worth of epidemiological population data from the United States, the United Kingdom, and Denmark. Using statistical and computer models, the authors found that in all of these locations there was a statistical significance (P < 0.001) that showed that measles incidences were associated with childhood mortality. A gamma‐distribution and linear fit showed a strong significance (P < 0.00001) between childhood mortality and MV immunomodulation. The data suggests that this MV‐associated immune amnesia period centers at a 28.3‐ month duration, suggesting that the number of measles incidences predicted a two to three year immunomodulation period that finally culminated in death of measles or another infectious disease.
Measles infection suppresses the immune system and leaves the infected individual vulnerable to pathogens and other infectious diseases. While there is little empirical evidence to support the immunological hypothesis, previous research suggests that measles vaccinations stimulate the immune system’s T cells and trains innate immunity to inherit immunological memory. These results, however, explain short term and not necessarily long‐term effects.
Interestingly enough, Mina and colleagues found that females had a higher association with these effects than males did. The year analyzed, however, had no statistically significant effect: regardless of changed health policies, the association between measles incidences and a two to three year immune amnesia endured.
To ensure that this immunosuppressive impact was unique to measles, the researchers tested their analysis on another vaccine‐preventable disease called Pertussis. Unlike with measles, there was no correlation between incidences of pertussis and infectious disease mortality, suggesting that pertussis is not immunosuppressive.
Between all the factors that were analyzed, the main factor that was strongly significant in reducing overall childhood infectious diseases mortality was the introduction of the measles vaccination. Additionally, the measles immunomodulation effect is supported in that intensive MV exposure correlated most strongly with increased mortality.
Mina and colleagues acknowledge that some of the previous studies that sought to analyze the long‐term immunological effects of measles were inconclusive or failed to come to the same immune amnesia conclusion. However, the authors point out that these studies focused on low‐income West African countries, where high rates of infectious disease infections and deaths prevent longitudinal studies of the effects of a measles infection years down the line. While scientists continue to gather enough empirical evidence to support the immunological hypothesis, this study shows compelling evidence to reinforce the unique importance of measles vaccinations.
Mina, M. J., Metcalf, C. J. E., Swart, R. L. de, Osterhaus, A. D. M. E., & Grenfell, B. T.(2015). Long‐term measles‐induced immunomodulation increases overall childhood infectious disease mortality. Science, 348(6235), 694–699.