Measles is an extremely contagious disease — on average, 90% of those exposed to the virus become infected, unless they’ve been vaccinated, or have already contracted and resolved the infection. The virus spreads from person to person, is airborne, and hangs around up to two hours — in the air and on surfaces — after the infected individual that coughed or sneezed leaves the area.
Although there is no treatment or cure for measles, most people who catch it do survive the infection. However, measles is not a harmless childhood disease — it can, indeed, cause lifelong disabilities, or even kill. Severe complications include, among others, ear infections that can lead to hearing loss, pneumonia, and encephalitis. According to the CDC, for every 1,000 children who get measles, one or two will die from it.
The major cause of death in infants infected with the measles virus is immunosuppression — the virus suppresses the function of the immune system, making it unable to fight other infectious diseases. More specifically, the virus wipes out immunological memory — the ability of the immune system to remember previous encounters with infectious microorganisms and respond to them more rapidly and effectively.
Since its introduction in the 1960s, mass measles vaccination has reduced childhood mortality by 30 to 50% in resource-poor countries, and by up to 90% in the most impoverished populations. Such dramatic declines in mortality cannot be explained only on the basis of the vaccine ability to prevent primary measles virus infections alone — instead, it could be an indirect effect. By preventing measles infection, the vaccine may also prevent the loss of immunological memory, and thus prevent secondary infections and complications.
Now, results from a new study show that children infected with the measles virus remain vulnerable to other potentially deadly infections up to three years after clearing measles. Lead author Michael Mina said he was motivated to carry out his study because of previously published research showing that the measles virus attacks memory lymphocytes — lymphocytes responsible for immunological memory. By attacking these memory lymphocytes, measles leads to immune amnesia — the immune system no longer remembers previous encounters with pathogens.
C. Jessica Metcalf, one of the researchers involved in the new study, said: “We already knew that measles attacks immune memory, and that it was immunosuppressive for a short amount of time. But this paper suggests that immune suppression lasts much longer than previously suspected. In other words, if you get measles, three years down the road, you could die from something that you would not die from had you not been infected with measles.”
Researchers examined measles cases and death rates from other infectious diseases before and after widespread measles vaccination in the United States, England and Wales, and Denmark. They found that the number of measles cases in a given period of time correlates with the number of deaths caused by non-measles infectious diseases during the following two to three years.
Mina told Nicholas Bakalar (New York Times): “With mathematical analysis of all of the epidemiological evidence we have, it seems that when measles was prevalent, it would go through a population, and that population would be at increased risk for mortality from other diseases for about 28 months, and in proportion to how many people were infected with measles.”
However, due to the design of the study, the researchers could not prove a cause-and-effect relationship — indeed, their results only identified a link between measles infection and increased risk of other infectious diseases. Even so, the study results are intriguing, and provide a convincing explanation for the ability of measles vaccination to prevent infectious diseases caused by many other infectious pathogens.