Back in the 1960s, the U.S. started vaccinating kids for measles. As expected, children stopped getting measles.
But something else happened.
Childhood deaths from all infectious diseases plummeted. Even deaths from diseases like pneumonia and diarrhea were cut by half.
Scientists saw the same phenomenon when the vaccine came to England and parts of Europe. And they see it today when developing countries introduce the vaccine.
"In some developing countries, where infectious diseases are very high, the reduction in mortality has been up to 80 percent," says Michael Mina, a postdoc in biology at Princeton University and a medical student at Emory University.
"So it's really been a mystery — why do children stop dying at such high rates from all these different infections following introduction of the measles vaccine," he says.
Mina and his colleagues think they now might have an explanation. And they published their evidence Thursday in the journal Science.
Now there's an obvious answer to the mystery: Children who get the measles vaccine are probably more likely to get better health care in general — maybe more antibiotics and other vaccines. And it's true, health care in the U.S. has improved since the 1960s.
But Mina and his colleagues have found there's more going on than that simple answer.
The team obtained epidemiological data from the U.S., Denmark, Wales and England dating back to the 1940s. Using computer models, they found that the number of measles cases in these countries predicted the number of deaths from other infections two to three years later.
"We found measles predisposes children to all other infectious diseases for up to a few years," Mina says.
And the virus seems to do it in a sneaky way.
Like many viruses, measles is known to suppress the immune system for a few weeks after an infection. But previous studies in monkeys have suggested that measles takes this suppression to a whole new level: It erases immune protection to other diseases, Mina says.
So what does that mean? Well, say you get the chicken pox when you're 4 years old. Your immune system figures out how to fight it. So you don't get it again. But if you get measles when you're 5 years old, it could wipe out the memory of how to beat back the chicken pox. It's like the immune system has amnesia, Mina says.
"The immune system kind of comes back. The only problem is that it has forgotten what it once knew," he says.
So after an infection, a child's immune system has to almost start over, rebuilding its immune protection against diseases it has already seen before.
This idea of "immune amnesia" is still just a hypothesis and needs more testing, says epidemiologist William Moss, who has studied the measles vaccine for more than a decade at Johns Hopkins University.
But the new study, he says, provides "compelling evidence" that measles affects the immune system for two to three years. That's much longer than previously thought.
"Hence the reduction in overall child mortality that follows measles vaccination is much greater than previously believed," says Moss, who wasn't involved in the study.
That finding should give parents more motivation to vaccinate their kids, he says. "I think this paper will provide additional evidence — if it's needed — of the public health benefits of measles vaccine," Moss says. "That's an important message in the U.S. right now and in countries continuing to see measles outbreaks."
Because if the world can eliminate measles, it will help protect kids from many other infections, too.
DAVID GREENE, HOST:
Back in the 1960s, the U.S. started vaccinating kids for measles. And then something surprising happened; childhood deaths caused by all infections plummeted. As NPR's Michaeleen Doucleff reports, scientists think they have finally figured out this connection.
MICHAELEEN DOUCLEFF, BYLINE: Scientists saw the same measles-related decline in England and parts of Europe, and it still happens today. When a developing country starts to give kids the measles vaccine, right away other diseases become less deadly.
MICHAEL MINA: So it's really been a mystery. Why do children stop dying at such high rates from all of these other infectious diseases following introduction of the measles vaccine?
DOUCLEFF: That's Michael Mina, a postdoc in biology at Princeton University. He and his colleagues have been using computer models and medical records to figure out what happens to children after they get measles. Now, there's an obvious answer here, right? If children are getting the measles vaccine, they're probably already in a position to get better health care, maybe more antibiotics or vaccines. But Mina says that's not what's going on here.
MINA: What we found is that measles infections predispose children to all other infectious diseases for up to a few years.
DOUCLEFF: And that's because the virus zaps a child's immune system. That makes them more vulnerable to new infections. Mina and his colleagues published the findings this week in the journal Science. But there's something else really intriguing. Measles seems to erase immunity previously built up. So what does that mean?
Well, say you're 4, and you get the chickenpox. Your immune system figures out how to fight it so you won't get it again. But then you're 5, and you get measles and it looks like measles might wipe out your immune system's memory of the chickenpox. So if you get exposed to chickenpox, you might get it again. Mina says it's like the immune system gets amnesia.
MINA: The immune system comes back. The only problem is that it's forgotten what it once knew.
DOUCLEFF: So after measles, a child's immune system has to start over, rebuilding its protection system against diseases it has already seen before. Over at Johns Hopkins University, William Moss has studied the measles vaccine for more than a decade. He says more research is needed on this idea of immune amnesia, but the new findings do suggest measles damages the immune system for two to three years.
WILLIAM MOSS: Hence the reduction in overall child mortality that follows measles vaccination is much greater than previously believed.
DOUCLEFF: Which Moss says should give parents more motivation to vaccinate their kids because if the world can eliminate measles, it will help protect kids from many other infections, too. Michaeleen Doucleff, NPR News. Transcript provided by NPR, Copyright NPR.