A dose of measles vaccine is seen at the Miami Children's Hospital. The Centers for Disease Control and Prevention announced that they are seeing the most measles cases in 20 years as they warned clinicians, parents and others to watch for and get vaccinated against the potentially deadly virus. (Joe Raedle/Getty Images)

It’s now official: Measles cases in the United States are at their highest level since 2000, when the disease was declared eliminated — meaning that regular, year-round transmission had been interrupted. Five hundred fourteen cases have been reported this year — more than double the previous high, in 2011 — and the number is expected to continue to climb.

The measles vaccine is among the most effective, and the vast majority of Americans have either had the measles (and are therefore immune to a second case) or have been vaccinated against it. So why are so many people being infected?

One big reason is vaccine refusals: The overwhelming majority of current infections occur in unvaccinated people, most of whom declined the injections for religious, philosophical or personal reasons, according to the Centers for Disease Control and Prevention.

But 10 percent of the cases occurred in vaccinated individuals. Even worse, a case study published in February about a woman dubbed “Measles Mary” described a scenario that scientists had not known was possible: In 2011, the 22-year-old New York theater worker, who had been vaccinated, not only contracted the disease but also passed it along to four others, two of whom had also been vaccinated.

While this was very rare — it was the first documented case of a vaccinated person passing measles along to others — the case illustrates how the disease can pose a threat even to those who have had their measles vaccine.

View Graphic How to recognize measles

Two ways to fail

What’s going on? Briefly, there are two ways vaccines can fail. To understand them, it helps first to understand how vaccines work.

Vaccines are made from a deactivated or weakened piece of a

disease-causing pathogen — a virus or bacterium. When the vaccine is introduced into your body, it tricks your immune system into thinking it’s the real deal. The immune system ramps up and creates anti­bodies, which remain in your system until the day the real pathogen arrives . . . and then BAM! The disease is dead on arrival. That process — the body’s generating antibodies in response to a vaccine — is called seroconversion.

But sometimes the person getting the vaccine does not seroconvert: Antibodies are not created, and the person is not protected. This is the first way vaccines can fail, and it happens about 5 percent of the time with the MMR (measles, mumps and rubella) vaccine. That’s the reason for the second shot in the MMR series: After getting it, an additional 3 to 4 percent of people seroconvert, leaving 1 to 2 percent unprotected.

Parents who don’t want to vaccinate their kids may ask, “Why does it matter to you that my kids aren’t vaccinated if yours are?” The reason is that an unvaccinated child is more likely to catch a disease and then pass it on to others, including to someone who has been vaccinated but — because of failed seroconversion — is not fully protected.

So if vaccines can fail right off the bat, why bother with them?

Well, they fail in only a small percentage of the population. Seat belts don’t prevent death 100 percent of the time, and life jackets don’t prevent drowning 100 percent of the time, but they do reduce the risk of death; that’s why we use vaccines — for risk reduction, not risk elimination.

But if a person seroconverts after the first MMR shot, does he need the second? The short answer is yes: Most of the time, it’s a good idea. For one thing, a person may seroconvert for the measles but not the mumps or rubella after the first MMR dose. Even if a blood draw to check antibody concentration after the first shot reveals the person has high enough levels to protect against measles, mumps and rubella, the second dose may further increase those concentrations, acting like a booster. And having high concentrations may be good later on because of the second way vaccines can fail: the passage of time.

A mother holds her daughter as a pediatrician administers a measles vaccination. (Joe Raedle/Getty Images)

As a successfully vaccinated person ages, the concentration of antibodies decreases, causing immunity to wane. The best known recent example of this is the pertussis vaccine introduced in the 1990s, which waned more quickly than its predecessor vaccine and is partly responsible for the increase in U.S. cases of pertussis, or whooping cough, over the past decade.

Waning immunity is the reason for vaccine boosters. (The immunity provided by having caught an infection can also decrease over time.) The initial series of vaccine shots, such as the five on the childhood immunization schedule for DTaP (diphtheria, tetanus and pertussis), provide initial immunity at the highest efficacy possible. Then boosters, such as the Tdap (the adult version of DTaP), are given to strengthen a person’s immunity after it has probably waned.

In the case of Measles Mary, the vaccine failure was a result of waning. Researchers already knew that some people don’t seroconvert after contracting measles, but Mary had evidence of anti­bodies, so failed seroconversion wasn’t the problem. The problem was that her antibodies had waned, making her both susceptible to catching the disease and, apparently, capable of passing it on.

This finding is important, but it’s also notable that this occurrence is so rare — so far — that it merited a case study in the medical literature. It’s not the waning antibodies to measles that caught scientists off guard; it’s the fact that she was able to pass on the infection despite having antibodies. It’s the first time since the measles vaccine became available in 1963 that this process has been documented.

Lifetime protection?

What this finding means remains to be seen. Because the measles vaccine has been around only about 50 years, scientists don’t know if it can last a lifetime. The Measles Mary case implies not. In fact, since this woman was so young, it implies the vaccine’s effectiveness may wear off in some rare cases in just a couple of decades.

Until about six or seven years ago, waning immunity to measles would not have been a major concern. That’s because the United States had high levels of what is called herd immunity — meaning so many people are vaccinated that a disease has trouble spreading because there are few unprotected people left to pass it along. Not only does herd immunity protect those who are too young, too sick or too immuno-compromised to be vaccinated, it also protects those who have been vaccinated and think they’re protected but actually aren’t.

Herd immunity meant that even the measles cases introduced into the United States from abroad wouldn’t travel far. But the recent string of outbreaks spans the country — New York City, California, Seattle, Massachusetts, Dallas — and they will continue if immunization levels remain low enough to let the disease spread. If the case of Measles Mary indicates that waning immunity from the vaccine is more widespread, the situation may become even more dangerous unless vaccination rates pick up again.

Haelle is a freelance science writer.