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A vial of measles, mumps and rubella vaccine and an information sheet is seen at Boston Children's Hospital in Boston, Mas...

There’s a measles outbreak. Do you need another shot?

The U.S. could be eight months or so away from losing its “measles-free” status.

The measles virus was eliminated from the U.S. in 2000, meaning there was no longer continuous transmission of the disease for more than 12 months anywhere in the country. Since then, the disease has occasionally sprouted up due to travelers, mostly Americans, getting infected abroad and returning home. Though those outbreaks create public health hazards, especially for children and pregnant women, the incidents tend to stick to single locations — Anaheim, California, in the winter of 2015 or Minnesota in 2017 — and then fizzle out.

That trend started to shift last October, when New York City — namely Brooklyn and Queens — began reporting continuous measles episodes. On Monday, as the epidemic in Washington State came to a conclusion, the Centers for Disease Control and Prevention announced new outbreaks in Los Angeles, Sacramento, Georgia and Maryland.

Outbreaks are now ongoing in nine counties in the country, and 2019’s case count of 704 is the nation’s highest since 1994’s tally of 963 cases. (Reminder: It’s only April.) On Friday, two Los Angeles universities quarantined more than 1,000 students and staff members, meaning they were confined to quarters on campus or sent home— and President Donald Trump, who formerly spread misinformation about vaccination and its false connection to autism, encouraged unvaccinated children to get immunized.

If the U.S. loses its “measles elimination” status, it will join Venezuela as the only other country in North and South America with this distinction. Measles was declared eliminated across the Americas in 2016, but within a year, an outbreak sparked in Venezuela that has persisted up to the current day.

For most Americans, these outbreaks are a bittersweet wake-up call about the importance of the measles-mumps-rubella (MMR) vaccine. Thanks to the success of vaccination programs, most people are unfamiliar with measles itself — which means they may be unsure about how to approach these outbreaks and protect themselves.

The PBS NewsHour posed these questions and concerns to two experts: Stephen Morse, director of the infectious disease epidemiology program at Columbia University’s Mailman School of Public Health and Dr. William Moss, a infectious disease epidemiologist and pediatrician at Johns Hopkins’ Bloomberg School of Public Health.

Who is most vulnerable during a measles outbreak?

Morse: Children are usually the major targets in part because the youngest children don’t have immunity at all.

Moss: Because the vaccine has been so effective in the United States and around the world, I think people have forgotten about measles and have underestimated the risk of measles. [Globally,] more than 100,000 children die each year, or about 300 children per day. Measles can also cause lifelong disability [such as deafness].

Morse: The virus can incubate slowly in the brain over years. Then suddenly, usually when the patient is much older [up to 10 years after a person has measles], the infection will reactivate and you get this very severe progressive inflammation in the brain called Subacute Sclerosing Panencephalitis (SSPE)

[SSPE has long been considered rare, but a 2017 study from the California Department of Health estimated 1 in 600 infants developed the condition after they caught measles.]

What if you’re a healthy but unvaccinated adult—should you be worried about catching the measles?

Morse: There are actually serious complications that occur in unvaccinated adults who catch measles, namely pneumonia.

Pregnant women are certainly at risk too. These issues may not be as well publicized as the Zika virus, but measles-related pregnancy complications exist [such as stillbirths, miscarriages and low-birth weight]. There were a lot of those going around in the old days.

[Pregnant women infected with measles are also more likely to be hospitalized, develop pneumonia, and die than nonpregnant women, according to the Centers of Disease Control and Prevention.]

That said, pregnant women should avoid getting the MMR vaccination for the same reason that measles infection is so dangerous for them: Their immune systems are compromised. Once their child is born, they can get vaccinated and do things like breastfeed without any concerns. Should anyone else avoid taking the vaccine?

Morse: People who have immunosuppression or some immunodeficiency, which are considered rare exceptions.

If you think you have the measles, what’s the best course of action?

Morse: Stay home and call your doctor or your health care provider before you head to a medical office or emergency room. If you do visit a doctor’s office or ER, immediately notify the staff, so they can take the proper precautions.

We don’t typically give the MMR vaccine right at birth? How come?

Morse: In general, the MMR vaccination was not recommended before about [one] year of age because the feeling was children younger than that would not respond and develop protection from measles — their immune systems were not sufficiently mature.

In outbreaks, doctors can push the envelope a bit because of the heightened possibility of exposure. They can give the vaccine at six months when there is some risk of exposure, but this dose doesn’t count toward the child’s vaccine schedule. In other words, the child still needs to get the other two doses when they’re supposed to.

For kids younger than 6 months, the recommendation has been basically to keep them away from anyone who might have measles or from places where they might catch it.

A vial of the measles, mumps, and rubella (MMR) vaccine is pictured at the International Community Health Services clinic in Seattle, Washington, U.S., March 20, 2019. Photo by REUTERS/Lindsey Wasson

A vial of the measles, mumps, and rubella (MMR) vaccine is pictured at the International Community Health Services clinic in Seattle, Washington, U.S., March 20, 2019. Photo by REUTERS/Lindsey Wasson

The practice of giving two doses of the MMR vaccine started in Sweden in 1982, but why?

Morse: They’d noticed for a number of years that children would come into school, and even though they’d had one vaccine shot, they might still catch measles if someone else came into the school with measles. It became evident that one dose simply wasn’t giving a high enough level of protection.

Right, and eventually the U.S. Centers for Disease Control and Prevention adopted the two-dose strategy in 1989, as the nation experienced a major resurgence of measles that lasted until 1991. But how old do kids need to be now for their MMR vaccines?

Moss: It was later that the World Health Organization recommended that all children in the world get two doses of measles vaccine. And the reason for the second dose is really because not all children who receive a single dose are protected.

In much of the world, the first dose is given at nine months of age, and we generally see that about 85% of children will be protected after that first dose. The second dose is, in large part, really to immunize the 15% that didn’t respond to the first dose.

Now, in the United States, we give the first dose at an older age — it’s usually between 12 and 15 months of age. There, a higher proportion of children will respond to the first measles vaccine, on the order of 90 to 95%. So the second dose is really to get that small proportion, maybe 5 to 10% of the children who don’t respond to the first dose.

Both inside and outside the U.S., that second dose is really critical for elimination, because measles is a very highly transmissible pathogen. We often say it’s the most contagious “directly-transmitted pathogen,” meaning it spreads from person to person.

Right, if one person gets measles, they will spread it to 18 others if those people are unvaccinated. So, because of that, herd immunity or community immunity for measles can only be maintained when 90 to 95% of a population is immune.

Moss: In order to actually stop transmission, to eliminate measles, we need very high levels of immunity in our communities. That is achieved with the two doses of measles vaccine.

But what if you’re an older adult and you had measles as a child? Do you still need to get a shot?

Moss: This is somewhat arbitrary, but we generally say that people born before 1957 are immune, because almost everyone got measles before then. The vaccine was introduced in the United States in 1963.

Morse: And we believe that if you actually had it and recovered, you have lifelong immunity, which is good.

Moss: So, there’s no good evidence that once a person has developed protective immunity to measles, either because they had the infection before or from the vaccine, that that protection wanes over time.

The principle of community immunity applies to control of a variety of contagious diseases, including influenza, measles, mumps, rotavirus, and pneumococcal disease. Infographic and caption by National Institute of Allergy and Infectious Diseases, National Institutes of Health

The principle of community immunity applies to control of a variety of contagious diseases, including influenza, measles, mumps, rotavirus, and pneumococcal disease. Infographic and caption by National Institute of Allergy and Infectious Diseases, National Institutes of Health

But America has experienced recent bumps in, another disease, the mumps, which I heard was due to the immunization wearing off?

Moss: That’s a great question, and you’re exactly right. What we’ve learned in the past couple of years because of large mumps outbreaks, particularly on college campuses, is that it does appear that immunity to mumps virus wanes. It’s recommended during mumps outbreaks that individuals who’ve had prior mumps vaccine get an additional dose if they’re at high risk for exposure.

There’s no evidence that the immunity to rubella wanes.

And if waning immunity was a real phenomenon with measles vaccine, we would be seeing these outbreaks spreading out into the general population and particularly affecting older adults, and we’re just not seeing that.

Right. So to recap: If an adult catches the measles, it most likely means that they never had the disease as a child or they only received one dose of the vaccine in their life. That means if you were born between the late 1950s and 1989, then you might want to get another MMR shot?

Morse: Yes. What we say to people is if you’re not sure about your vaccine status, especially if you’re traveling, take another MMR shot.

What do you do if you can’t remember if you were vaccinated or lost your documentation?

Moss: There is a blood test to check for measles immunity. It can measure if your body is making antibodies to the measles virus. Those antibodies have a pretty good correlation with protection. It is what we call a serological test.

It’s used, for example, to test health care workers. We want to ensure that the employees within hospitals — nurses, doctors, other employees — are immune from measles, to not only protect that individual but to prevent the spread of measles within a hospital.

At the moment, it’s not widely used during outbreaks or in the general population. But it is being increasingly used to identify susceptible clusters of individuals outside of the United States.

If you get a second MMR shot after childhood or in adulthood, how long does it take to become effective?

Moss: When we’re talking about vaccine effectiveness, what we’re talking about is its ability to prevent disease.

…right, one dose yields 95 to 98% effectiveness, while two doses leads to 99% protection…

Moss: And, it usually takes two to four weeks for a person to develop protective antibody levels.

True, but that means some people can still get the measles even after two doses?

Morse: Given the nature of both statistics and the variation in human immune responses, it’s possible.

Three percent of the measles cases in Rockland County [New York] have apparently had two immunizations and should therefore not have caught the measles. They should have been fully protected.

An unidentified boy receives an early version of a measles vaccination at the Fernbank School in Atlanta, Georgia in 1962. Photo by the CDC

An unidentified boy receives an early version of a measles vaccination at the Fernbank School in Atlanta, Georgia in 1962. Photo by the CDC

Do these cases suggest that the measles virus has evolved over time, like the seasonal flu virus but in slow motion, rendering vaccines received decades earlier less effective?

Moss: The short answer is that there’s no evidence that measles virus has evolved over time. The measles vaccines that are used in the United States and throughout the world were derived from measles virus isolates from the mid-1950s.

Now, people have looked to see whether there is has been evolution of the virus, which we see with some other viruses — like influenza virus, which changes dramatically and we have to develop a new vaccine every year.

We don’t have to do that with measles and there’s no evidence to date that the virus has evolved away from its vaccine-induced protection.

Much of New York City’s outbreak centers around Orthodox Jewish communities. How come?

Morse: Virtually every religion, including Islam, almost all Protestant sects, Catholicism and even ultra-Orthodox Judaism allows immunization without religious or theological objections. It’s considered to be a good thing. It saves lives.

This year’s outbreak is associated quite a lot with a subgroup within this ultra-Orthodox Jewish community.

That’s reflecting an insularity of one particular group that doesn’t trust government — really for the same reasons that you get vaccine hesitancy or vaccine refusal in any other group.

They don’t think the risk [of contracting measles] is that great. They’re willing to take the chance. But it’s a big chance to take, as we can see.

Editor’s note: This story was updated April 29 to reflect the latest case numbers from the Centers for Disease Control and Prevention and to add advice on what people should do if they think they’ve contracted the measles.