There is a measles epidemic. Do you need another shot?

The United States could be in about eight months of losing its "measles-free" status.

The measles virus was eliminated from the United States in 2000, which means that there has been no continuous transmission of the disease for more than 12 months in the country. Since then, the disease has sometimes been caused by travelers, mainly Americans, infected abroad and returned home. Although these outbreaks pose risks to public health, especially for children and pregnant women, incidents tend to be limited to single sites – Anaheim, California, Winter 2015, or Minnesota in 2017 – then fizzle out.

This trend began to change last October, when New York City – Brooklyn and Queens – began reporting ongoing measles episodes. Outbreaks are underway in five other counties across the country and the number of cases in 2019 is the highest that the United States has had in 25 years. (Reminder: it's only April.) On Friday, two Los Angeles universities quarantined over 1,000 students and staff, meaning they were confined to campus premises or sent home. them. President Donald Trump, who had previously leaked inaccurate information about immunization and its false link to autism, encouraged unvaccinated children to get vaccinated.

If the United States 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 throughout the Americas in 2016, but within one year, an outbreak triggered in Venezuela has continued until now.

For most Americans, these outbreaks are a sweet-and-sour awakening of the importance of the measles-mumps-rubella (MMR) vaccine. With the success of immunization programs, most people do not know about measles itself – which means they may not know how to tackle these outbreaks and protect themselves.

PBS NewsHour asked 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, Epidemiologist of Diseases. infectious and pediatrician at the Bloomberg School of Public Johns Hopkins. Health.

Who is most vulnerable during a measles outbreak?

Walrus: Children are usually the main targets, partly because younger children do not benefit from any immunity.

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

Walrus: The virus can slowly incubate in the brain over the 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 will cause this very serious progressive inflammation of 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.]

And if you are a healthy but unvaccinated adult, should you be afraid of contracting measles?

Walrus: In fact, serious complications occur in unvaccinated adults who get measles, which is pneumonia.

Pregnant women are certainly also at risk. These problems may not be as well known as the Zika virus, but there are pregnancy complications related to measles [such as stillbirths, miscarriages and low-birth weight]. There were many in ancient times.

[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 vaccine for the same reason that measles infection is so dangerous to them: their immune system is compromised. Once their child is born, they can get vaccinated and do things like breastfeeding without worry. Should someone avoid taking the vaccine?

Walrus: Immunocompromised or immunodeficient persons considered rare exceptions.

We do not usually give MMR vaccine at birth? How to come?

Walrus: In general, MMR vaccination was not recommended until about [one] year because the feeling was that children were younger than it would not respond and develop against measles – their immune system was not mature enough.

In the event of an outbreak, doctors may push the envelope a bit because of the increased possibility of exposure. They can administer the vaccine at six months when there is a risk of exposure, but this dose does not count in the child's immunization schedule. In other words, the child must always receive the other two doses when he is supposed to do so.

For children under 6 months of age, it has been recommended to keep them away from anyone who may get measles or catch it.

The practice of administering two doses of MMR vaccine began in Sweden in 1982, but why?

Walrus: For several years, they had noticed that the children were going to school and even if they had only one vaccine, they could still get measles if another person came to school with measles. It became clear that a dose simply did not provide a sufficient level of protection.

That's right, and finally, the US Centers for Disease Control and Prevention adopted the two-dose strategy in 1989, when the country was experiencing a significant resurgence of measles that lasted until 1991. But how old should children now have their MMR vaccine?

Foam: It is then that the World Health Organization has recommended to all children in the world to receive 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 most countries of the world, the first dose is administered at the age of nine months and it is generally found that about 85% of children will be protected after this first dose. The second dose is largely intended to immunize the 15% who did not respond to the first dose.

In the United States, we administer the first dose at a later age, usually between 12 and 15 months. There, a higher proportion of children will respond to the first measles vaccine, in the 90-95% range. The second dose is therefore to reach this small proportion, perhaps 5 to 10% of children who do not respond to the first dose.

Both in the United States and outside the United States, this second dose is really essential for elimination because measles is a highly highly transmissible pathogen. We often say that it is the most contagious "directly transmitted pathogen", that is, it is transmitted from person to person.

Yes, if a person gets measles, it will be transmitted to 18 other people if they are not vaccinated. Thus, because of this, the collective or immune immunity against measles can only be maintained if 90 to 95% of the population is immunized.

Foam: To really stop transmission and eliminate measles, we need very high levels of immunity in our communities. This is achieved with both doses of measles vaccine.

But what if you are an elderly person and have measles as a child? Do you still need a shot?

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

Walrus: And we think that if you had really recovered, you have immunity for life, which is a good thing.

Foam: There is therefore no valid evidence that a person who has developed a protective immunity against measles, either because he had already contracted the infection or with the vaccine, this protection disappears over time.

But America has recently been affected by another illness, mumps, which I heard was due to the fact that vaccination was fading away?

Foam: That's an excellent question and you're absolutely right. What we have learned in recent years because of major mumps outbreaks, particularly on university campuses, is that it appears that immunity against the mumps virus is decreasing. During mumps outbreaks, it is recommended that people who have previously received the mumps vaccine receive an additional dose if their risk of exposure is high.

There is no evidence that immunity to rubella decreases.

And if the weakening of immunity was a real phenomenon with the measles vaccine, we would see these epidemics spread to the general population and particularly affect the elderly, and we simply see it not.

Right. So to summarize: if an adult catches measles, it most likely means that he has never had the disease as a child or that he has only received one dose of the vaccine in his life. This means that if you were born between the late 1950s and 1989, you may want to get vaccinated again?

Walrus: Yes. What we are telling people is that if you are not sure about the status of your vaccine, especially if you are traveling, take another MMR vaccine.

What do you do if you do not remember if you have been vaccinated or have lost your documentation?

Foam: There is a blood test to check the immunity against measles. It can measure whether your body makes antibodies against the measles virus. These antibodies have a fairly good correlation with protection. This is what we call a serological test.

It is used, for example, to test health care workers. We want to ensure that hospital employees – nurses, doctors, other employees – are safe from measles, not only to protect this individual, but also to prevent the spread of measles in hospitals.

For the moment, it is not widely used during epidemics or in the general population. But it is increasingly used to identify clusters of vulnerable individuals outside the United States.

If you receive a second MMR vaccine after your childhood or at adulthood, how long does it take to become effective?

Foam: When we talk about the effectiveness of vaccines, we talk about their ability to prevent diseases.

… On the right, a dose gives 95 to 98% effectiveness, while two doses provide 99% protection …

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

That's true, but does that mean that some people can still get measles even after two doses?

Walrus: Given the nature of the two statistics and the variation of human immune responses, it is possible.

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

Do these cases suggest that the measles virus has evolved over time, like the seasonal flu virus, but slowed down, making vaccines received decades earlier less effective?

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

Now, people have been trying to find out if there had been an evolution of the virus, as we are seeing with other viruses – like the influenza virus, which is changing radically and we need to develop a new vaccine every year.

We do not have to do this with measles and there is no evidence to date that the virus has evolved beyond its vaccine-induced protection.

Most of the epidemics in New York are concentrated around Jewish Orthodox communities. How to come?

Walrus: Virtually all religions, including Islam, almost all Protestant sects, Catholicism and even ultra-Orthodox Judaism allows vaccination without religious or theological objections. It's considered a good thing. It saves lives.

This year's epidemic is often associated with a subgroup of this ultra-orthodox Jewish community.

This reflects the insularity of a particular group that does not trust the government – for the same reasons that you have hesitation to vaccinate or refuse to vaccinate in another group.

They do not think that the risk [of contracting measles] it's great. They are willing to take the risk. But this is a great chance to seize, as we can see.