AMNA NAWAZ: Measles, one of the world's most contagious diseases, was declared eliminated in the U.S.

more than 25 years ago.

Today, an outbreak that started last year is continuing, even as the U.S.

officially ends its membership in the World Health Organization.

Stephanie Sy explores the ramifications and the reasons behind the outbreak.

STEPHANIE SY: Measles cases have been skyrocketing since last year.

Take a look at this chart.

In 2023, there were just 63 cases.

That number quadruples by 2024 to more than 280 cases, then, last year, a huge spike, 2,240 cases.

We're only three weeks into this new year, and already 416 cases have been confirmed.

Outbreaks in South Carolina and along the Utah-Arizona border are jacking those numbers up, all this as vaccination rates continue to decline, and that accelerated during the COVID pandemic.

Caitlin Rivers, an epidemiologist at Johns Hopkins University and author of "Crisis Averted: The Hidden Science of Fighting Outbreaks," joins us now with more.

Caitlin, thank you so much for joining the "News Hour."

So, more than 200 measles cases were confirmed in just one week this month.

Are these outbreaks likely to stay relatively local or are we moving toward a point where the U.S.

could see a widespread outbreak?

DR.

CAITLIN RIVERS, Epidemiologist, Johns Hopkins Center for Health Security: We are seeing that some of the larger outbreaks like that currently centered in South Carolina, it is being exported to other states.

And the longer these outbreaks spread, the more opportunities there are for the virus to travel.

That being said, public health officials are working very hard to contain them and stop transmission, but there have been a lot of cases in the last year,and I do expect that to continue in the year ahead.

STEPHANIE SY: Who is most vulnerable in these outbreaks?

DR.

CAITLIN RIVERS: People who are unvaccinated are by far the most vulnerable.

Around 94 percent of cases found last year were in people who were unvaccinated.

And so the strongest protection to prevent infection is that MMR vaccine.

STEPHANIE SY: We have been hearing from parents of babies who are getting measles.

They are those that may not be vaccinated yet or the vaccine may not have taken full effect yet.

And then there's the question of the elderly and whether they should be getting vaccine boosters at this point.

What do you think?

DR.

CAITLIN RIVERS: It's true that, infants, who are too young to be vaccinated, people who are immunocompromised, they're going to be at higher risk.

And that's why it's so important that we as a community take all of the steps that we can to prevent transmission, because stopping transmission overall is the best way to protect those people.

Now, older adults is a little bit of a different story.

A few decades ago it was the policy that people would only get one MMR vaccine.

That provides about 94 percent protection from infection.

That two-dose, that second dose, brings up the level of protection to closer to 97, 98 percent.

So if you are older and you are considering getting a second vaccine or wondering if that's right for you, one thing you can do is ask your doctor to check your titers, check your blood and see whether your immunity to measles is high enough.

If not, they may offer you that second dose.

STEPHANIE SY: You talked about the local and state response to containing these outbreaks, but I want to talk about the federal impact.

The chair of the federal advisory panel on vaccines, himself a pediatric cardiologist, has said in just the last few days that the polio and measles vaccines should be optional, including at schools.

What is your reaction to that?

DR.

CAITLIN RIVERS: That's really not the role of the Advisory Committee on Immunization Practices.

They do not have the power and, in fact, the entire CDC agency does not have the power to require anyone to get vaccinated.

What they're meant to do is provide the best advice to the American people about how to keep themselves and their communities safe from infectious diseases, and I'd like to see them get back to that in 2026.

STEPHANIE SY: Is that the best advice to make these vaccines that have long protected public health optional?

DR.

CAITLIN RIVERS: Absolutely not.

There's no question that the measles vaccine, the polio vaccine and in fact, the entire childhood vaccination schedule does good.

It is good for people and communities, because it prevents preventable infection.

But what's at issue here I think is mandates.

And I want to underscore that the federal government does not make vaccine mandates.

They do not have that authority.

And so when -- I think it's a red herring to pit individual choice against vaccine recommendations, because individual choice is not at issue here.

STEPHANIE SY: We have heard quite often that decreasing vaccination rates can be attributed to vaccine hesitancy.

But, Caitlin, you have pointed out that equal attention should be paid to health care access and affordability.

Expand on that.

DR.

CAITLIN RIVERS: There's no question that there is rising vaccine hesitancy.

And I think we're right to think about that and to worry about that.

But when you look at which demographic groups have lowest vaccine coverage, it is people who are uninsured.

And so I think access and affordability needs to be on the agenda when we talk about ways to increase community vaccine coverage.

STEPHANIE SY: The U.S.

at this point may be on the path toward losing what in your field is called measles elimination status.

How big of a deal is that and what would it mean?

DR.

CAITLIN RIVERS: It's really a sign that something has gone wrong in our public health system.

Measles was eliminated in the United States in the year 2000.

So it's been over 25 years of maintaining that elimination status.

And I think we will likely lose it in the year ahead.

Now, I want to make the point that a vast majority of Americans are vaccinated and support vaccination.

And so it's really this minority of people who are thinking about ways to erode our vaccine confidence and reducing community access.

And, unfortunately, we are seeing the effects of that.

But I also want to contrast that with the point that vaccines are very popular.

STEPHANIE SY: The U.S.

left the World Health Organization officially this week.

I'm curious whether you think that might affect the measles outlook, either domestically or globally.

DR.

CAITLIN RIVERS: Measles is probably not top of the list of places where we will see the impact of our withdrawal from the World Health Organization.

But, regardless, it's clear that that is a move that will reduce the United States is preparedness for epidemics and pandemics.

The WHO is the main way that we receive information about what is happening in the world.

And we provide information and support as well.

And so I think it's really a mistake to sever those ties.

STEPHANIE SY: That is Caitlin Rivers, an epidemiologist and author of "Crisis Averted: The Hidden Science of Fighting Outbreaks," joining us.

Thank you.

DR.

CAITLIN RIVERS: Thank you.