AMNA NAWAZ: The CDC has now confirmed more than 4,000 cases of measles this year as the country grapples with the largest outbreak of the highly infectious virus in 35 years.

Nearly a fifth of cases comprise children under the age of 5, and more than 90 percent are among people who are unvaccinated.

Pennsylvania is the epicenter of the outbreak with more than 1, 100 cases.

That includes five measles-related deaths and potentially a sixth still being investigated.

For more on what we're seeing and its impact, we're joined now by Dr.

Evan Shirey.

He's a pediatric infectious disease doctor based in Hershey, Pennsylvania.

Dr.

Shirey, welcome and thanks for joining us.

DR.

EVAN SHIREY, Penn State Health: Thank you for having me.

AMNA NAWAZ: So you're in Hershey in Central Pennsylvania, the middle of your state's measles epicenter.

Just tell us what you're seeing at your hospital right now.

DR.

EVAN SHIREY: Yes, unfortunately, I have been seeing a lot of children and a lot of babies who have been admitted to our hospital and have been very sick.

I am seeing the severe consequences that can come from this virus.

AMNA NAWAZ: Tell us about those consequences.

What are the measles like in those young children you're seeing?

How does it present?

DR.

EVAN SHIREY: So there are a number of complications that can come about from measles.

One that we really worry about is encephalitis, and that's when the virus causes infection of the brain.

Children with encephalitis from measles in some cases may be unresponsive.

In other cases, they may be awake and aware, but confused.

They don't know where they are.

They're scared, in some cases, not even able to recognize their own parents.

These cases are really, really tragic to see.

We see infections where the virus infects the lungs or the airways, and we call that a bronchopneumonia.

These children might come in breathing really fast, breathing really heavily, gasping for air, in some cases needing different forms of respiratory support and oxygen, and in some of the extreme cases we have seen have even need to be put on a ventilator.

We have seen babies who have come in who are unable to nurse or unable to take bottles, and just all forms of various different kinds of suffering from the manifestations of these infections.

AMNA NAWAZ: And, Dr.

Shirey, do we know what's driving this outbreak, how it's gotten so severe in Pennsylvania?

DR.

EVAN SHIREY: So we know that, historically, the measles virus caused a lot of infections.

We had three or four million infections every single year in this country, and that also resulted in 50,000 hospitalizations, around 500 deaths every year.

So we know this is really bad virus.

We know the measles virus can spread like wildfire.

It is famous for how infectious it is.

One person with measles can infect up to 18 others.

The reason that we have not been seeing measles cases up until now is because we had a very safe and a very effective vaccine that we had been using for decades that actually resulted in elimination of the virus from this country.

The virus is back because of a failure to vaccinate.

AMNA NAWAZ: Given that we thought measles was eliminated about 25 years ago, is this something you ever thought you would be seeing during your time practicing as a doctor?

DR.

EVAN SHIREY: Absolutely not.

I spent a lot of time learning about measles as a medical student and as a resident and as an infectious diseases fellow going through all of my training.

I always expected that measles was going to stay in the textbooks, and I did not expect that I would live in a world where I was in an ICU taking care of patients in real life.

AMNA NAWAZ: Can I ask you about the rhetoric we're hearing from some of the nation's leaders?

I mean, famously, the HHS secretary, Robert F. Kennedy Jr., has had some anti-vaccine views and vaccine skepticism he's expressed.

How does that complicate your job on the ground?

DR.

EVAN SHIREY: I think it's a real shame.

I think that this outbreak just really highlights how important it is that, when we communicate about these important health issues, that we lead with honesty and the truth.

I think it's -- overall has been a shame that the anti-vaccine movement has gained the power that they have and that they have gained the influence in reaching ordinary Pennsylvanians.

I see in my clinic when I talk to people about vaccines that the types of fears that people have over these last five, 10 years are really just a different dynamic from the way things used to be.

And I think that is largely -- unfortunately, it's related to the success of the anti-vaccine movement as a whole.

AMNA NAWAZ: Whenever we cover these kinds of stories, I should mention to you we hear from people who say, look, if you or your children are vaccinated, you will be fine, so there's not much to worry about.

What do you make of that argument?

DR.

EVAN SHIREY: So, first and foremost, I want to be very clear that the vaccine is extremely safe and extremely effective.

We have been using the MMR vaccine really in the same form since the late 1960s.

So we have decades of experience with this.

Billions of people have received this vaccine.

So I lead with reiterating the truth that the vaccine is very safe and very effective.

For people who are vaccinated, I do want to give reassurance, if you are vaccinated, the vaccine is protective.

The unfortunate thing that I think people need to understand is that not everyone can receive the vaccine.

And so the vulnerable among our communities have been relying for decades on the rest of us to be vaccinated at a high enough rate to achieve herd immunity.

One really important point that I can share personally, infants are an important group that can't be vaccinated.

And I'm a father of a 2-month-old.

He is living in an area where this outbreak is active.

And there's nothing that I can do to protect him, rather than -- other than limit his exposures to public places where he may get the virus.

AMNA NAWAZ: Dr.

Evan Shirey from Penn State Health joining us tonight.

Doctor, thank you so much for your time and your insights.

We appreciate it.

DR.

EVAN SHIREY: Of course.

Thank you.