AMNA NAWAZ: We are entering peak flu season,# and a new flu variant known as subclade K is## spreading quickly across the country, driving# a sharp rise in illness and hospitalizations.

Public health officials say this# current wave may last for weeks to come.

William Brangham has the latest.

WILLIAM BRANGHAM: That's right, Amna.

Just this week, according to the CDC, f.. and jurisdictions.

That's nearly double from# the week before.

More than seven million people## nationwide have been infected, and over 3,000# have died so far this season.

Only about a quarter## of adults and just 17 percent of children# have received the flu shot as of November.

So, for more on the variant and what# to look out for this flu season,## I'm joined by Andrew Pekosz.

He's a virologist at# Johns Hopkins Bloomberg School of Public Health.

Andrew Pekosz, thank you so much for being here.

We know flu is a shape-shifter.

It's why# we have to design a new vaccine every## single year.

But this particular strain,# this subclade K, is proving to be quite a## challenge.

Tell us a little bit about it.

Is# it more contagious?

Is it more dangerous?

ANDREW PEKOSZ, Johns Hopkins University: Yes,# well, the first thing we know about this subclade## K virus, which is part of a larger group of# viruses we call H3N2 viruses, is, we know that it## has mutations at a couple of what I call hot spots# on the virus that are the targets of your immune## system and that can generate those protective# antibodies that minimize infection and disease.

So, this clade K virus has changed from# the viruses that circulated last year.

And,## importantly, it also has changed based# on the vaccine that was distributed## earlier this fall and is still available.

So# we think that the first thing that's going## on with this clade K virus is, more people are# susceptible because it's found a way to evade## that preexisting immunity in the population that# is induced by either infection or by vaccination.

WILLIAM BRANGHAM: So is that why we are hearing# reports that the current flu shot that people## can get at the drugstore is considered not# as effective against this particular strain?

ANDREW PEKOSZ: Yes, this year is# what we call a mismatch between## the vaccine strain and the circulating virus.

We know there's a few mutations that will# cause some of that vaccine-induced immunity## to not recognize the currently circulating# virus.

And what usually results from that is,## we end up seeing a drop in the vaccine# efficacy, particularly against infection.

Now, it's important to note that, even in a# year when there's a mismatch of this sort,## we oftentimes still see a signal of protection# against severe disease that's associated with## vaccination.

So just because there's a mismatch# in this clade K virus doesn't mean you shouldn't## take the vaccine.

If you haven't gotten the# vaccine already, still a good idea to take it.

WILLIAM BRANGHAM: It's still --# there's still time to get some## protection out of it for the rest of this season?

ANDREW PEKOSZ: Yes, the influenza vaccine# takes about two weeks for you to generate a## strong immune response to it.

Flu seasons usually# last anywhere from four to six weeks or longer.

So, even if you're in a part of the country that's# starting to experience a rise in influenza cases,## not a bad idea to go get vaccinated because## it'll protect you against the# tail end of the flu season.

WILLIAM BRANGHAM: I want to ask you a# question about public health messaging.

Recently, Dr.

Mehmet Oz, who runs the# Center for Medicare and Medicaid Services,## one of the most prominent doctors in the Trump# administration, was on TV and he was asked about## flu prevention.

And he ticked off a whole slew# of things that people should do, getting more## exercise, getting more sunlight, eating well,# taking vitamin D, sneezing into your elbow.

The only thing he said about the vaccine,# the flu shot, was something like,## we know it doesn't always work# that well and it's controversial.

What do you make of that as a messaging strategy?

ANDREW PEKOSZ: Well, certainly,# the healthier that you are,## the better are you will be# at fighting off an infection.

But there's nothing really controversial about# the influenza vaccine.

We know that it works.## We know that it's safe.

It does vary from# year to year in terms of its efficacy.

But## that's primarily because we have to choose# a flu vaccine six to nine months before our## flu season in order to have enough time to# manufacture enough vaccine to distribute.

And with this clade K variant, for# instance, it emerged after the vaccine## was chosen for the Northern Hemisphere last# February.

So there really was no chance for## us to make a better matched flu vaccine# because of that -- manufacturing issues.

So, again, the vaccine works.

It could# be better, but certainly it works.

And## there's no reason why most individuals# shouldn't take the influenza vaccine.

WILLIAM BRANGHAM: So who is most# at risk from this flu right now?

ANDREW PEKOSZ: Well, everyone can get infected,# but, when it comes to that severe disease, we're## seeing a typical pattern, which is, individual# -- young individuals, pediatric populations,## kids under the age of 11, are one prime# area that is suffering from severe disease.

And then individuals over the age of 65 are# also suffering from more severe disease.

Now,## it's important to note that right now the data# is saying that we're seeing a lot of flu cases## and a proportionate rise in the amount of severe# cases.

So, we're not seeing any data so far that## says the clade K variant is more dangerous to# people than other H3N2 viruses that we have seen.

It's just that more people are susceptible.# And, therefore, we will be seeing more cases## and a corresponding or proportionate# rise in the number of severe cases.

WILLIAM BRANGHAM: Do we know why some areas# seem to be having bigger outbreaks than others?

ANDREW PEKOSZ: Yes, we think that, every year,# influenza, particularly here in the U.S.,## it appears in different states at# slightly different times of the year.## New York was unfortunate to be one of the first# states that really got hit by this clade K virus,## and so we have seen the most experience# in the U.S.

in New York in terms of how## quickly that virus can spread# and how many cases it can cause.

Over the last two weeks, we have# seen many states that have gone## from basically no cases or very few# cases to a very high number of cases.## And we will probably see the rest of the country# see the same thing over the next two weeks## or so.

So it's not uncommon to see influenza# appear in different states at different times.

Sometimes, that's just bad luck.

That's# where the virus lands first and starts## spreading first.

And then it moves from# that area into other parts of the country.

WILLIAM BRANGHAM: All right, that is Andrew Pekosz## of Johns Hopkins Bloomberg# School of Public Health.

Thank you so much for being here.

ANDREW PEKOSZ: My pleasure.