Science Friday - Why Are Flu And Other Viral Infection Rates So High This Year?

Episode Date: February 25, 2025

It’s been an unusually tough winter virus season. Rates of flu-like infections are higher than they’ve been in nearly 30 years. And for the first winter since the start of the COVID-19 pandemic, flu deaths have surpassed COVID deaths. Add to that a higher-than-average year for norovirus, a nasty type of stomach bug.Then there’s the emerging threat of avian flu. While there is no evidence of human-to-human transmission of the virus, about 70 people in the US have contracted the virus from livestock since April 2024.To make sense of the latest viral trends, Host Flora Lichtman talks with Dr. Katelyn Jetelina, epidemiologist and author of the newsletter “Your Local Epidemiologist”; and Dr. Erica Shenoy, chief of infection control at Mass General Brigham hospital.Transcripts for each segment will be available after the show airs on sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

This is Science Friday. I'm Flor Lichten. Today in the podcast, we're digging into some viral trends. You know, this winter, we had a very small COVID wave. There's this hypothesis that because we're having such a big flu season, these flu cases are pushing COVID aside. It has felt like a brutal season for sickness. More people have gotten hit with flu symptoms this year than in death. decades. Hospitalization rates are the highest they've been in 15 years. And on top of that, neurovirus is retching new highs. There's been a surge in walking pneumonia cases. Of course,

there's RSV and COVID and now concerns about bird flu. It's a lot to worry about. So is this year, in fact, very virus-y? And if so, why? Joining me now to explain are my guests. Dr. Caitlin Jedalina, epidemiologist and author of the newsletter, your local epidemiologist. She's been, based in San Diego, California. And Dr. Erica Chenoy, Chief of Infection Control at Mass General Brigham, based in Boston, Massachusetts. Okay, Caitlin, is it me or is it actually an unusually bad year for viruses? No, it is not you. There's a lot of sick people out there. Like you mentioned, influenza-like illnesses, which is about fever, cough, runny nose, haven't reached these heights since the 1990s. And the main culprit, it is flu. Flu usually gives us a rollercoaster because of changing weather and different strains,

but as of now, flu hospitalization rates are higher so far this season than any time during the past 15 years. And interestingly, this is also the first winter that flu deaths past COVID deaths. So it's definitely palpable out there and a lot of people are feeling it. Okay. Why is it such a bad year for the flu. You know, a nasty flu season is usually due to a few different factors. One is that it just could be a bad flu year. It happens every couple years because of waning population immunity. Second is fewer kids are getting vaccinated against the flu than in pre-pandemic times. The latest numbers are about 44% of kiddos are vaccinated for the flu. This is compared to 58% in 2019. And the third reason is just the match between the flu virus that is circulating and the vaccine is just

okay. One of the strains that's circulating is H3N1, and that is just a notorious strain that our flu vaccines have a hard time targeting. So because of all of that, we are seeing severe disease on a population level. Erica, tell us about what you've been seeing at your hospital. So we've seen basically a replication of the national data in terms of the number of flu cases and hospitalizations. I do think at least in my professional career, this harkens back to 2017 and 18, which was prior to this one, a pretty severe flu season. Additionally, the number of influenza tests that have been reported to the CDC is much higher than in prior years. at the hospital, I see a little bit of glimmer of hope just based on some of the more recent numbers that we're seeing. The CDC data is delayed by a little bit, but in terms of our daily

numbers of flu cases and patients being admitted to the hospital with flu, I think we may be close to the plateau. I never want to like, you know, use a crystal ball. But there are several things, including wastewater data that seem to make it seem like we're headed towards potentially the peak and then we'll have to make our way down that. That tall, tall mountain. Yeah. I mean, how has it been in the hospital? Has it, have these higher rates of flu and flu-like illness made the hospital more crowded? You know, what are the effects of that for you on the ground? Well, we were definitely experiencing even prior to COVID, extreme capacity, very busy emergency rooms. So additional influenza activity of a severe season.

puts additional strain. So I would say that on a hospital level, when you're operating at really the margins in terms of capacity that having a severe flu season has strained across the board, we're making our way through it like we do every year, and in trying to make sure that people are aware of their options, getting treatment early, so that they can avoid hospitalization. Caitlin, you mentioned that flu deaths surpassed COVID deaths for the first time since the onset of the pandemic. Does that tell us anything about the COVID virus or how it's evolving? I mean, I think that's the billion dollar question. And honestly, we just don't know. You know, this winter, we had a very small COVID wave. In fact, the smallest since COVID first came

on scene. And this could be due to several things. One is we just had a pretty big COVID wave during the summer. And so population immunity. built up over time. And so that's preventing some infections. COVID also hasn't really mutated in a big way to escape our immunity. It's still making small incremental changes, but not these huge jumps like we saw from like Delta or Omicron, for example. The third thing that I think is, most interestingly, I'll put my geekiness hat on, is that there's this hypothesis that because we're having such a big flu season, these flu cases are pushing COVID aside. And so will COVID reemerge when flu goes down? Is COVID really turning into more of its cousins, like these coronaviruses that are just

become the calm and cold after we've built so much immunity? I mean, these are a lot of questions. We just don't have answers to yet. When you say pushing COVID aside, like the flu's out competing COVID, If we're the ecosystem, they're colonizing us and there's no room for COVID? I mean, yeah. So what we saw the past couple winters is this really interesting phenomenon where we had RSV peak and then flu took over and then COVID took over. They never really peaked at the same time, which really peaked a lot of our interest as epidemiologists to understand if there's like a competing demand here.

On a biological level, it kind of makes sense, right? if your body is fighting flu, your immune system is already heightened and maybe preventing even more COVID transmission than before. But again, these are all just educated guesses right now, and we'll see how this all plays out. Let's go deep on neurovirus, not a sentence I ever wanted to say. My family had it. Everyone I know has had it this year. It's this really nasty stomach bug. It's also been circulating at higher levels this year. Is that right? That is right. I mean, we just can't catch a break this winter. We've had incredible levels of norovirus, which is like you said, it's nausea, it's diarrhea. It's just not something you want to have. It's also very highly contagious.

So if it hits a household, for example, typically that household all gets sick. And I think another question is why, right? A lot of people are wondering why we're having a bad norovirus season. And typically when we have a bad norovirus season, it means that norovirus mutated a bit to escape our immunity, just like flu and COVID can do that. We haven't had the genetic testing, not that I'm aware of, to know for sure that's what's happened, but that that would be my guess what's driving this. Now, norovirus trends looks like it has peaked. It's still above average levels right now, but it looks like it's on the way down. And I think all of us won't be complaining about that.

After the break, how living through a public health emergency has made us think differently about winter bugs. One silver lining, at least from this epidemiologist of the pandemic, is that people are paying more attention to viruses. Here's the question I want an answer to. Is it a coincidence that we are having this terrible year for norovirus? We're having this walking pneumonia surge. Flu is higher than it has been in decades. Is it a coincidence that this is all happening at the same time? Or is there, do they interplay in some way?

Yeah. I mean, one thing that I've seen rumbling on social media is questioning whether the COVID pandemic, COVID infections are weakening our immune systems. And we just have not seen the data to support that at a large population level. That may be happening among immunocompromised, for example, but that doesn't seem to be driving it. Another really interesting thing was behavior. And I think one silver lining, at least from this epidemiologist of the pandemic, is that people are paying more attention to viruses. And I think that they now realize there are things we can do to stop flu transmission.

that can prevent noravirus. And so a lot of the headlines and talk, I think, is also a result of us going through a major public health emergency. And we have increased awareness and education around all of this. That's fascinating. You know, it's interesting because I've seen on social media friends be like, oh, this was our adventure with flu A. And I was like, I never remember people calling out the flu family that they have. You know what I mean? Like there is, I do. do you think that there's something to that? I was just going to say, I think that I was looking back at CDC data over, just looking at the total number of tests perform for influenza. And I also think there's a change in the way we are approaching respiratory viruses overall. I would

love to look at, you know, overall volume of testing. And if we're just testing more than we used to in the past, so that comment about flu A, you know, many of the kind of rapid tests that people would do if they even got tested for, flu in the past would not have perhaps even told them which version of flu they had, but now people are getting much more testing. Some of that is indicated. And some of it is just not really necessary from a clinical perspective to know which particular virus one might have. While we're on testing, Erica, you know, no viral conversation today is complete without talking about H5N1 bird flu. Is your hospital testing people for bird flu? So we are not.

testing broadly for bird flu. And in fact, I'm not aware of any facility that's doing that. Maybe I'll go through it piece by piece. So so far, about 70 cases in humans in the U.S., almost all of those have had some notable exposure to animals where we know bird flu has been circulating. But there hasn't been documented person-to-person transmission. And at this point, that exposure history is really key in terms of risk factors. In terms of available testing, my own view is the focus of the testing really should be in individuals who have a relevant exposure history. And what's really challenging, I think, for most health care facilities is how do you elicit that exposure history and someone presenting to your facility in a way that's timely? And then you can figure out how to get the testing done.

There are some of the tests that we have available at the hospital that will get down to the flu subtype. And they will say, in an example of someone were to present and who actually had H5, it would be not typable. And that would be a signal to dig further for that patient. But I think the vast majority of testing that's being done out there is not going to differentiate between strain types. So you really have to be attuned to that exposure history right now and then work with your local public health to sort out next steps in terms of additional testing and confirmation. When we think ahead to the future, I mean, do you think this is something that we need to be working on, creating more accessible tests for Bird Fluict specifically?

I think testing is so key. And I'm hoping that we'll start investing at this point in testing that is actually available and that can be put into the regular clinical workflows. So while there are commercial labs out there where you could send out a test to or you could go to your public health lab and they could do the testing, that really does not work for the way health care is delivered and the timeliness in which we need these sorts of results. So right now, while it's really limited in specific risk populations, we need to be leaning forward and thinking about how do we get assays at the bedside or in the health care facility so that should we see more activity or should we need to make those sorts of decisions that we have

the tools available to us. I can share that in our facility, there is a research team that developed research-only H-5 assay, and they've been running it over the course of last summer, testing thousands of discarded samples looking for H-5, and then intermittently since that time. And at least in our area, which makes a lot of sense if you look at the national data on H-5 detections in wastewater and then the cases, there really hasn't been much H-5 around. But it's very geographical. And so in certain parts of the country, if you were asking someone in my position, they may have a different answer because they are dealing with their local epidemiology.

Caitlin, what are you looking for in terms of bird flu? What are you monitoring? I mean, we don't have a perfect data system in epidemiology. So we have to triangulate all sorts of data sources. One being wastewater. So our wastewater systems are able to test for H5N1. And this is one way we know, for example, that this huge flu wave we're seeing, you know, seasonal flu wave we're seeing right now is not due to H5N1 because the wastewater system isn't lighting up across the nation. The other thing we are very dependent on is, like Erica is saying, testing in hospitals as well as syndromic surveillance, which is basically people coming to the emergency department all across the United States and trying to find patterns of, for example, red eye and flu. And so we're trying to piece this puzzle as much together.

But I will say a lot of epidemiologists, including myself, are very frustrated because it does seem like we are flying blind. And if we were to start seeing human-to-human transmission, our systems are certainly not fast enough to stop it in its tracks. You're flying blind because of current changes and availability of data on CDC websites or why? There's a couple reasons. One is that there is a significant distrust in government among the highest risk. And the highest risk right now are farm operators as well as farm workers. And these are majority undocumented workers.

They are majority, you know, agriculture, and they don't trust CDC coming in and asking them to test once they're exposed to sick animals, for example. So there's a really big challenge with that. And you're right. There's also been very little communication in the past four weeks from this new administration on what is going along with H5N1 and very little trans. And so us in epidemiology always think that there could be more testing. And of course, it is a balance between resources and willingness and feasibility. Erica, any final tips for staying healthy for the rest of flu season? Well, if you haven't gotten vaccinated, you should absolutely get vaccinated. There's still time. I think the other part that some people may not realize is that in

terms of treatment for the flu, there is a treatment available. The treatment works best when it's given early in the infection. If you're an individual who may be at risk for severe complications of the flu, so if you have asthma or other underlying lung disease, diabetes, heart disease, if you think you have flu, reach out to your provider, they can even treat you without a positive test. And I think that sometimes people don't realize that for influenza, especially this time of year, with this amount of flu, circulating that we don't want testing to be a barrier, and we can certainly prescribe and make a clinical diagnosis that you have flu and prescribe the right treatment. I want to thank you both for taking the time to break this down for us. Yeah, thanks for having me. Yeah, thanks for having me.

It's fun. Dr. Caitlin Jedalina, epidemiologist and author of the newsletter, your local epidemiologist. She's based in San Diego, California. And Dr. Erica Chenoy, Chief of Infection Control at Mass General Brigham based in Boston, Massachusetts. And that is about all we have time for. Lots of folks helped make the show happen, including... Dee Petersmith. Felissa Mayors. Emma Gomez.

Jackie Hirschfeld. I'm Flora Lichtman. Thanks for listening.