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Bapu JENA: Let me set the scene for you. It’s mid-August. My son, Aiden, is about to turn 3 years old. And, like most kids his age, he goes to the doctor right around his birthday for an annual checkup. I took him to his pediatrician. Everything was fine. But as we’re about to leave, the nurse stops me at the door and says,

NURSE VO: Don’t forget to come back in a few weeks. We’ll have the flu shot ready by then. Your son can get his.

I thought to myself, “Man! If only Aiden had been born in September! If he had, we would have walked out of that office all set, flu shot in arm, with no need to return for another year if we’re lucky.”

But, okay, okay, we can call and schedule him to come back. Fast forward a few weeks. The flu vaccine is ready.

But appointments at his pediatrician’s office fill up in a couple of minutes. I call. I call again. Basically, no appointments are available until November. And that’s three months away.

So I try Walgreens, C.V.S., you name it. But no, they only vaccinate adults and kids who are older than Aiden, at least in the state where I live — Massachusetts. Now, I know states do vary in their policies. But lots of them insist you get vaccinated at your doctor unless you’re over 4 years old.

While my wife and I were scrambling, trying to find him an appointment, that’s when it occurred to me: This is actually a real pain. And it made me start to wonder if being born in the summer means a kid might be more likely to actually get the flu. 

Welcome to Freakonomics, M.D. I’m Bapu Jena. I’m an economist, but I’m also a medical doctor. And in each episode, I dissect a fascinating question at the sweet spot between health and economics.

Today, we’re tackling flu shots — a seemingly straightforward, easy-to-come-by, run-of-the-mill vaccine — unless you’re a baby or a toddler. As an adult, if I want to get my flu vaccine, I can get it at my doctor’s office. I can get it at a walk-in flu clinic. I can get it from my employer.

But little kids can usually only get a flu vaccine at a doctor’s office. And kids who are healthy tend to only go to see a doctor once a year, usually around their birthday. So, do kids with summer birthdays miss getting flu shots more often? And if so, what are the ripple effects?

I just want to state for the record that I don’t always make my team at Harvard conduct research based on things that happen in my day-to-day life. Freakonomics, M.D. is a very new podcast, and already you’ve heard at least two research studies that were inspired by a question that struck me during a really otherwise mundane activity, like bringing my kid to the doctor.

That said, quite a lot of the work done by our team begins with a question that hits one of us, and that person brings it back to the office.

Chris WORSHAM: We throw out ideas, some of them good, many of them bad.

Chris Worsham is a physician, researcher, and my partner-in-crime here at Harvard. We’ve spent a lot of time in idea meetings together. Here’s his version of how the sausage gets made.

WORSHAM: We’ll go through dozens and dozens of ideas until we find a diamond in the rough that’s something we can look into, that’s something that’s interesting, and something that we have the data to answer.

That last part is important. Not only does the idea have to be good, but we’ve got to be able to answer the question that we’re posing completely. It turns out that a lot of interesting questions just get tossed out because we realize the study is not entirely feasible. Like, for example, one of my favorites was to study whether patients think that doctors with British accents are smarter — I certainly do — but I wasn’t about to launch a randomized trial to study that!

Back to this idea that I had about kids and flu vaccines. Chris also has a kid with a summer birthday and had noticed the same issues. And we came up with this basic question:

WORSHAM: We wanted to see if kids like ours, who were born in the summer, have trouble getting a flu vaccine at their annual exam. We know that it’s going to be a pain in the butt to come back. And when things are a pain in the butt to do, they’re less likely to happen. And so that was sort of the foundational behavioral economic principle behind this, is that there’s a barrier to getting this shot because you have to come in for an additional visit. And our goal was to find out how much of a barrier is that? Can we measure it?

Really quick, let’s do a little flu 101. Every year, millions of people get the flu, hundreds of thousands of people are hospitalized with it, and tens of thousands of people actually die from it. The flu vaccine is revised on a seasonal basis, which is an attempt to react to the flu strains that we predict will be circulating in that year. And the vaccine works like other vaccines. It encourages your body to make antibodies before you get infected with the virus. And remember, the flu shot is safe to get starting with babies as young as 6 months old. I’ll let Chris explain how we did the research.

WORSHAM: We used insurance claims from a large insurance database. And we have information that links a child to parents and other family members, because they’re all on the same insurance plan. And when they go in for their annual physical or they go in for any visit, we can see what happened at that visit. Was a procedure performed, like a flu shot? Or did the doctor diagnose them with the flu at that visit?

It was literally that simple. We had data from two consecutive flu seasons. And we looked at the kids who were getting their annual doctor’s visits around the time of their birthdays. We looked at whether they got the vaccine. And we also looked at whether they were treated for the flu and whether or not other people in their families were also treated for the flu. And lo and behold: our hypothesis was right. Chris and I both got that tingly feeling like we were onto something real. Summer birthdays meant fewer flu shots. And did that result in more cases of the flu?

Yeah, it did.

WORSHAM: So when we started to look at the flu diagnoses and we saw the pattern that we would have expected, that because the kids in the fall were more likely to get their flu shot, they were less likely to get the flu than the kids born in the summer, that’s when we knew that all of our data made sense.

Okay, you might be saying to yourself — big deal! Most kids don’t get that sick from the flu. And, you’re right. That is true, thank goodness. But if you’re asking the question: “who cares?” Well, let me introduce you to this guy.

Amol NAVATHE: My name is Amol Navathe. I’m a physician and health and behavioral economist at the University of Pennsylvania.

In short, Amol studies how physicians and patients make decisions.

NAVATHE: Oftentimes in my field, we’re always asking the so-what question. So what about this result?

Okay, so what about these results? Why should you care if summer kids don’t get their flu shots as often?

NAVATHE: Well, plainly it’s a public health issue. Right? So, one thing that is very different about flu about other diseases that are transmittable from one person to another is how your neighbor behaves, the decisions that they make end up impacting you. It’s a strong social responsibility of every individual in every family to go get their flu vaccine.

So many other things we, as physicians, try to do to get people to improve health really only impacts them — like telling people to exercise more, eat better, take their meds. But this, and all vaccines, this impacts everyone.

Plus the study found pretty significant numbers. We found that 40 percent of kids with summer birthdays got flu shots. And 53 percent of kids with fall and winter birthdays got flu shots. That’s a 13-percentage-point difference.

 NAVATHE: If you think about it from a relative magnitude, it’s actually bigger.

What Amol is saying is that a full third more children with fall and winter birthdays got the vaccine. That’s a lot.

NAVATHE: But the biggest deal here is that there’s evidence of these sort of ripple effects.

Ripple effects being whether kids with summer birthdays, who were less likely to get the flu shot, were more likely to get the flu themselves and pass it on to family members. We found just that. Kids with July birthdays were about 15 percent more likely, in relative terms, to get the flu than kids born in September, and so were their family members. These weren’t trivial infections. We’re not talking about people with sniffles at home for a few days. We’re not talking about self-reported infections with the flu. We’re talking about people who are sick enough that they actually went to the doctor’s office to get care. And that, by the way, is how they ended up in our data. Here’s Amol Navathe again.

NAVATHE: Probably some small proportion of them got really sick and maybe even died from it. That tells me that, man, we got to care about this. If it weren’t real, then we wouldn’t really see those ripple effects.

And on the topic of ripples, our data only allowed us to show transmission of the flu to family members. But we all know that kids with summer birthdays were likely to have interacted with a lot more people outside their families too. So our data only measures part of the flu spread caused by these kids. Coming up, we turn to history to understand why it’s so hard to get a flu shot for a toddler, and one solution that could make it easier.

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Okay, so we were able to see this link between kids with summer birthdays and flu vaccination rates. Now what? Can we do anything about it? The answer, I think, can be found if we combine what we know about how people behave and how the medical system works. Amol Navathe again.

NAVATHE: In behavioral science, we have a simple saying which is, if you want to get somebody to do something, make it easy. Make the best path the path of least resistance. And so, to the extent that structure of our health system or structural barriers play an impeding role, removing those barriers and improving the structure of how we deliver healthcare is going to be effective because it makes it easier.

But beyond our natural desire to stop a virus from spreading, this study shows us that our approach to delivering medicine — something that we often take for granted — could stand to be re-examined. Remember at the beginning of the episode, when I was talking about trying to find my son a flu shot? No place outside of his pediatrician’s office would give him one. It literally took me hours to sort this out. And it was weeks before he got the flu shot.

But let’s contrast that to Covid think about all the places you can go to get the Covid vaccine. Doctor’s offices, sure. And I know it was rocky at first, but there have been these massive efforts to make it as convenient as possible for people to get their Covid shots. You can get them at all kinds of stores. We’ve transformed sports arenas into vaccination sites, and there’s even drive-thru options. Now, I know Covid is a crisis, and there’s no holding back right now. But even for the flu, we actually do a lot of the same stuff to make vaccines more convenient for adults like make them available at the grocery store or pop-up clinics at offices and factories.

So why is it so hard then to get the flu shot for kids? Let’s get a brief history lesson on this.

Back in the day — and I don’t mean just before Covid, although that does feel like a long time ago, right? No, I mean way back. Back then, most vaccines were part of public health initiatives. And we just needed to get shots in arms.

           Alyna CHIEN: In those days there were more of these mass vaccination programs.

That’s Alyna Chien.

CHIEN: I’m a pediatrician health services researcher based at Boston Children’s Hospital and Harvard Medical School.

When she says, “Those days,” she’s referring to the days of Smallpox, like in the late 18th Century and more recently (but not that recently), polio in the 1950s.

News READER: The three-year statistics run 50,000 polio cases, 103,000 cases, 122,000 cases. Where will it end?

Vaccines that were especially important for young children.

CHIEN: But as we started to have more vaccines, we started to have to decide: do we reach these babies through public health programs, or is it better for them to also be in physician offices? Because then they can tend to the medical problem that might be at hand in addition to providing a vaccination. So there was a choice back in the day. 

And you can see why the choice to focus on giving vaccines at the doctor’s office, it seemed like a good idea at the time. Like Alyna says, getting people in for checkups could help catch other issues early. But it also meant that the system for all medical care in the U.S. — vaccines included — it increasingly tended to be provided only in doctor’s offices. We showed Alyna our study about how this was affecting kids getting flu vaccines. This was her reaction.

CHIEN: It’s provocative of this tension between wanting something that is mass, fast, and perhaps more complete, versus something that is a little bit more meditative, tailored, and one-by-one.

Alyna Chien is saying that our individualized approach to kids’ medical care doesn’t leave a lot of room for public health and that’s a problem. Amol Navathe agrees. And he says there are ways we could make a huge difference.

NAVATHE: This isn’t just an observation that we should hang our heads and feel sad about and walk away and say, “Man, I feel defeated.” We can start to think about how do we design programs that address each of these pieces? So, for example, you know, what if we offered and made it the default in preschools or schools to offer a flu vaccine from September to October. Now, if you, as a parent, really don’t want your kid [to] have a flu vaccine, for whatever reason, you can opt out. So we’re not impinging on people’s choice or religious freedom, anything like that. But we’re creating a default. We’re saying this path, the path of least resistance, is to get the flu vaccine. And if you don’t want to get it, you can certainly opt out. These types of defaults are extremely powerful. They’re extremely powerful.

So Amol isn’t saying we should require the flu vaccine for anyone. But man, we could make it a lot easier to get. But I’d actually go one step further. In Massachusetts, during the pandemic, they changed the rules and said kids needed flu vaccines just like they needed other childhood vaccines if they wanted to go to school. We should think about that. And we need to change how flu vaccines are delivered to our youngest kids.

It’s fine to still get them in the doctor’s offices, but we also need delivery systems with far fewer barriers, just like we’ve seen with the Covid vaccine. That means a re-emphasis on a public health approach to vaccinating kids against the flu. Fewer people would need to take time off from work, or spend hours on the phone, like I did. And, and I’m fortunate to have that time, the resources and, you know, the type of job that allows me to spend time thinking about these kinds of things. But many parents don’t, and they can’t afford to take time off from work for even one doctor’s visit for their kids, let alone multiple ones.

Of course, most importantly, these inconveniences mean barriers to getting kids vaccinated. If we could make that easier, thousands more children would not get and spread the flu. 

That’s it for this week’s episode of Freakonomics, M.D. Thanks for listening, and I hope you subscribe to or follow the show.

Next week, what do grocery store prices and heart surgery have in common?

If you have thoughts on the show, ideas, anything at all, I’d love to hear from you. You can email me at

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Freakonomics, M.D. is part of the Freakonomics Radio Network, which also includes Freakonomics Radio, No Stupid Questions, and People I (Mostly) Admire. This show is produced by Stitcher and Renbud Radio. You can find us on Twitter and Instagram at @drbapupod. This episode was produced by Katie Simon and mixed by Adam Yoffe. Original music composed by Andrew Edwards. Our staff also includes Stephen Dubner, Alison Craiglow, Greg Rippin, Joel Meyer, Tricia Bobeda, Emma Tyrrell, Lyric Bowditch, Jasmin Klinger and Jacob Clemente. If you like this show or any other show in the Freakonomics Radio Network, please recommend it to your family and friends. That’s the best way to support the podcasts you love. As always, thanks for listening.

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  • Chris Worsham, physician and researcher at Harvard University.
  • Amol Navathe, physician and health/behavioral economist at the University of Pennsylvania.
  • Alyna Chien, pediatrician health services researcher at Boston Children’s Hospital and Harvard Medical School.


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