It actually makes things worse.

I'm going to start this week with a thought experiment.

I'm going to give you $1,000,000. Here's the catch. You can't keep any of it. You need to use it to get as many people vaccinated against COVID-19 as possible. How would you do it?

The sky's the limit here. You could pay Jalen Hurts a million dollars to do an Instagram story about getting vaccinated. Or you could buy billboards all around your local area, encouraging people to get the shot.

Or you could send direct mail to people's houses. Or robocalls! It's up to you.

Some of you, brainstorming about this, might decide just to cut out the middleman. If I have a million dollars to get people vaccinated, maybe I'll just pay people to get vaccinated. I could offer 100,000 people 10 bucks each or 10,000 people 100 bucks each.

This is the central insight behind financial incentives in public health. We spend a lot of money creating public service announcements to get people to eat healthy and exercise. We create websites full of accurate, reliable information about the benefits of quitting smoking or reducing alcohol intake. But instead of paying for an ad to convince people to stop smoking or something, what if we just pay the person to stop smoking?

Direct financial incentives are a fascinating intervention for a variety of behaviors we want to encourage that require a little extra motivation. And they've been tried, with some significant successes, across a variety of behaviors - from quitting smoking to taking HIV medications.

But would it actually work for vaccination? Turns out - probably not.

Here's the situation. It's November of 2021 - about a year after the COVID vaccines came out - and you are German. More specifically, you live in Ravensberg, a charming city in the south of the Country.

Ravensberg which means "Ravensberg". Source: Wikimedia.org

One day, a flyer comes to your door signed by the Mayor himself. It gives you information about 7 upcoming free covid vaccination events. Are you going to go?

OK - alternative version. The same flyer, but this time it comes with a little barcode. Go to a vaccination event and bring the barcode, and, after your shot, you'll get 40 euros. Are you going to go now?

Final version. The same flyer, but this time one person in your house gets the barcode but you… don't. You just get the information about the vaccination event - no promise of 40 euros. But you know that some people are getting 40 euros. Are you going to go now?

What this thought experiment illustrates is the concept of "spillover" in public health interventions. It is the effect of an intervention directed at one group of people on the people the intervention was not specifically directed towards. The problem is, a lot of studies only look at the targeted population - not the entire population - and failing to take the big picture here could lead to some incorrect results.

This phenomenon is ably described and evaluated in this study, appearing in JAMA Network Open, from John Ternovski and colleagues. I blatantly stole the thought experiment I just gave you from their study design. But they actually did it, so I can show you the results.

Source: Ternovski et al. JAMA Network Open. 2025

First a bit more detail on how the study worked. The entire city of Ravensberg was randomized at the household level - so you have intervention houses and control houses.

One adult in each house was picked as the "representative" adult in the house. In control houses, each adult was sent the flyer just providing information about the upcoming vaccination events. In intervention houses, the representative adult got the flyer with the bonus payment, but every other adult got just the informational flyer.

This set-up allows three really interesting comparisons.

We can use the "representative" adults from each house to see how effective the bonus payment would be on increasing vaccine uptake among the targeted group. This is the so-called "direct" effect. I'll just point out that most research studies would stop here. They would just say - ok people who got the flyer with the promise of money were more likely to be vaccinated without considering the other people around them.

Or we can use the non-representative adults from the two types of households - all of whom got the normal flyer without a bonus - to assess spillover effects. Would simply living with someone who got the bonus offer change your likelihood of getting vaccinated one way or another?

And finally, we can use everyone in the two types of houses to assess the overall, population-wide effect of the intervention. From the public health point of view, this is all that matters. Why increase the rate of vaccination in one group if it will decrease it in another group?

With that said, let's look at the results. We'll start simply, by comparing just the representative adults from the various households. Did a promise of money increase the vaccination rate among those who were promised money?

Surprisingly - no. Numerically, in fact, the offer of money reduced the rate at which people got vaccinated - by just over 0.2 percentage points. That's not enough to declare this statistically significant, but it is certainly enough to rule out any strong effect of financial incentives in this space.

Now, some of these folks were getting their first dose of vaccine, and some were getting boosters. Interestingly, you see that the negative effect of offering money was a bit stronger for booster vaccines.

What does this tell us about the psychology of vaccines and financial incentives? I think one interpretation is that people may view vaccination as an act of public good - you are not just getting vaccinated for yourself, but to help others as well. Perhaps offering money cheapens that act? That's why the negative effect of the offer was worse among those who were on the booster shot - they already got vaccinated once without being paid, why would you want to pay them now. Or, more cynically, perhaps the act of offering money suggests to people that the vaccine is dangerous or ineffective. It's sort of a "what aren't they telling us?" vibe.

What about the other household members? Here, we see evidence of spillover, particularly when it comes to booster shots. In the case of boosters, seeing someone in your house get offered 40 euros, when you don't, reduces your likelihood of getting the booster by 0.3% compared to people in houses where no one was offered money.

And the overall effect just ends up being a weighted average of the direct and spillover effects - a net negative, at least for booster shots.

So what is going on here? People like money, don't they? Why would offering money reduce the likelihood of vaccination for both those offered the money and those who are merely living in the same house as people who were offered money.

I already suggested that many people might be getting vaccinated for altruistic reasons - and the offer of money may simply make that seem less attractive.

More prosaically, the authors note that the 7 vaccination events they advertised were very crowded, with long lines and wait times. People who knew that folks were getting paid to get vaccinated might have simply decided to sit these events out in favor of getting vaccinated elsewhere. There is some empiric evidence for this. The authors show us that there was an uptick in booster vaccinations in additional events outside of the 7 advertised in houses that got the financial incentive.

Overall, though, it's hard to view this as a novel new intervention to get more people vaccinated. If anything, it teaches us about the complex interplay of psychology and biology and economics that can affect health decisions.

And of course, there are a million other variables we could play with that might lead to different results. What if the offer was 100 euros instead of 40? What if it scaled up the more members of your household you brought with you? Or - for the cynics - what if you could get paid to NOT get vaccinated? Would that paradoxically increase the vaccination rate because people suspect the vaccines must be pretty special if the powers-that-be are trying to get us not to take them?

This is what makes this area of research so complicated and so fascinating. I'm not entirely ready to hang this up - I think there is still room for direct financial incentives. For some stuff. For something as important, and charged, as vaccination though, the off-target effects may not be worth the cost.

A version of this commentary first appeared on Medscape.com.

Methods Man Blog

Why Paying People to Get Vaccinated Doesn't Work

It actually makes things worse.

The Methods Man