Future Tense

Which Vaccine?

Making sense of the differences between COVID-19 vaccines, for public and personal health.

People's feet beside a yellow V painted on the floor, with a syringe in each arm of the V.
The opening day of Berlin’s sixth vaccine center, located in Hangar 4 of former Tempelhof Airport Michele Tantussi/Getty Images

A version of this article first appeared in Emily Oster’s newsletter, ParentData.

Throughout the pandemic, I’ve had a recurring complaint about the public health messaging. Put very broadly, I think it is too eager to give definite answers when the evidence is still uncertain, and too willing to issue sometimes patronizing platitudes.

This has come up again in the context of comparing across vaccines. The U.S. has now approved three vaccines for COVID-19. The Pfizer and Moderna vaccines are very similar and have been approved since December. The Johnson & Johnson vaccine was only recently approved and is beginning to be rolled out.

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The efficacy numbers in the trials of these vaccines differ: J&J’s is lower. I have more to say on what these numbers mean below, but if you look at the top-line study results, this is what you see. At the same time, we are hearing “All the vaccines are awesome!” and “The best vaccines for you is the one you are offered.” This last line worries me especially since I think it runs the risk of suggesting that some people deserve to get a less-good vaccine.

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So I’d like to dig into this a bit more deeply so we can understand better what we do know, what we don’t yet know, and how to think about the differences across these options.

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Before getting into this, I want to emphasize: These vaccines will not eliminate COVID-19, at least not for decades. They will also not prevent all cases of symptomatic or asymptomatic illness. They will prevent a lot! But not all. What is most impressive and most important about all of the vaccine options is that they do a great job preventing very serious illness. If we can get to a point where COVID-19 is relatively rare and where people who do get it experience mild symptoms, that will be winning. This is where we are aiming. Not zero COVID-19.

Now, let’s start with the question of what the difference is between the vaccines. Both the Pfizer/Moderna vaccines and the J&J work through the mechanism of delivering instructions to your cells to manufacture the COVID-19 spike protein. Your immune system produces antibodies to recognize this spike protein and kill it. If the COVID-19 virus is then introduced to your body, you’re ready for it.

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The difference lies in how instructions are delivered. The Pfizer/Moderna vaccines deliver instructions through mRNA, and J&J delivers them through DNA embedded in another type of virus (an inactivated adenovirus that cannot make you sick). An advantage of the DNA delivery mechanism is that DNA is more stable than RNA, which is why the J&J vaccine can be stored at more normal temperatures.

The difference in delivery mechanism opens up the possibility for differences in efficacy. These vaccines work better if they generate a stronger immune response, which will happen if the delivery mechanism is better able to get the virus into your cells. If you’ve already been exposed to the adenovirus used in the J&J vaccine it may work less well (because you’re immune to the delivery vector). On the other hand, the mRNA delivery mode in the Pfizer/Moderna vaccines is new, and it’s possible it works better for some people than others.

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When we talk about efficacy of these vaccines, the first thing we want to run down is—based on what we see in trials—how good a job do the vaccines do at preventing various stages of illness.

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I’ve put a simple table below, comparing roughly where these efficacy numbers are based on trial data and other things we know. (I pulled these from widely published trial numbers and, in the case of asymptomatic estimates, from early data such as that found here.) These are estimates, especially in the case of asymptomatic infections.

A chart touting the efficacy of the Pfizer and Moderna vaccine in comparison with the Johnson & Johnson COVID-19 vaccine.
Emily Oster
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The vaccines are all extremely effective at preventing death. The J&J is slightly less effective in the trial data against serious illness and against symptomatic and asymptomatic illness.

It is important to be clear on what these numbers mean. A 95 percent efficacy does not mean that 5 percent of people in the trial with the vaccine got symptomatic COVID-19! What it means is there was a 95 percent reduction in the risk from the baseline. If your baseline risk of getting serious COVID-19 without the vaccine is, say, 1 in 100, then your risk with the Pfizer or Moderna vaccine is 1 in 2,000. With the J&J it would be 1 in 666.

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These numbers are all extremely good. Flu vaccines, by contrast, often have efficacy numbers against symptomatic illness that are under 50 percent. It’s still very useful to have them, but they pale in comparison to any of these options.

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Based on the simple table above, it does look like the J&J performs slightly less well. However, we cannot directly conclude this because these were not tested in head-to-head trials. The conditions under which they were evaluated differed in a number of ways, the most important of which is that the J&J was tested in a period with more variant activity. All of the vaccines are likely to be at least slightly less effective against variants, so this could drive some of the differences. (Put another way: The Pfizer/Moderna numbers could be a bit lower in the variant space, or the J&J numbers could be higher without the variants.)

In addition, although these trials are enormous, there is still statistical noise—error—in their estimates. The data in all trials are consistent with a range of values, so the numbers above do not tell the whole story. To really figure out which is “better” (if one is), we’d need to test them head-to-head in a trial. We might eventually get that, but there isn’t much point at the moment given that all are performing well.

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Bottom line: Based on my read of the data and many conversations with people who know a lot of stuff I don’t, I think the J&J is likely to be slightly less effective against symptomatic illness than the others, although I think the differences are smaller than the numbers above suggest.

So, what does that mean for both public health and personal choices?

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From a public health standpoint, the goal is and should be to vaccinate as many people as possible with any vaccine that we have as fast as possible. Every day we do 2 million or 3 million shots of anything is a good day. If everyone magically got the J&J vaccine today, we would be in a totally different world. Given that demand still outstrips supply, the public health value of having more vaccines available cannot be overstated.

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The J&J is also incredibly useful for public health because of its one-shot, easy-storage nature. Many states are talking about using it to vaccinate people who come to the ER or taking mobile vaccine clinics to homeless encampments or other difficult to reach populations. These groups are harder to serve with vaccines that require two doses.

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From a public health standpoint, this approach is smart, but I will admit to some concern that this will exacerbate the “this is the less good vaccine” view. However, the idea of using the J&J vaccine in these populations has nothing to do with vaccine quality and everything to do with the ease of use. If I were in charge (which I am decidedly not), I would consider also using the J&J on other “transient” populations with less of a stigma. My choice might be on-campus college students: mass one-shot vaccination before they leave for the summer.

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A lot of people are thinking about this more as a personal choice, though. With the frame of something like: Should I get the J&J if offered, or should I wait for one of the others?

It might be easier to tell you how I’m thinking about it.

My No. 1 point is that I would get any vaccine offered. I’m teaching in person and we’ve had outbreaks, and even though I understand my personal risks are small, I’d feel better if I were vaccinated. If someone told me I could get the J&J today, I would stop writing and cooking stuffed shells right now and go get it. Honestly, I’d probably get the Russian Sputnik V vaccine if it was offered. Novavax? Sure. I’m not throwing away my shot, is my point.

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And importantly, I would consider myself vaccinated regardless of which vaccine I got. Which doesn’t mean hot basement singing parties or taking off my mask in the grocery store, but does mean I’d feel more comfortable seeing family and friends in my house, without masks. I wouldn’t make these changes differently if I had the J&J rather than the Pfizer/Moderna.

Second point: If a menu of options was in front of me and I got to choose, I’d pick Pfizer or Moderna, since I think their efficacy is slightly higher.

Underlying my feelings here is the realization that this is not my last COVID-19 vaccine; it’s not any of ours. We are going to be having boosters for a while, as the virus evolves and vaccines evolve to address variants. If you get the J&J now, you’ll probably end up with another shot in a year, maybe from them, maybe from Moderna or Pfizer, maybe from some other source. This may be an annoying revelation, but I think it also dials down a lot of the importance here. You aren’t committing to some vaccine for life.

Future Tense is a partnership of Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.

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