Covid Boosters: Will We or Won’t We?

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It has now been 6 months since the first wave of Americans received their first shot of a COVID-19 vaccine. Proving just how effective the vaccines are in the U.S., we’ve seen the virus spread down to levels not seen since the start of the pandemic. But one thing we still don’t know: how long immunity will last.

As Pfizer and Moderna move forward for full FDA licensure, executives at both companies have cited the need for a likely booster. Pfizer CEO Albert Bourla, DVM, PhD, predicted the need for a booster to be “somewhere between 6 and 12 months” after first being vaccinated. Explaining the key role variants will play in the decision, Moderna CEO Stéphane Bancel predicted it could be as soon as September for those at highest risk of severe infection.

NIAID director Anthony Fauci, MD, told a Senate subcommittee that he would expect the need for a booster, but declined to put a timeline on his prediction. “I don’t anticipate that the durability of the vaccine protection is going to be infinite,” Fauci said. Researchers within his institute at the NIH recently began looking into, not just if boosters are necessary but also how to possibly mix vaccines and the impacts on protection.

On this week’s episode, William Schaffner, MD, professor of Preventive Medicine and Infectious Diseases at Vanderbilt University Medical Center in Nashville, joins us to help break down what the science tells us now, and what to expect.

The following is a transcript of his interview with “Track the Vax” host Serena Marshall:

Serena Marshall: It still seems like it’s kind of unclear on how long immunity lasts and if we’ll need boosters?

William Schaffner: Serena, that’s exactly correct. We don’t know yet. You know, we’re writing this textbook as we’re reading it, if you will. So we’re just following people who’ve been vaccinated. We know that protection lasts at least 6 months and the clock is still ticking. The sorts of things that will be happening is: we’ll be watching to see several things.

One is antibody levels. So there are populations that have been vaccinated whose antibody levels will be measured over time. But that’s inferential. The other thing we’ll be doing is looking for breakthrough infections. How often do people who are vaccinated then subsequently become infected? If that starts to tick up then we know that our immunity, our protection, starts to wane.

Serena Marshall: But we’re starting to see some of that data, Right? We just have heard recently from the CDC that breakthrough infections are only 0.01%. So shouldn’t that give us some sense of maybe where we’re headed?

William Schaffner: Oh yeah! It’s very, very encouraging, indeed. There have been some recent studies that have come out of the laboratories where immunologists have looked, actually, at immune cells and they seem to have a very robust memory for exposure to COVID. Which also bodes well down the road. But we’ll have to watch what happens in the real world, in people, in order to make a final decision about that.

Serena Marshall: Six months in, Dr. Schaffner, what does the data in the real world, in real people, look like so far?

William Schaffner: Six months in, protection is very, very solid. And obviously, breakthrough cases are really uncommon. And even in that context, we have to recognize that at their best, these vaccines were thought to be 95% effective. I didn’t say 100%. So we knew that there would be some people who, after vaccination, would get exposed, who would develop an infection.

But so far, these breakthrough infections have been genuinely very, very rare and are occurring more often in people who are immunocompromised or very frail older persons. And, obviously they couldn’t respond as well to the vaccine when it was given.

Serena Marshall: So why are we talking about boosters really at all? What are the actual chances we’ll need one if it’s going so well so far?

William Schaffner: I think it’s, first of all, the normal human condition to wonder what’s happening. People are mentally playing chess, right, and wondering, is this going to resemble the influenza vaccine, which we have to give each year? But there is another consideration. And that’s those rascally variant strains.

So far these variants, the ones that are circulating in the United States, are well covered by our vaccines. The vaccines prevent them, but there are some strains out there, particularly the South African strain, where the vaccines we’re using offer partial protection — at least that’s what we think. Fortunately, the South African strain is, at the moment, pretty unusual in the United States. It’s not spreading very readily.

So we’re looking at strains, doing genomic sequencing of strains that are occurring, to see which variants are out there and whether they have implications for requiring a future vaccine that might have to be tailored the way our flu vaccines are to what’s going around.

Serena Marshall: So, Dr. Schaffner, is the whole booster conversation really stemming from the variants? When we hear Dr. Fauci, for example, say that he does think we’ll need a booster shot, but declined to say when, or when we hear the pharma companies talking about how they’re working on them already and possibly as soon as September, is it totally dependent on the variant or immunity waning over time, because especially with the mRNA vaccines, they’re new technology?

William Schaffner: Well, I’m chuckling because I think the answer to your question is yes. It’s both those issues. How long is the protection going to be robust, so duration of protection, as well as watching the variants. Those are the two issues that will impact whether and when we’ll need a booster down the road.

Serena Marshall: So when you mentioned the flu vaccine, just a moment ago, we know that that’s decided by for the most part, the World Health Organization, right? They look at the different hemispheres and kind of make an assumption about which strain will be happening. How will that work with COVID?

William Schaffner: Well, as you say, the mechanism for making decisions about new flu vaccines is well established. Goes back decades. The World Health Organization and our Food and Drug Administration participates in these decisions. They make decisions twice a year: once for the Northern hemisphere, once for the Southern hemisphere. Now, who will make the decision about a booster dose for COVID?

Obviously, our Food and Drug Administration will be involved. And since we will be relying on data from the field, so will the CDC. The manufacturers will be part of that discussion, of course. Exactly how that decision will be made, I’m not entirely clear about. I don’t know what the structure will be for that. When the time comes, one probably will have to be created.

Serena Marshall: A system will have to be created for a booster globally or within the United States itself?

William Schaffner: The need for boosters’ duration of protection is a global phenomenon, right? Human beings are pretty much the same around the world, their immune systems. On the other hand, the variants could be quite regional. And so exactly how much of an international participation there will be in making these decisions, that’s yet another question. I know the companies will be very interested in that because their vaccines are of course distributed beyond our four borders.

Serena Marshall: But Dr. Schaffner, are you saying, am I understanding this correctly, that we could see the need for a U.S.-based booster if you’re here and then if you’re traveling to India or South Africa or South America, you might need a different booster?

William Schaffner: Many things are possible. And I would see that at different times of this pandemic being possible. We don’t really have any precedent for that, except that international travelers going to certain parts of the world need yellow fever vaccine because yellow fever is only endemic in certain countries. So it might be a system kind of like that.

Serena Marshall: When you have a booster system that’s based possibly on where you travel, is it also possible to have that booster system be based on the type that you’ve received? Meaning an mRNA versus an adenovirus vector vaccine?

William Schaffner: Well, everything is possible, right? But at the moment, these various vaccines are behaving in a reasonably similar fashion. There may be nuances among them. Some already are seemingly more effective than others. The Sinovac vaccine that’s manufactured in China, its effectiveness is a little bit lower than the mRNA vaccines, for example.

So some of that may play into whether boosters are necessary. And I mentioned Sinovac because this is an issue, or at least the variation of that issue, which is one that we’re going to have to address. Let me set this up for you.

Serena Marshall: Okay.

William Schaffner: Universities are opening up this fall, and most of them will be in-person. So many of their students who have gone home will be returning and many of those students are international students.

And so we’ve already, at Vanderbilt, received inquiries from students in China who have received the Sinovac vaccine and they want to know: Is that going to “count” as immunized or will they be obliged when they come to the United States to receive one of the vaccines that’s licensed here in the United States?

When I called my friends at the CDC as to whether they had encountered this issue, they laughed. And they said hourly they’re getting that kind of call. And they will be providing recommendations for us down the road. Yes, let’s leave it at that at the moment.

Serena Marshall: At the moment there’s no recommendation. So what would you say in this scenario you studied?

William Schaffner: Well, with the Sinovac vaccine, we know that it doesn’t work quite as optimally as the mRNA vaccines. And I would say in the absence of data and being rather comfortable about the safety issues from the point of view of effectiveness, I would say come back and get revaccinated with one of the licensed vaccines we have available here in the United States.

Serena Marshall: Could that be a situation that plays out even for international travelers? Not people coming here to live long-term, but vacationers or businessmen and women?

William Schaffner: Serena, in the beginning of this conversation, you asked me: gee, why are people worried about boosters? And here we are playing chess.

Serena Marshall: Yes.

William Schaffner: Thinking several moves down the road when these issues haven’t even come up yet in the real world. But the short answer to your question is: it’s possible. That people moving, as we said before, from one part of the world to another, particularly if in the other part of the world a different variant is the dominant strain. We might have to get vaccinated to protect ourselves against that variant if we chose to travel to that country.

Serena Marshall: Well, let’s take it back to this part of the world for just a second. What about if, you know, I got a J&J vaccine and my husband got an mRNA vaccine — will we want to be boostered with a different kind? Would that make a difference in our immunological response?

William Schaffner: We’re in a no data zone, a data-free zone. And the CDC’s Advisory Committee on Immunization Practices does not like to make recommendations just on the basis of “expert opinion.” They’re really adverse to that, particularly on the safety side. So we would anticipate that there would be at least small datasets that would offer some guidance for that.

We’ve already said that if you begin your vaccine series with one mRNA vaccine and you’re in a location and you want to get that second dose, but you can’t remember, or the same vaccines are not available, you can complete your two-dose series with the other mRNA vaccine.

We really haven’t said anything officially about mixing and matching the J&J vaccine with the mRNA vaccines. And so all of those kinds of questions would have to be addressed immediately because, you know, 10 minutes after a booster recommendation is made, those are the sorts of questions that will come in, in the email.

Serena Marshall: So what about mixing? I know it’s not available here, but since we were talking globally, an AstraZeneca adenovirus vector with a J&J? Or using one or the other as your booster?

William Schaffner: Well, that’s the question that comes in with all these international students. Many of them have indeed received AstraZeneca. Some have received Sputnik 5, the Russian vaccine. A bunch have received Sinovac, which is the vaccine made in China. None of those are licensed here.

Were they to come back, the question is: could they receive the mRNA or the J&J vaccine? And as I said, those recommendations are anticipated and I’ve told my friends at the CDC: the sooner they come, the more comfortable we’ll all be. Because we’re being asked these questions on behalf of returning students, literally every day.

Serena Marshall: Yeah, I can imagine. So what do we know about boosters when it comes to other vaccines? Like for Tdap, we kind of talked about the flu already. That essentially that’s what you’re getting every year. But the duration of how long you’ll need it — in a year or 2 years, that’ll come with the data. But what can we say based on existing boosters and especially when it comes to existing boosters with the current established vaccines, like for example, Ebola with the adenovirus vector?

William Schaffner: Well, we know very little about Ebola boosters. They’ve been used in outbreak situations when the whole focus is in getting vaccines into the defined population at risk. Now Tdap — tetanus diphtheria acellular pertussis vaccine — obviously is one where the current recommendations are every 10 years.

And that’s driven by a combination of looking at the several components and we probably don’t need a 10-year booster for tetanus. And that’s probably a little long for pertussis. But you know, you just kind of …

Serena Marshall: Kind of meet in the middle.

William Schaffner: Yeah. Kind of like that. Yeah.

Serena Marshall: What about with COVID? I mean, with pertussis, you know, pregnant women get a Tdap whenever with every child. Will you have something like that with COVID?

William Schaffner: Let me see, I put my crystal ball here …

Serena Marshall: Yes. I’m asking you to predict the future, come on, doctor.

William Schaffner: Still a little cloudy. My prediction is that it won’t be that frequent. I think we are going to see a more robust, lengthy protection from these vaccines than, for example, with influenza vaccine. So I’m certainly loath to predict what the interval is, but it might not be necessary every year.

Serena Marshall: Or whenever you get pregnant?

William Schaffner: Or whenever you get pregnant, that’s correct.

Serena Marshall: Okay. Dr. Schaffner, let’s talk about natural immunity and boosters. If you’ve had COVID and didn’t get a shot, we don’t need to talk about boosters. But if you’ve had COVID and then gotten the shot, would you still need a booster, or is that shot essentially like your booster?

William Schaffner: I think if you’ve had COVID and then had the vaccine, you’re like people who’ve had two doses of vaccine. The reason we vaccinate people who’ve had COVID is because the vaccine actually produces higher levels of antibody then does the natural infection.

That’s kind of amazing. And the higher the level of antibody, usually the longer the duration of protection.

Serena Marshall: But we’ve heard from folks that say, “oh, well, when I, I had COVID and then I got my first dose of one of the two-dose shots, and I had a terrible reaction.” Which is similar to those who haven’t had COVID and got the second shot. So wouldn’t their — those who’ve had COVID — second-shot kind of be like someone who hasn’t had COVID third shot, their third exposure?

William Schaffner: Well, a lot of people have asked about that, and it’s one of those things that still has not been studied systematically enough for the CDC to change its recommendations.

Serena Marshall: Okay. Well, what is the magic number? What would be the antibody number that if your immunity level fell below, we would say: yep, that’s the time that we need to talk more about boosters?

William Schaffner: So you have mentioned an antibody level, a number. That’s something we call a correlate of protection. And for some vaccines, such as the measles vaccine, we have an excellent correlate of protection. For others, we don’t, and we don’t yet know what the correlate of protection is for COVID. So I can’t tell you what level of antibody is sufficient to give you protection or for how long.

Serena Marshall: Well, when we first started talking about vaccines, when they first started to roll out, before we had any of the data, the number we heard from the CDC and NIAID was at least 50% efficacy. And we are so much higher than that. So why wouldn’t they have a number somewhere if, you know, the efficacy falls that we would say okay, yeah, now we need boosters?

William Schaffner: And that’s because we haven’t had enough people who’ve gotten the vaccine and failed. And then we could measure their antibody levels and see below which, actually, the antibody was not sufficient to prevent infection.

We don’t know this for every vaccine. For example, we don’t know what it is for flu, and that’s because the flu virus changes so frequently that we can’t do this for each and every strain because we know there’s strain-to-strain variation.

Serena Marshall: So that third shot, that possible booster, what would be the same or different? Would it be the exact same one, the same dosage that you received when you first got vaccinated?

William Schaffner: I’m just sitting here smiling, because …

Serena Marshall: I keep asking you questions we don’t have answers to yet?

William Schaffner: That’s exactly right. We don’t know that. And remember, if there are variant viruses out there, they may require, actually, more protection than some other variants. And so that will all have to be worked out very pragmatically at the time.

Serena Marshall: So what’s in the vial could be variant specific?

William Schaffner: That’s absolutely correct. Or it could be a combination. A booster of the current virus, the dominant strain, plus some material that would give us a protection against the variant.

Serena Marshall: If what’s in the vial, though, is different, wouldn’t it have to go back through FDA approval and that whole emergency authorization and/or full approval?

William Schaffner: Oh sure. There’s no doubt that it would have to go through FDA approval and the more different it is, the more you would want it to go through FDA approval, looking at both safety and effectiveness. Now influenza is so standard and is so similar from year to year, that process is very much facilitated.

Serena Marshall: At what point, though, does it become not necessarily a booster, but a brand new vaccine?

William Schaffner: That depends on how different the variant is from the original strain.

Serena Marshall: I see. Dr. Schaffner, it sounds like there’s just so much that we still don’t know. It makes sense why there’s so much conflicting information out there …

William Schaffner: True. But the amount of information we have learned about COVID since the beginning is extraordinary. As I like to kind of joke or illustrate, you know when COVID hit, it was the first time this virus was in the human population. We all opened up our textbooks to COVID, figuratively speaking, and there were just blank pages.

Well, we’ve been filling in those blank pages with research: laboratory research, diagnostic research, therapeutic research, public health research. And along with the vaccines, we’re learning about the vaccines and what they can and cannot do. So, everyone has to cut the scientists and the clinicians and the public health people a little slack while they applaud, because we’re learning as we go.

And we’ve been trying to be as transparent as possible. You know, there are a lot of people who took a minimum of science in high school, and they’re learning a lot of virology and clinical medicine in this pandemic period. Good for them! It’s hard for them to keep up. And it is for many people who would like definitive answers — difficult for them to understand that the answer I give you today is my best answer, but you know, next week I may have to modify that answer. That’s harder for some people than for others.

Serena Marshall: Well, Dr. Schaffner, we appreciate your time.

William Schaffner: Well, thank you very much. And while we’ve been talking about boosters, let’s remember, Serena, there are still many people who haven’t gotten even their first dose. So let’s all urge them to get their first dose.

Roll up those sleeves everybody. Let’s get vaccinated.