Is COVID-19 Herd Immunity Even Possible Anymore?

23:40 minutes

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This story is a part of Science Friday’s coverage on the novel coronavirus, the agent of the disease COVID-19. Listen to experts discuss the spread, outbreak response, and treatment.

Since the start of the pandemic, we’ve equated getting out of this mess with the concept of herd immunity—when a certain percentage of the population is immune to a disease, mostly through vaccination.

With COVID-19, experts have said we need somewhere around 70 to 90% of the population to be immunized to meet this goal. Now that all adults in the U.S. are eligible for the vaccine, how far are we from that goal? And what is our trajectory?

Some experts now say with variants and vaccine hesitancy, herd immunity may not be possible here in the U.S. Joining Ira to break down this and other coronavirus quandaries is Angela Rasmussen, research scientist at VIDO-InterVac, the University of Saskatchewan’s vaccine research institute in Saskatoon, Saskatchewan.

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Segment Guests

Angela Rasmussen

Angela Rasmussen is a research scientist at VIDO-InterVac, the University of Saskatchewan’s vaccine research institute in Saskatoon, Saskatchewan.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Since the start of the COVID-19 pandemic, we’ve equated hope in getting out of this mess with the concept of herd immunity. Some people call it community immunity. That’s when a certain percentage of the population is immune to a disease, mostly through vaccination.

With COVID, experts have said we need somewhere around 70% to 90% of the population to be immunized to meet this goal. Now that all adults in the US are eligible for the vaccine, where are we at? And has that 70 to 90 number changed? Some experts now say with variants and vaccine hesitancy, herd immunity may not be possible here in the US. Well, here with me to break down this and other recent COVID quandaries is my guest, Dr. Angela Rasmussen, research scientist at VIDO-InterVac, the University of Saskatchewan’s vaccine research institute in Saskatoon, Saskatchewan, Canada. Welcome to Science Friday.

ANGELA RASMUSSEN: Thanks so much for having me back, Ira.

IRA FLATOW: Welcome back. Angela, where do you fall on the herd immunity optimism scale? Do you also think that it’s unattainable?

ANGELA RASMUSSEN: So I think that some of that depends on what the herd immunity threshold actually is. And some of that also depends on other factors, such as how much transmission is actually occurring in the community. Now, one thing I think that people have been confused about with regard to the herd immunity threshold, which is that number that you just referred to at which a virus would not be able to spread within a population because too many people are immune– you know, that’s an estimate based on transmission dynamics, based on susceptibility in the population. And for that reason, it’s very difficult to nail down a specific number.

But I think some of the confusion has been that people think we absolutely have to get to that number, and then once we get there, things will magically change and the pandemic will be over. But unfortunately, it’s really not that simple. We can look at a country like Israel, for example, that has now vaccinated about 60% of the adult population. And we can see that they’ve had long, sustained reductions in transmission in the overall community.

Now, 60% is nowhere near the herd immunity threshold. And there probably is some population immunity from people who were naturally infected with COVID adding to that, but I think that an important point that we should all think about is that we might not have to hit the herd immunity threshold, which currently is calculated at somewhere above 80% of people immunized, to see sustained reductions in transmission like what Israel is seeing.

Now, at the beginning of their vaccination campaign, they had a pretty restrictive lockdown. And that’s not the only way to do it. But if you get transmission down enough while boosting vaccination enough in the population, you can actually get to a place that is functionally like the herd immunity threshold before you actually get there, just because there’s not very much virus in the community and you have a growing number of people who are immunized.

So it’s a really tough concept. I think it would be a lot easier for people if we could just say, hey, we’re aiming for 80%, 85%, 90% vaccinated. And of course, it’s great to vaccinate as many people as we possibly can, but that’s not necessarily the metric that we have to hit in order to start relaxing some of the restrictions and being able to return to, quote, unquote, “normal.”

But herd immunity is really not a national concept. Herd immunity is really regional. And as you pointed out, some people call it community immunity for this reason. If you have, overall, 70% of people vaccinated, there still may be communities where there’s a very low vaccination rate that’s far under that, in which you will have a lot of susceptible people.

IRA FLATOW: Interesting point. I know that right now more than half of the US adults have gotten at least one dose of a vaccine. But some states have only been open to all adults for less than a month. Is it too early to really tell how many people will get vaccinated and how much to open? What’s your view on that?

ANGELA RASMUSSEN: So if you look at New York, for example, which has planned to fully reopen by May 19, I do think that that is premature, again because of this herd immunity being a regional issue. And in some places in New York state, you’re going to have people with very high levels of immunity who have been able to access vaccines more readily and who were very enthusiastic about getting it, but you will definitely have other communities, including within New York City, where people have not been able to access vaccines even though the vaccination efforts are now open to all adults. So I think that it may be a bit premature.

What we would ideally like to have is people who are fully– the majority of the population in any given state being fully vaccinated before deciding to completely reopen everything, and potentially, looking to make sure that all communities are hitting that 60% to 70% metric. I think that it’s also not appreciated that in many communities, there are a number of people who want to be vaccinated but have not been able to access the vaccines. So we need to make that more accessible, and this would be people who are homebound, for example, people who may not have access to the internet or may not be comfortable using some of the often Byzantine online systems for scheduling a vaccine appointment. I think if we can do a better job at making sure we’re getting vaccines to those people who are being left behind by the current vaccination efforts, we’ll be on track to open. I think that it’s very reasonable to expect that we could start to reopen this summer.

IRA FLATOW: So what are strategies, though, for getting to these people? What do you think would work?

ANGELA RASMUSSEN: So it’s going to be a combination. Certainly, for the people in the communities that I just mentioned that can’t access the vaccine, we need to do more in terms of bringing the vaccine to them. So we’ve done really well in terms of having mass vaccination sites, in terms of distributing vaccines in local neighborhood pharmacies and pharmacy chains like Walgreens and CVS, but that’s not going to reach everybody. In some cases, we may need to do targeted vaccine pop-ups, for example, in given neighborhoods. And in some cases, we might actually have to bring vaccines to people’s homes, the places that they’re living, in order to make sure that they can access them. Not everybody has a car and can go to a drive-through mass vaccination site.

In terms of people who are reluctant to take a vaccine, I think that that’s a much tougher challenge, because a lot of that is just going to involve personal conversations with people that they trust. So I think that for science communicators and scientists like myself, one really important thing is going to be to reach out to community leaders in all these different communities and make sure that people who are not public health people or scientists, but who are trusted members of the community– you know, a community leader, a faith leader, things like that, family doctors– that they are able to empower people with information and hopefully win some hearts and minds to the cause of getting vaccinated.

IRA FLATOW: You know, we have seen studies that show that what changes people’s minds– well, they’re the minds of their best friends and people they trust. And maybe some people are waiting to see, hey, you know, you get it first and let me see how you do, and then I’ll get it.

ANGELA RASMUSSEN: Yeah, I think there’s a lot of that. And I think that people tend to confuse what’s called vaccine hesitancy, which really is just that– it’s people who are hesitant about the vaccines. They have questions about the vaccines. They may have some doubts about the vaccines.

Those are all completely reasonable given that this has occurred in the context of a national prolonged emergency. And it’s really important that those people have their questions taken seriously and answered by people that they trust. And that person that they trust is not always going to be a virologist, as much as I’d love to be everybody’s trusted messenger.

IRA FLATOW: Since we last talked, you’ve moved to our wonderful neighbor to the north. And things are not going, vaccination-wise, as well as you would hope, has it?

ANGELA RASMUSSEN: No, it really hasn’t. And unfortunately, some of this is the fault, really, of my home country, the US. The US has purchased a lot of vaccine doses, and they haven’t been as eager to share. And in addition to that, Canada does not have, currently, domestic manufacturing capacity for vaccines at scale. But right now, Canada is really dependent on importing vaccines that are manufactured in the United States. And unfortunately, they just haven’t gotten as many doses.

Now, the good thing is that Canada’s population is about a 10th of the size of the US’s, so they won’t need as many doses. And the doses have increased in recent weeks. They’ve also approved Moderna for use here, so that is going to be coming in shortly. There will be more options for Canadians, but it has been very slow. And currently, vaccination is not open to all adult Canadians yet. I believe it’s only 40 years and older.

IRA FLATOW: I want to ask about your vaccine experience, because I hear that you got the Johnson and Johnson vaccine before its use was paused. Just to be clear, the CDC and FDA recommended it be paused because of a very rare blood clot in some women who received the vaccine. I know a lot of people were nervous about this vaccine after it was paused. What’s your experience with it?

ANGELA RASMUSSEN: Yeah, absolutely. So I was really relieved to get the Johnson and Johnson vaccine on April 5, because I knew that I was moving to Canada on April 20. And if I had gotten the Pfizer or Moderna vaccines, I don’t know when I would have been able to access the second shot, being in Canada. And because of the travel restrictions between Canada and the US, it would be very, very difficult for me to come back to the US just to get my booster shot.

So when I saw the opportunity to get an appointment for Johnson and Johnson, I jumped on it, since it’s really one and done. Now, that said, six days after my vaccine was when the news about the blood clots came out, and the vaccine was paused. And I thought to myself, well, I’m under the age of 50, I’m a woman, and I have had the vaccine within six to 13 days, so I’m right in that window where, potentially, if this side effect is going to happen to me, it could.

But then I thought more about the relative risk. And even though we know now that these blood clots are quite rare, and they’re a very specific type of blood clotting abnormality, I have done other things in my life that have drastically increased my risk of blood clots. I took hormonal birth control pills for over 20 years, starting when I was a teenager. You know, I’ve flown quite a bit. And I used to smoke. So all of those things really dramatically increase your risk of blood clots.

And then finally, I know that getting COVID-19 would dramatically increase my risk of a blood clot, including a serious blood clot like a DVT, a pulmonary embolism, or a stroke. And I, overall, doing that risk-benefit analysis, am still profoundly relieved to have gotten the Johnson and Johnson vaccine. And given that case trajectories are up in a lot of Canadian provinces, including Saskatchewan, where I live now, I’m very, very relieved to have the protection that that vaccine affords me. I would do it all over again in a heartbeat.

IRA FLATOW: Having heard you say this, I imagine that you are very much in favor of the CDC and the FDA saying that this vaccine can be used again.

ANGELA RASMUSSEN: Yes, I am. I also think, though, the pause was appropriate. I think that even though this was a very small what’s called safety signal– it was a total of seven cases, one during the phase 3 clinical trial and six since the vaccine’s been administered. That’s a very low incidence. And even if it turns out to be a little bit higher, as we’ve seen with the AstraZeneca vaccine in Europe, that’s still a relatively low risk.

So I think that the benefits of using this vaccine, especially considering that the one-dose vaccine makes it easier to give it to people in some places– the lack of a strict cold chain requirement also makes it easier to get this vaccine to places that don’t have ultra-cold freezers– this is a really important part of our vaccine program, including globally. So I think that it’s really important that our regulatory systems and ACIP, the Advisory Committee for Immunization Practices, really took their time to look through all of the data thoroughly. But given that they did, I think that what the data shows is that there are way more benefits to continuing to use this vaccine than there are drawbacks.

IRA FLATOW: Just a quick note that I’m Ira Flatow, and this is Science Friday from WNYC Studios. Variants, our listeners tell us, are still on a lot of people’s minds. I want to play a question we got from listener Ken in Cleveland on our Science Friday VoxPop app.

KEN: How well, if at all, do the current vaccinations protect us from the variations, especially the B117?

ANGELA RASMUSSEN: So all of the data so far suggests that all of the vaccines that are available currently are very protective against B117. And that’s great news, because as you know, there are several variants, and I know it can be tough to keep them all straight given that their names aren’t exactly really catchy. But the B1351 variant from South Africa, which does have some potential capabilities to evade immunity– some immunity. They’re not completely– they don’t render the vaccines useless at all.

That variant is also in the US. And conveniently, Johnson and Johnson did one of their phase 3 clinical trials in South Africa when that variant was circulating widely. And that vaccine is still protective against the most critical outcomes, so hospitalization and death.

The Johnson and Johnson vaccine performs very well against B117, pretty much just like it performs against all other SARS-coronavirus-2 variants that are not variants of concern. But the ones that people are worried about, like B1351, which, again, may be able to evade some antibody responses– the vaccines still work against that. Johnson and Johnson, Pfizer, Moderna– all of the vaccines that have been tested are still effective against all the known variants in terms of protecting against the most severe outcomes, and in many cases protecting against disease altogether.

IRA FLATOW: I want to shift gears a bit and talk about antibodies, because a new study in the Journal BMC Infectious Diseases says that after 100 days, only 44% of COVID-19 patients still had antibodies. What do you think of that?

ANGELA RASMUSSEN: Well, so this is one of the troubles with doing antibody studies like this. Just because you can no longer detect antibodies with the test that you’re using doesn’t mean that those people don’t have some degree of long-term immunity. And there’s a part of the immune system that’s called the anamnestic response in which somebody who has been infected or vaccinated with another pathogen previously will be re-exposed to that, and they may not have detectable antibodies, but they do have memory B and T cells in their immune system that will recognize that. And the anamnestic response is when those memory B cells start making just a ton of antibodies, so they rapidly crank up antibody production really quickly.

In many of those people who tested negative for antibodies after recovering, within 100 days, they may well have memory immunity that is just more difficult to measure because they don’t have a lot of antibodies actually circulating in their blood. That doesn’t necessarily mean, though, that they’ve gone back to square one and they’re completely susceptible. But this is an area of ongoing study where we don’t really know what the typical immune response looks like, especially over the long term, for somebody who was infected with SARS-coronavirus-2. We do know that there is a pretty big range of different antibody responses that people can have.

So I don’t make too much out of that other than to say that if you had COVID before, you shouldn’t rely on your natural immunity. You should get a vaccine, because we know that vaccines reliably induce very high titers of neutralizing antibodies. And they can only boost whatever immune response you will have carried over in your immune memory from a natural infection.

IRA FLATOW: Do you get the impression, from following the course of this disease and the immunity and the antibodies, that sooner or later we’re going to need a booster shot?

ANGELA RASMUSSEN: I think it’s possible. The one thing that we didn’t look at in the clinical trials is durability and how long the protection that the vaccines confer last. Right now, I think that the possibility of needing a booster is higher because of potential loss of durability over the long term. The same way that we will get an MMR shot when we’re, I think, two, and then we’ll get another one when we’re 10, we may need a booster like that.

Right now, anyways, it doesn’t look like we’re going to need a booster specifically to address variants, because, as I mentioned before, the vaccines do still work and are protective against all the known variants of concern. But that’s a possibility, too, if new variants should emerge that are even more capable of evading vaccine-induced immunity. Right now, that’s not the case. So I do think, though, that there’s a good likelihood that we might need a booster in the coming years just to maintain those long-term protective immune responses.

IRA FLATOW: And what can we do for people who are immunocompromised and for whom the vaccine may not work?

ANGELA RASMUSSEN: That’s pretty simple. We can get vaccinated. And that really is the benefit of herd immunity, community immunity, population immunity. If there are enough people vaccinated, then the virus can’t spread within the population, and that indirectly protects people who can’t take the vaccine.

And it’s not just immunocompromised people who aren’t responding well to the vaccine or aren’t developing immune responses because of it. There’s also people who can’t take vaccines for a variety of reasons. Maybe they have some kind of allergic condition. Maybe they don’t have good vaccine reactions. There are many reasons why people may not medically be able to take a vaccine. So that’s why it’s even more important that people who can take vaccines do take vaccines.

IRA FLATOW: Let’s talk about, for a moment, the grave shortages of vaccine among developing countries. What are your thoughts on how to overcome that?

ANGELA RASMUSSEN: So this is a really tough problem. And this has been, unfortunately, exacerbated by countries like the US and Canada, and the European Union, all of which have purchased a number of vaccine doses and really put their own people first. And while this is somewhat understandable, and it’s also reflecting the political realities of each of those countries or groups of countries, it’s not how we should be approaching global health going forward. We really should be doing these efforts from day one. We should be planning ways to distribute vaccines equitably among all the countries of the world, because a pandemic, by definition, is a global crisis.

So right now, the COVAX Initiative has purchased a number of vaccine doses, basically for countries that are not able to purchase it for themselves. Those are primarily AstraZeneca, Johnson and Johnson, and the Novavax vaccine, which is currently finishing phase 3 clinical trials, because those three vaccines are easy to distribute in a number of different situations regardless of needing specialized infrastructure such as ultra-cold freezers. I think that over time, unfortunately, many countries that are depending on COVAX have started to see that since they’re getting AstraZeneca and Johnson and Johnson, they’re worried that they are getting sort of the B-plus vaccine instead of the A vaccine that’s the mRNA vaccines, both in terms of efficacy and in terms of their safety records.

So that’s something that we really do need to combat. We need to make it clear– and that’s one of the reasons I’ve been so vocal about getting Johnson and Johnson as a privileged American who was able to effectively choose my vaccine. I want to really get the message out there that these vaccines are fantastic. And again, after doing the risk-benefit calculation, it didn’t stop me from getting it. I would hope that these vaccines are distributed to the rest of the world. But this is something that I think needs to be priority number one after the emergency phase of this pandemic is over and we all sit down to try to figure out how to not go through this again.

IRA FLATOW: What about relaxing the patents on these vaccines so that the countries themselves can make them?

ANGELA RASMUSSEN: Well, it’s not that simple. And I would say that, first of all, before I say anything about this, I am not an expert on intellectual property or technology transfer. But I do think that it is a good idea to not only relax the patents– the TRIPS waiver, which is a waiver of intellectual property rights that’s been called for by India and South Africa, is one mechanism for doing that. It’s not the only mechanism for doing that.

But we not only need to relax the patents, we also need to share the technology for manufacturing these vaccines, because it’s not just about getting the ingredient list. It’s about getting the actual recipe and being able to build the infrastructure to manufacture these things at scale. I think that that is often left out of this discussion, because people say, well, why don’t they just share the patent rights? That wouldn’t cost them that much other than profits, potentially.

But it isn’t that simple. It really is about this larger tech transfer issue and building capacity in different countries that want to have these manufacturing capabilities. And I leave that to the experts to figure out the best way to do that. But in general, I do think that people should not be worrying about patent protections or intellectual property rights during a global crisis like this. If it can help get more people worldwide immunized faster, we should do that.

IRA FLATOW: Angela, as always, great talking with you. Thank you for taking time to be with us today.

ANGELA RASMUSSEN: Absolutely, Ira. Any time.

IRA FLATOW: Dr. Angela Rasmussen, research scientist at VIDO-InterVac, the University of Saskatchewan’s vaccine research institute in Saskatoon, Saskatchewan, Canada.

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