The Future Without Herd Immunity

herd immunity. There are a range of reasons and causes for this, and they go from vaccine hesitancy, to the contagiousness of new variants, and beyond to the underlying science. As a consequence, we're all trying to figure out what this means for what happens next. Will COVID become a manageable, self contained disease not unlike the common coal, Will it alternatively develop new variants that require us to go back to the drawing board and develop new vaccines, or will

it be something in between. To help us understand these pressing questions, we're joined by doctor Mark Lipsitch Our go to COVID expert here on the program and indeed throughout the country. Mark is a professor of epidemiology at the Harvard Shan School of public Health. He's affiliated both with the Epidemiology Department and with the Department of Immunology and Infectious Disease. He runs the Center for Communicable Disease Dynamics

at Harvard. Regular listeners of the podcast will know that Mark was one of the first voices anywhere in the United States or indeed in the world, drawing attention to the future trajectory of COVID nineteen. He joined us as early as February twenty twenty to tell us where the disease was going to go, and he's managed to join us intermittently ever since, taking time out of his extraordinarily busy schedule to set us right about where COVID is,

where it's been, and where it's going. Mark, thank you as always for joining us. In general, I've tried to keep you as a guest, reserved for those moments when something really important seems to be shifting in the trajectory of COVID nineteen and news reports suggest that maybe we're getting there at this moment as well, and that that's something has to do with our acknowledgement finally that herd immunity may not be coming our way in the United

States and the feseable future. It's something you have been warning about really almost from the start. So I want to start by just asking you to remind us, all for definitional purposes, of what herd immunity is defined in

terms of our zero the replication rate of the virus. Sure, I think what you're asking is about the herd immunity threshold, and I'll come back to that distinction in a minute, because I think it's actually helpful to talk about it, But because the herd immunity threshold is to find as that level of immunity in the population from whatever means, usually infection or prior infection or vaccination or a combination, that it can't spread widely because there are so many

immune people that chains of transmission don't sustain themselves and they fizzle out before the virus can spread too far.

That is quantified by the basic reproductive rate or basic reproductive number are zero are not, which is the number of secondary cases that each case creates, and when everyone is susceptible, and when you do a little bit of algebra with that expression and try to find a level of immunity where instead of infecting, say three or four other people, the average case infects less than one other person, meaning the number of cases is going to go down

over time. If you do the math, you end up getting the proportion that have to be immune is one the reciprocal of that number our zero one one over our zero. And so, to give some examples, if our zero is three, then that's two thirds. If our zero is four, that's three quarters. You've been saying ever since people started talking about the possibility of herd immunity being created partly through the spread of vaccines, that it would be optimistic. I think that's the word you used to

actually imagine we could get there. As I understand some public statements that you've made recently, including the New York Times, you're now willing to shift a little bit from it's unlikely that we'll get there, too. It seems extraordinarily improbable that we're going to get there in the foreseeable future in the US, at least not through vaccinations as opposed to vaccinations plus natural infections. What has shifted your empirical observation.

I don't think your theory has changed, but what has caused your perical observation to evolve to that point? Well, I think I do want to now mention the concept that hurt immunity and that threshold are slightly different. The threshold is the point where there is so much hurt immunity that we don't have ongoing transmission when we return to normal behavior. Herd immunity is a quantity, it's between zero and one, and we are getting more and more of it, So we are getting hurt some hurt immunity.

But my statements have been about whether we're going to reach a level of immunity that means that we can expect zero or nearly zero transmission despite pre pandemic types of behavior. So, first of all, thanks for that clarification, because I and I think most other non scientists have been using the term herd immunity when what we should have been saying is the threshold of enough hurt immunity. So you could say, well, right, it's like the blueness

of the blue thing. It could be very faintly blue and it would still be blue. But the question is how blued you need it to be for whatever your purpose is. Right, Okay, that's important. That's important for I mean, everybody is, including scientists, are sometimes sloppy about using her immunity is shorthand for the threshold. The reason I try to be very careful about it is that herd immunity is a good thing. Even a little bit is good,

and a little more is a little better. And being below the threshold does not mean it's not doing any work. It is doing a lot of work. So that's actually some of the good news is that we are building up a lot of herd immunity. And the reason to answer now your question, the reason why I think it's extremely unlikely that we will get to the threshold in

the United States is a few things. One is, when we started out, we had no data on whether these vaccines were protective against infection and a transmission, and that's the part of vaccine effectiveness that matters to her immunity. If everybody gets a technis shot, that doesn't protect the next person from getting tetanus because it doesn't do anything to transmission of the bacteria which comes from the soil. If the vaccine isn't stopping transmission, then but it's just

stopping disease, it's not doing anything for her immunity. These vaccines do a lot for her immunity. Initially, it was unclear how much the first number I think that was meaningful that came out with sixty one percent, but that was from one dose and probably an underestimate for technical reasons that aren't that interesting for the Maderna vaccine, And now the numbers have come out through various sources of evidence.

It's still uncertain, but it seems like it's probably somewhere between about sixty and ninety percent, probably seventy and nine protective against the process of transmission. So that's really good news, but it's not one hundred percent. And that distinction matters because to get to the herd immunity threshold, we have to cut transmission to one minus one over our zero,

not just vaccinate that number of people. So the more the closer to one hundred percent effect of the vaccine is, the more we can interchange how many people have been vaccinated and how many people are totally immune. But we can't totally do that yet. It's because we have to multiply one by the other. Right, the question is not how many people have the vaccine, and it's not how much does the vaccine do, it's how much does the vaccine do for the number of people who have been

vaccinated exactly. Yeah, okay, so we now understand that number, and though it's surprisingly good seventy nine, it's still not going to get us necessarily where we need to go when you consider it as a percentage of all of the people who've been vaccinated, which is the part that's not so high. Right. So the other bit of good news that makes it easier to get close to the threshold is that children really don't seem to be as important for transmission, and so that one one over our

zero is an average. And as you learn more about the population, if you leave out some of the people from vaccination who are less important to transmission, that matters less than leaving out people who are more important to transmission. So that probably buys us a little bit of margin. How far did those numbers go mark on kids not

being involved in transmission? And the reason this is relevant is one of the conversations that I've heard repeated a lot among all kinds of people is people say, well, I'm vaccinated, but my kids aren't vaccinated, and so I'm worried that there is some possibility that I might transmit to them, or that they might transmit to me, and even though they wouldn't get that sick, most probably they

would nevertheless transmit it. And so if the number of kids not transmitting is relatively high, then that worry is much less salient. It seems to increase with age, So a fifteen year old is probably effectively an adult from sars Kovi two's perspective, roughly speaking, and a five year old is definitely considerably less I'd say that's the sort of rough picture. Okay, So those are the bits of

good news. Mostly, the bits of bad news that make it unlikely that, say, the United States will quickly get to the herd immunity threshold are First, that variants are more contagious than the original strain of sars Kobe two. So everyone agreed pretty much that the original strain had a reproductive number of about two and a half or three. I think that might have been too simple, and maybe it's higher in some places, but let's say for the sake of argument, it was two and a half or three.

The B one one seven variant that's now very common in many parts of the world is thought to be about fifty percent more contagious, more transmissible, so that pushes it up to maybe four and a half, and so that pushes the herd immunity threshold close to eighty percent.

So that means that say we need say it's seventy five percent, say we need to immunize seventy five percent of the population, and say that the vaccine is eighty percent of transmission, then we need to get very close to whatever seventy five divided by eighty is, so very close to one hundred percent coverage in the population in

order to reach that threshold. And given the combination of hesitancy poor access, which I think are two sides of the same coin, and so far not vaccinating children under sixteen, soon it will probably be under twelve, I think it's just very hard to imagine getting to that level. So now let's turn to what this means in practical terms.

There are a few different dimensions. Let's start by thinking about modeling, which is one of the things that you do for a living and that you've done so skillfully in this entire epidemic process. When you think about what models might look plausible for how the current COVID virus is going to proceed. Circumstances where we're not at the threshold of herd immunity. What's the modeling space, as it were,

What are the possibilities that you see as most plausible. Yeah, I think the most plausible scenario, not the only plausible one, but really the most plausible one is one that was described in science in a paper by Rustamantia and Jenny Levine and Autar Bjornstad. And what they do is essentially

make an analogy to other coronaviruses. So the most important difference probably between this coronavirus and other ones is that everyone in the world has been infected with the other coronaviruses many times by the time they're say a teenager, and so people get infected over and over and it just circulates. They just circulate. And those are mostly they're not all of the common cold viruses, but they are

within the family of the common cold viruses. They are justifiably obscure except virologists because they don't make people very sick.

And so I think a very likely outcome is that this one behaves similarly, and that what vaccines are going to do for us is to get us over the hump of the period in history, namely the one we're living through where there are a lot of people who would get very sick from a first exposure to this virus, and everybody's getting first exposures because it's a new virus,

and therefore that's a bad combination. So if the vaccines can be used to put us in a position where nobody is getting or a few people are getting exposed for the first time when they are also old or have comorbid conditions, then the vaccines will have prevented a

lot of illness and death. And then it's quite possible that the residual circulation of the virus among the people who aren't vaccinated, or among the people in whom the vaccine immunity slowly wanes, is going to give us longer term protection against severe disease, so that five years from now, you and I and our children and our parents will have probably been infected by the coronavirus if we don't continue to vaccinate very heavily for the next five years,

but it won't bother us because we will have all been vaccinated once and or had it before at an age when we were not likely to get very sick, and so it will behave much like the other coronaviruses. So the good news part of that is that at least over time, that describes a kind of normalization where childhood infection is protective in a lot of good and

healthy ways. I assume the downside of that is that among those who are not vaccinated and who are more vulnerable, a substantial number of people who have not been vaccinated. Among that class of people could get very sick and even die in the next let's say five who would

not if they were vaccinated. Right, So the public health goal if that scenario is going to play out, is to vaccinate as many of the people known to be at high risk as possible, and probably also as many of the other people as possible, in order to slow down transmission and just give people's immune systems a chance to catch up. If that scenario plays out, then the vaccines are sort of a bridge to a more peaceful future rather than something we have to do all the time.

So that's the vaccine as bridge scenario. And it's not so bad relative to the other possibilities, and you think it's the most probable, So that's good news. Talk to

us about some of the other possibilities. Well, the other possibilities would be that immunity to severe disease from the vaccine and or from natural infection is not long lasting, either because the immunefectors wan over time, or because the virus changes and is not as strongly affected by the immunity, or both, And so then it's a sort of more ongoing war because we have to keep trying to protect people from infection, not only from their first infection, but

from their second, third, fourth infections. And is possible. I think infection immunity very likely will wane, But the question is whether there is this long term, relatively long term immunity that gets you from infection to infection and then

gets boosted again against getting really sick. So that's one of the two subpossibilies that you just describe, that infection immunity itself WANs, whether from natural infection or from the vaccine, and as you say, whether that's a disaster or not really depends on whether the virus is circulating in enough quantity that you can get a natural booster from being exposed.

Then there's the possibility that you mentioned that there might actually be an evolutionary process in which the virus evolves

to the point where it is vaccine resistant. And I guess what I want to ask you about is you've done plenty of modeling of other viruses that have evolved to be vaccine resistant, presumably, and there are lots of bacteria where this has happened over time where there's pretty good data I think on trying to figure out that, and so there must be familiar and available models to

you when you think about vaccine resistance. Is there any way to think intelligently about the probability of something like that happening? Yeah, I mean flu certainly is the sort of classic example of a viral infection that changes over time to resist our natural immunity, and at the same time it happens to also escape from existing vaccines, and that's why we keep changing the flu vaccine. So I

think that's the example or the analog. The viruses are different and they evolve differently, and I'm not sure how to put a probability a sort of how far the virus, this virus can go in escaping our immunity. It's like we've chased it down an alley, and the question is at the end of that alley, is there a brick wall, and it's stuck and we've got it. It's escaped as far as it can escape, and now there's nowhere more for it to go, or is there a big vista

of other things for it to do? And I don't I don't know any way to put probabilities on that. I think the best way to know it probably would be to try to do it in the lab and see where it goes from the variance we've already seen, which is itself a somewhat dangerous thing to do, but

probably a very valuable one. That's a fascinating topic, which I would like to come to in just a second, but before we do, I'm sort of surprised to hear you say that there isn't a kind of ability to make a prediction about probability of evolution, especially in light of the fact that we've now observed the virus evolving pretty quickly with respect to the variants that are out there.

I mean, is there no I guess there isn't a really way to extrapolate from the virus's capacity to evolve, to evolve these new variants that are more capable of infection than the kind before. To extrapolate from that to whether it would be able to evolve in ways that would escape the vaccine, Well, I think there are a few things. One is, it can mutate. I mean it's generating mutations all the time, and the existing variants are generating new variants that are more different from the original

strain than the say B one one seven itself. So B one one seven is making new versions of itself that are further mutated. So that's not in doubt. That's just RNA viruses do that. The question is what will the impact be on escaping our immune responses, and even for the existing variants, that remains a little bit unclear. It is very clear that they escape neutralizing antibodies to

varying degrees depending on the variant you're talking about. But whether that means that they totally escape our ability to control them. That's mostly unobserved and still being documented. So they can change, that's not in doubt. But whether they can change in ways that really get around to a large degree, our ability to fight them off with our antibodies and other forms of immunity, I think is still

an open question. Even for the existing to maybe one three five one the South African variant or the Brazilian

variant or the Indian variant. So that's a question. It's not whether they can evolve, it's whether how far can they go in their biological properties, And that's something that you can't really answer, mostly because evolution is not a perfectly modelable process, right, I mean, it has contingent features, and you know there aren't necessarily absolute is that will tell you in relation to both the underlying biology and then the way it interacts with the world outside what's

going to happen. If it were, we could do a lot more predictive work on evolution than we actually are able to do. Yeah. Yeah, I mean there are a number of us, including our group with bacteria, trying to chip away at this problem. But this is a major hard problem in biology, and we aren't there yet. We don't know what the space of possibilities is and how it relates to the sequence. Talk to me about what in the real world it would look like in this

scenario where let's call it the flu like scenario. If we started with the vaccines as a bridge, this is the flu like scenario where we have to go back and try to get a new vaccine each time a dangerous new variant emerges. Why do we do as poorly as we do with respect to the flu vaccine, which I guess only prevents about forty to sixty percent of the flu each time it's redvised. Yeah, it's a good question. I think that in itself is a scientific discussion that

isn't totally resolved. Why the flu vaccines have been so hard to make as good as say, these existing coronavirus vaccines. Partly it's about predicting which sequence to use, but even when it's well matched, it's not always very good. So I think there's some reason for optimism here that the first guests at what to do about a coronavirus vaccine turned out to be as much as ninety five percent effective, and that's much better than flu vaccines. So it may

be that coronaviruses are easier. But this coronavirus may continue to change, And then again, the question is whether it changes in a way that evads our infection immunity, which would mean that we have to consider trying to control it, or whether it evades our severe disease immunity, which would

be a much bigger deal. And what I mean by that is if we didn't have flu vaccines, as most of the world doesn't use flu vaccines, there are people, many people who die because they don't get flu vaccines, but it is not the same kind of global catastrophe as this has been. And that's because existing immunity to flu, even though the flu viruses are changing, is still protecting people to a large degree against getting very sick and dying.

So the sort of flu analogy is a worse scenario, and we probably would try to make better vaccines that keep up with the new strains, but it's still not a new pandemic every year. It's in the realm of public health problem rather than public health catastrophe. We'll be

right back mark. When you see the virus running maybe checked is the wrong word, but running hard and fast through a population, as seems to be happening right now in India, Apart from the understandable concern about the human suffering that that's going to entail, do you think to yourself, oh, boy, that raises the probability of new variance emerging that could be ultimately vaccine resistant, or do you think, rather, look, this is what the virus was always going to do.

If not restrained and people are catching it, and so there's no particular reason to think that it would need to evolve new forms of resistance, because, after all, it's spreading successfully through the population with whichever variant that it presently has. Yeah, I mean, I think it's a fairly central principle of evolution that large populations give rise to more variation and select the most fit variants more effectively

than small populations. And in infections, that means lots of infected people are both more of a breeding ground and more of a efficient filter for variants that are good at transmitting. So having virus spreading widely in any place is not good for any place else, even places that have vaccine access, and that is one of many reasons why trying to help India to control the virus is in the interest of other countries, in addition to being

the humanitarian thing to do. When you're wearing your public health recommendations, hat, what sensible recommendations do you think policy regulators should be considering, Leaving aside the question whether they can practically be adopted or not for a world where the virus seems to be more or less calming down in places with relatively higher vaccination rates, while it's simultaneously spreading pretty rapidly through places in the world which have

very low vaccination rates. I mean, what's the picture of what kinds of opening are appropriate, in what kinds aren't, and can we really sustain and tolerate a world where you know, Westerners would be saying, well, in our countries where we've got reasonable amount of vaccination, we're more or less reopening. But we're urging you India, Brazil, you know, we're urging you to engage in serious lockdowns. I mean, so, first of all, is that the sensible thing to be

saying to them? And second of all, is it plausibly sustainable to say that, even even if it's true. Wow, that's way way above my pay grade. I mean, I think in reality, it is the decision of the countries what to do, and if we're not forthcoming as a global community with aid to blunt the impacts of having to close your societies and vaccine doses in the quantities that would make them, make the rest of the world

able to close. Speaking as a sort of citizen and someone who's married to a philosopher, I would say that seems very unfair to say our public health is at risk because of your virus, so why don't you close your society down. I think there are many things in

between neglecting and telling people what to do. But I don't think that our desire to keep variants from emerging is or should be a very compelling argument to countries that are not receiving or able to generate on their own the resources to control the virus in a humane way. I mean, I don't think it would be framed that way. It would be framed as, look, you still have not that many people vaccinated, so you should be engaging in the same kinds of lockdowns as we were engaged in

until now. I mean that is presumably the line coupled with and by the way, we'll give you more vaccines or will enable you to make more vaccines. Yeah, this is where I think, as a scientist, I find myself at a of a loss because these are not really scientific problems. These really are problems of global equity and disparate impact, and I think I will pass on trying

to solve them. Fair enough, I mean, I guess what I wanted to say is that they are classically social scientific questions in that well, they of course require some normative assessment. But that normative assessment, you know, of what we ought to do is based a little bit on facts.

But in any case, I won't press you. Let me ask one more question mark before I close by going back to your mention of lab experiments, and the question is basically this, when you look at where the US is now and where it plausibly will be in the next six months as we vaccinate twelve to fifteen year olds, how much prophylactics shouldn't remain in place in your view?

I mean, at what point will it be plausible to say we no longer need to mask up when we're indoors, or we no longer need to maintain social distancing in schools. I think that is a great question. I think what's going to happen is that we're going to sort of feel our way in that direction, and there's going to be a lot of variation across any given rich country in how fast people go, and we'll get some sense from those experiments, those non planned experiments as to what works.

I mean, in trying to frame how to think about it, I would say the goal is that when there are very very few people at high risk still in the line of the virus and unvaccinated, then permitting more spread is a much safer thing to do. It's not totally safe, but it becomes a reasonable trade off with the fact that we all want to have economic, social, educational, and cultural lives back. But to give a number at this stage,

I'm not really sure. I mean, in some ways, one of the more optimistic, the most optimistic information is coming out of Israel right now in the sense that they have when you talk to people really reopened to a striking degree, and their cases continue to go down, and their hospitalizations and desks to stay at low levels, and they have not gotten anywhere close to what seems to

be the herd immunity threshold. And so to me, that's to go back to our earlier discussion, that might be the evidence that some of these concerns are overblown, that we can actually control spread with a little bit lower

levels of vaccination. But what I think is also happening in Israel, as I understand it, is that if you're vaccinated, you have lots of opportunities to go to the theater and to go to crowd indoor events and whatever, and if you are unvaccinated, you have fewer of those privileges, and or you might well be someone who was in a high risk group for infection before and are benefiting from natural immunity. So I'm wondering to what extent Israel really is an example of reopening, and to what extent

it's partial reopening. But I think they will set a pace that we can sort of watch and see what works and what doesn't. Mark before I let you go, you made an intriguing comment about the scientific value of doing in lab experiments and trying to see what happens to COVID nineteen in the lab, And obviously there's a lot of nervousness around this, especially given the ongoing belief. I don't know if I would call it a paranoid

fantasy or not. That might be too strong formulation, but the ongoing, let's call it unsubstantiated belief among some that perhaps COVID nineteen originated itself in a lab and then unintentionally escape from that lab. How much could laboratory based, responsible careful research reveal and what would make it worth doing given the presumably existing risks of an accidental leak

or outbreak. Well, I think the beliefs on all theories are all hypotheses for the origin are unsubstantiated right now, And I've said publicly and continue to say that I think investigating the Lab league hypothesis is the responsible thing to do, as has for example, the head of the World Health Organization, the group that put out the report, downplaying that theory. That's quite helpful actually, by the way, So what I'm reading in code is that people like

me shouldn't say, oh, that's crazy. It might or might not be We don't have the data. Yeah, it's not crazy, it might be wrong. But the arguments put out so far to say that a LAB leak is way less likely than the competing hypothesis I think don't hold any water. Very good to hear. Okay, so that's a very helpful corrective.

But to answer your specific question, I spent much of the decade of the twenty tens working very hard in the sort of science policy arena on an effort to curtail experiments so called gain a function of concern, experiments that make at that time it was mostly influenza viruses more contagious or more deadly and the basis of that concern or of that activity was a calculation of risk and benefit that the science doesn't save enough lives in

expectation to be worth the possibility of creating a novel pandemic pathogen that could get out of a lab by mistake and spread, not to mention the possibility of fading deliberate attempts, the possibility that experiments like dangerous experiments could also aid deliberate attempts to misuse biological agents as weapons.

But I was focused more on the safety aspect. And one of the people who was in some ways supportive of my efforts and those of my other colleagues was Jesse Bloom, who was one of the several groups that did experiments with this coronavirus to see what would happen when you asked the virus to escape human serum, to escape the immunity and human theorem, the antibody immunity and

human sorem. And what they found was that you generate mutations very much the ones that we have been seeing in the variants that have become globally famous, for example, the four four K mutation. And that was incredibly valuable information because it allowed vaccine developers to understand that this was not just a freak that happened in one or two variants, but was actually the sort of typical thing

that happens even in a laboratory. And other mutations were also identified, so you could ask the question, would it be a good idea to now take the existing variants

that have those mutations and do that again. I would very much expect that people are doing those experiments as we speak, if they haven't already done them, And in this case, I would be supportive of doing that kind of potentially dangerous experiment because first, it has a very clear public health rationale that it's not just scientific curiosity that would be satisfied, but it would help to prepare us for vaccination and other countermeasures. And also that we

have a big problem in front of us. It's not that the virus being created there is utterly novel. The analog of that experiment is being done in people's body these around the world. So I think the incremental risk of doing the experiment is smaller and the incremental benefit of doing it is greater than those other experiments that I was very much against. That's extremely helpful. You've given us some good news and some bad news, and as always,

you're the soul of balance. Can I ask you, if you think back to where we were in February of twenty twenty, is the overall picture worse than you thought it was going to be slightly better than you thought it was going to be more or less the same. Well, I'll give you a balanced answer. So the part that is much better than I think anybody believed was possible in February of last year is that we have in large parts of the world, though not enough of the world,

substantial supplies of very effective vaccines. I mean, I saw a prediction from April from a university research group that said absolute best case scenario as of April last year was that we would be beginning around now to start rolling out vaccines, and we began rolling out vaccines four or five months ahead of that. And the vaccines are not just fifty percent effective, which was the floor set by the FDA, but up to ninety five percent effective.

That's all just stunningly good news. And to be fair, part of the reason for that good news is that the US, Brazil and the UK created a global public good by having rampaging epidemics that allowed vaccine trials to go quickly, and the whole world has us to thank for not controlling our virus very well, with the consequence that we were able to get quick answers about how well the vaccines worked, So that part, I think is

much better than expected. I think the speed with which variants have come up is a little worse than expected, and the fact that they probably do have significant consequences

for immunity, even if they don't totally defeat it. And then the most confusing part is in February, everybody was saying that the sorts of disparities we see within rich countries where the worst off are hit harder by the virus, would be true globally and the sub Sharan Africa, India, etc. Basically the global South would be hit much harder and

it would be a catastrophe. And then that didn't happen for the first year or so, and everyone started scratching their head trying to understand whether it was a reporting artifact or not true, or really was true, and if so, why, And now we see South Asia, not just India but its neighbors as well, really really struggling under a renewed epidemick, and I would say that part is just puzzling and sort of gives a real humility to scientists who thought

we understood what would happen, then it turned out not to happen, and now it's turned out to happen a year after everybody else, and I just I can't explain it. Mark, thank you as always for your clarity, your rationality, your judgment, and for putting us in the picture. I'm really grateful to you. Thank you. It's nice to talk to us. Ever, doctor Mark lipsych is always the reasonable man. He gives you the good news as well as the bad news, and in this conversation with him, he gave us a

reasonable dose of each. To start with. His top line observation is that we are really not going to reach the threshold for her immunity in the United States in the foreseeable future. Mark had warned of that possibility, indeed that probability over the last year plus, and now he says that it's pretty certain that that's the outcome that

we've reached. What does that mean in practice? Mark lays out several scenarios, the first, which he thinks is the most likely and is not a bad scenario as these things go at all is it. Vaccines function as a bridge for us, protecting those people who would be badly harmed by a first exposure to the coronavirus, while simultaneously allowing young people to be exposed to the virus for the first time at an age where it's very very

unlikely to harm them. As a consequence, we would all eventually develop our own immunities to the coronavirus, and over time, assuming that we are exposed frequently enough without it harming us, COVID nineteen would become very much like other coronaviruses out there in the world, but not much remarked because not

doing anybody very much harm. Yet, Mark also recognizes the possibility of another much more worrisome scenario in which either the immunity that we get from being exposed to the vaccine or to the virus doesn't last very long, or still worse, in which the virus mutates and evolves to a point where it escapes the vaccines that we have

for it. In those circumstances, COVID nineteen would start to look a lot more like the flu, and as we know, our flu vaccines are a bit of a challenge because they face a moving target in addressing the flu as it evolves each year, and as a result they don't successfully suppress the flu. Given that covid can be much

more harmful than the flu, that is a genuinely worrisome scenario. Ultimately, even when I pressed Mark to be a little less balanced, he gave us the reasonable balanced analysis that says that in certain respects we've done much better in this vaccine than anyone could have expected, but in other respects we've done really pretty badly, especially with reference to the fairness and justice of where the disease has harmed people and where it's likely to continue to harm people going forward

into the future. My main takeaway from listening to Mark now and the past is that we have to continue to be cautious in thinking about where we're going with respect to this disease. We might be seeing the light at the end of the tunnel, but we cannot know for sure that we are. We might be able to say that we're on our way to a return to normal, but we still cannot be utterly certain of that. Ongoing

vigilance is still going to be necessary. The challenge for all of us is to live as near to normal as we can, as little based on fear as we can, while simultaneously being rational about the risks and challenges that lie ahead. Until the next time I speak to you, I guess I'm afraid we have to continue to be a little bit safe, we have to continue to be a little bit careful, and I really hope that we can all continue to be well. Deep background is brought

to you by Pushkin Industries. Our producer is Mola Board, our engineer is Ben Tolliday, and our showrunner is Sophie Crane mckibbon. Editorial support from noahm Osband. Theme music by Luis Gara at Pushkin. Thanks to Mia Lobell, Julia Barton, Lydia Jeancott, Heather Faine, Carlie Migliori, Maggie Taylor, Eric Sandler, and Jacob Weisberg. You can find me on Twitter at Noah R. Feldman. I also write a column for Bloomberg Opinion, which you can find at Bloomberg dot com slash Feldman.

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