Watch the AMA's daily COVID-19 update, with insights from AMA leaders and experts about the pandemic.

In today’s COVID-19 update, AMA Chief Experience Officer Todd Unger and AMA CEO James Madara, MD, talk about the association's pandemic response and the math and science behind herd immunity and COVID-19.

Learn more at the AMA COVID-19 resource center.


  • James Madara, MD, chief executive officer, AMA

AMA COVID-19 Daily Video Update

AMA’s video collection features experts and physician leaders discussing the latest on the pandemic.

Unger: Hello, this is the American Medical Association's COVID-19 update. Today, we're talking to Dr. James Madara, the AMA's CEO in Chicago, about the AMA's pandemic response and the math and science behind herd immunity and COVID-19. I'm Todd Unger, AMA's chief experience officer in Chicago. Dr. Madara, let's start by talking about the AMA's pandemic response. How was the AMA positioned to address many of the challenges that we've seen with COVID-19?

Dr. Madara: Well, thanks, Todd. Let me first say that the coronavirus pandemic presented a challenge unlike anything we'd faced in this country for generations, but we also have to be clear that COVID-19 didn't create the problems in our health system that we now see. It revealed them in a way that can't be ignored any longer. So, having said that I'm really glad that the AMA was able to respond effectively and do so because our focus was on the right things prior to the pandemic, things that included health equity, hypertension, physician wellness, and really importantly, as you pointed out, our standing for data, for science, and for evidence. These are all things we had been working on in some instances for long before COVID was a pandemic.

Unger: Well, you mentioned standing behind data and science, which has been no small challenge, given the sheer amount of misinformation that we've seen throughout the pandemic. One idea I'd like to focus on as something we've been hearing a lot about recently, and that's herd immunity. One thing we hear is that we might be close to achieving herd immunity, and I want to ask you, is that true? And how do we explain what the real situation is?

Dr. Madara: Well, to achieve herd immunity, and that's where transmission of the virus is impeded because so many people have immunity breaks the chain of people available to catch the virus. To achieve that herd immunity state, we can estimate that something on the order of 70% of the population needs to be immune, and they can be immune either following infection or by successful vaccination. Now, if we take the example of measles, it's highly contagious, one needs about 95% of the population being immune to achieve herd immunity. So, guesstimating 70% of the population being immune seems to be reasonable, a reasonable estimate for COVID-related herd immunity. Now—

Unger: Are we anywhere near that right now?

Dr. Madara: We're nowhere near that. So let's first estimate what percent of our population has been infected with COVID, and keep in mind, these are all guesstimates right now. We call this a novel virus for a reason. We've just had under eight million cases in the U.S. documented by testing, and that's of this morning, it was about 7.9, so we'll round up eight million. And we know that that's likely an undercount, so we need some multiple of that eight million, and last week, the CDC gave us a clue to what the undercount was. And what the CDC did was take a scientific sample of individuals in the U.S. population for antibody responses to estimate the population of percent that had been infected, not just that were documented infected, but infected and created an immune response. And what they found was just under 10%, about 9% of the population was likely infected.

So, what does that 9% come to? That comes to more like 30 million that had been infected, almost four times that of documented infections. So, now, let's take that 30 million and assume that over the next few months, say six months, this history repeats itself and 30 million more are infected. So, the some six months out would be something more like 60 million Americans infected, and that's just under 20% of our total population, and we're trying to get that 70% for herd immunity. So—

Unger: We have a long way to go is what you're saying.

Dr. Madara: We'd need about 230 million Americans with an immunity, and that's 170 million more than that 60 that we estimated from the beginning of the pandemic to, say, six months from now.

Unger: So, you mentioned before, getting to herd immunity is both about those that have already had the infection and then those who have received an effective vaccine, so let's talk now about the mechanics and the math behind how a vaccine would help get us closer.

Dr. Madara: Okay. So, first of all, the good news, vaccine will get us closer, but it's going to be a while, probably, until we get there. If you look at the math we just went through, in addition to that estimated 60 million who would have the acquired immunity, we assume, from infection, six months from now we need to capture, say, another 170 million through vaccination. There are 270 million uninfected that are COVID naive, so that sounds pretty good. We need 170 million, we have 270 to draw from. But there are other caveats to consider. The first one is that vaccines, and specifically those for airway viruses, aren't 100% effective, and that's why both the FDA and the CDC, while they would hope for a vaccine that might be, say, 70% effective, they have set the acceptable target at 50%.

Unger: So, that's an important factor, then, as you think about how we get to that number. So, when you factor in the effectiveness part, that means that more people have to be vaccinated to achieve that and close that gap, so to speak. Can you talk about that?

Dr. Madara: Yeah. So, of those 270 million uninfected COVID-19 unexposed patients, we need about 135 or 190 million of that, that we would get through an effective vaccine. So, only that high optimistic end of this range provides that extra 170 million needed to be vaccinated and added to those 60 million infected so we can get the herd immunity. But not factored in, there's also a subgroup of Americans that tend to refuse vaccines, unfortunately, and this makes the math even more daunting.

So, for example, if 20% of the population refused the vaccine, we're now out of the range that gets us to herd immunity, and what can those vaccine resistant folks be like? What are they in the population? Well, for comparables, it's true that 90% of children get the MMR vaccine, measles, mumps and rubella, but for the annual flu vaccine, less than 50% of the population gets that vaccine. And for the senior population, only about a third gets the herpes zoster or shingles vaccine, so vaccine hasn't seen really complicates this problem further.

So, in sum, we could take this as like a 30,000 foot macro guesstimate, achieving herd immunity quickly seems to require a result that's near perfect in many dimensions. And this, then, puts a light on our need for continued development of therapeutics, such as monoclonal antibodies, continued preparation by the health care system for an influx of patients that we expect over the next six months, and then a continued push to better know where we are in this pandemic, and that is achieved through expanded testing. Now, it's probably more likely than not that even with a successful vaccine or vaccines, there still could be substantially more infection occurring in the population before herd immunity is achieved.

Unger: Well, I think it's really helpful to walk through that math and understand the challenge that we'll be facing in terms of achieving herd immunity. Is achieving that herd immunity, is it true of all communities, or could certain areas or communities achieve herd immunity before others?

Dr. Madara: Yeah, absolutely. I mean, those estimates I gave with this general math, it's just a general average direction, but for example, there could be communities where the infection rate was so high that herd immunity had been achieved. Probably not yet, though, even in communities where the infection rate is thought to have been high, it's probably more like the 25% range. But how are we going to find that out? The challenge is we don't know since we haven't done enough testing.

Unger: We've talked about the challenges being the effectiveness of the vaccine, and then we have resistance to getting the vaccine. What are some of the other challenges to achieving herd immunity?

Dr. Madara: Well, another one is understanding what the immune response to this virus is. We know, for example, that 16 million, say, have been documented six months from now, multiplying the 30 million that we know about today. But we don't know the other 44 million in that group, that's the proposed that if you know that 4X times what has been shown by testing. We don't know who they are specifically. So, there's another 44 million that would have to be tested in vaccine, and vaccinated, rather. So, in the absence of good and broad immune serologic testing, essentially, we would add that 44 million to the pool of those that need vaccination, so that's another challenge that limits our understanding of immunity. And we also don't know how long the immune response lasts after the infection or the vaccination.

For other viruses related to this family, the response typically lasts a few months to a couple of years. We don't know that. This is, again, a novel virus, but we would probably, having to guess, we would guess that it would not be lifetime immunity.

Unger: Well, when you take all of those things together, what would you say the key takeaways, then, are for physicians who are trying to message all of this to their patients?

Dr. Madara: First is this is not going to be easy. We wondered back in March and April if we would be back by the summer. Didn't work out that way. We will continue to have struggles, and there are no magical solutions and that includes vaccines. We can't imagine when there's a vaccine that we're always back to normal now. That's unlikely to be the case.

It's also important that we are transparent in our vaccine development. So, when a vaccine is approved, physicians feel confident in recommending it, and patients can feel confident in getting it. And until enough people receive the vaccine or have developed antibodies following infection, we need to keep following those public health measures, washing hands, keeping distance, avoiding crowds, wearing masks. We need to, as a community, control the spread.

And it's important to remember in this regard when we're laying out these challenges, that we as a nation have overcome challenges even probably greater than our current set. And while this could go on for some time, let's be aware of what this current period actually is. It's long and drawn out, but ultimately, it's a temporary time to be bridged in some way. And key to that bridge building for physicians is to be mindful and explicit in recognizing your own personal and emotional needs, and to get to that new normal, this will have to be key that we take care of ourselves through this most acute phase of the pandemic.

Unger: Well, Dr. Madara, thank you so much for that thorough walkthrough of the math that it will take to get us to the place of herd immunity and for your perspectives on the topics of transparent vaccine development. I encourage everyone to check out the videos that we produced from webinars with the FDA and CDC on AMA's YouTube channel.

That completes our COVID update for today. Again, thanks Dr. Madara for your leadership at the AMA and in the medical community and for being with us here today. For updated resources on COVID-19, visit Thanks for joining us. Please take care.

Disclaimer: The viewpoints expressed in this video are those of the participants and/or do not necessarily reflect the views and policies of the AMA.

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