Herd Immunity Is Still Possible, and I'm not Ready to Give Up

F. Perry Wilson, MD, MSCE


May 19, 2021

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This transcript has been edited for clarity.

Welcome to Impact Factor, your weekly dose of commentary on a new medical study. I'm Dr F. Perry Wilson of the Yale School of Medicine.

With mask mandates lifting around the nation, there is a palpable sense of relief and hope for the future.

The end of the pandemic, long promised, feels like it is in sight, at least in the US. But what does that mean?

For most of the pandemic, herd immunity — when enough people have been vaccinated or infected by SARS-CoV-2 that the virus doesn't have enough viable hosts to continue spreading — was synonymous with the end of the pandemic. Sure, we debated about how many people would need to get vaccinated — 70%, 80%, 85% — but we had a goal in mind. Real victory.

And then the vaccination program ran headlong into the reality of American life: vaccine hesitancy, politicization, and outright conspiracy theories.

And almost overnight, it seemed like herd immunity was no longer the goal. The New York Times ran with the headline, "Reaching 'Herd Immunity' Is Unlikely in the US, Experts Now Believe."

The article summarizes reasonable points from multiple public health experts. The consensus? We won't eradicate COVID, but it will become manageable; we'll live with it, like we do the flu. It's this capitulation to an infectious disease that helps explain the new CDC guidance: We're not getting rid of COVID, but vaccines are pretty good, so let's start getting back to regular life.

Well, I am not ready to go gentle into that good night. I still believe that we can have true victory over SARS-CoV-2 and not some sort of détente. But it won't happen if we don't try, which is why I'm so worried that we keep hearing that it can't happen. And though my words here may fork no lightning, I want to outline a few reasons why I think herd immunity is still achievable.

1. Vaccine Hesitancy May Not Be as Bad as We Think

Recent polls suggest that hesitancy is actually decreasing, but we still have 30% of adults surveyed saying they won't get the vaccine. The thing is, many of these people are actually still gettable. Sure, there are the diehards, the people you'll never convince, which might be 10% of adults or so. But for the rest, it's not so black and white. And thanks to the J&J vaccine, a single shot can move someone from the unaffiliated to the vaccinated, and that's that. That's why increasing access — making getting a vaccine "stupid easy," as my kids would say — helps so much. Get it at Walmart, your local drug store, your doctor's office.

2. It's in the Math

The #2 reason we can still get to herd immunity? Math. There's nothing magical about the herd immunity number. It is tied, via the equation you see here, to the basic reproduction number of the infectious agent — that's the average number of people an infected person subsequently infects under wild-type conditions.


If, on average, an infected person infects two other individuals, herd immunity would occur when 50% of people are protected.


We think the R0 of SARS-CoV-2 is around 2.5, maybe a bit higher given the variants, but you can see that the herd immunity threshold is not crazy here; 70% really might do it.


That, of course, would include kids (who make up 25% of the US population), so this isn't trivial. But it is achievable, especially as we expand vaccination to younger ages.

But you can actually look at the herd immunity equation another way. That immunity threshold on the y-axis means that we are changing the effective reproduction number, the number of people the average infected person infects under current conditions. If you get the effective reproduction number below 1, the pandemic runs out of steam and eventually ends. Getting 70% of people vaccinated gets the effective reproduction number below 1. But you know what else lowers the effective reproduction number? All the stuff we've been doing — the masks, the social distancing. In fact, most modeling suggests that even before vaccination started, we had the effective reproduction number of SARS-CoV-2 down to around 1.2 or so.

Source: EpiForecasts

In other words, if we can't get there with vaccines alone, we can get there with vaccines and a bit of caution. So, if you are not yet vaccinated, and aren't getting vaccinated, please continue to social distance and mask up to keep the effective reproduction number low. Starve the beast. And we can still win.

3. Even if You Don't Get Vaccinated, You Can Still Become Immune

I feel like people forget about this fact. Vaccination is not the only way to achieve herd immunity. Sure, it's the preferred way, since COVID-19 is a syndrome that can result in death and long-term disability. But hey, if we can't get enough people vaccinated, infections will occur — predominantly among the unvaccinated — driving the percentage of immune people up. I mean, it's not ideal, but it does get us closer.

4. Variants May Not Be as Bad as We Think

Variants are a bit of a wild card in all of this. We know that SARS-CoV-2 mutates. It's easy to think that the emergence of novel variants will break through vaccination. We even have cases of particular variants seeming to evade particular vaccines. But we can still beat this. For one, SARS-CoV-2 mutates slowly, compared with many other viruses, as you can see here. It's just that the sheer volume of cases gives a lot of rolls of the genetic dice, so to speak.


As we get cases down, the rate of variant emergence will slow down as well.

And let's not forget the astounding speed with which the vaccines were developed. Though it took a year to get shots into arms, Moderna had a version of its current vaccine produced before a single case was documented in the US. RNA vaccines may allow us to stay on top of variants if any do break through, but my hope is that if we can get cases low enough, it won't be much of an issue.

5. Other Countries

Other countries. Okay, this is an issue. Let's say we get to 80% protection in this country one way or another. What about the rest of the world? Right now, there have been 54 doses of vaccine given for every 100 people in North America. There have been 1.7 doses given per 100 people in Africa. Rich countries are vastly more vaccinated than poorer countries, but I promise you that COVID has no idea what a GDP is.

We need to get vaccines to other countries, and I see how ongoing high rates of infections around the world can be disheartening. But herd immunity starts locally. Patches of highly vaccinated people will act as a firewall to further viral spread. In other words, worldwide herd immunity will be a patchwork of local herd immunity; we can take care of our house here with that goal in mind and simultaneously help the world get closer to the target.

I think we're at a bit of a crossroads right now. The tide of public opinion seems to be shifting the goalposts from eliminating COVID to living with it. But we still have time to choose to do the hard work — to vaccinate this country and the world — and truly defeat a global threat. It's not hopeless. It's just hard.

F. Perry Wilson, MD, MSCE, is an associate professor of medicine and director of Yale's Clinical and Translational Research Accelerator. His science communication work can be found in the Huffington Post, on NPR, and here on Medscape. He tweets @fperrywilson and hosts a repository of his communication work at

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