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As Covid-19 vaccines are being rolled out across the U.S., Americans seem to be heaving a collective sigh of relief.

Yes, it will take months to get the vaccine to everyone. Yes, there were tremendous gaps in the Trump administration’s plans to distribute the vaccines, including promising doses that didn’t exist.

But it seems as if there is light at the end of the tunnel. As long as we maintain social distancing, keep wearing masks, and washing our hands, it feels to many as though we can hold on until we get vaccinated.


I’m sorry to be writing the words that follow, but here they are: We can’t vaccinate our way out of this pandemic. And the myopic focus on achieving herd immunity through mass vaccination may even make it tougher for America — and the world — to defeat Covid-19.

Don’t get me wrong: Mass vaccination is essential. But herd immunity is a numbers game. It is defined as the point at which community spread of a disease stops because unprotected individuals are surrounded by a “herd” of people who are immune to infection, making it difficult, if not impossible, for infected people to pass on the disease.


Many experts have said we will achieve herd immunity when about 70% of the population is immune to SARS-CoV-2, the virus that causes Covid-19, either through vaccination or by having had Covid-19.

How do we reach that number?

It’s harder than it seems. For starters, while the Pfizer/BioNTech and Moderna vaccines showed about 95% efficacy in the clinical trials, vaccine effectiveness — how well a vaccine performs under real-world conditions — is likely to be lower for several reasons. One is that the people who participate in clinical trials are an imperfect representation of the whole population. They tend to be healthier, and younger. Real-world factors such as vaccine transportation and storage can also reduce vaccine effectiveness.

Say the Moderna and Pfizer vaccines now being given across the country achieve 90% effectiveness. Vaccinating 70% of U.S. residents puts us at 63% immunity. So, we’ll need to vaccinate a full 80% of the population to reach the herd immunity threshold.

Additional vaccines are starting to be approved. Some of them have lower efficacy. For instance, the AstraZeneca vaccine has about 70% efficacy, and Johnson & Johnson has reported that its one-dose vaccine has 66% efficacy. Their real-world performance could be lower still. If these vaccines become part of the mix in the U.S., actual protection will be lower than the estimated 90% we’d get from just the Moderna and Pfizer vaccines.

There are other barriers to achieving herd immunity. Vaccine uptake — how many people actually get vaccinated — is far below the level we need, in part because Covid-19 beliefs have been politicized in the U.S. and a percentage of the population doesn’t even believe the disease is real. In a Kaiser Health News survey released near the end of January, 13% of Americans said they would “definitely not” get vaccinated, 7% would take the vaccine only if it was “required,” and another 31% would “wait and see how it’s working” before getting vaccinated. Not encouraging numbers for those hoping for a quick journey to herd immunity.

Even when ample vaccine supplies are restored — perhaps by President Biden invoking the Defense Production Act — other factors will further drive down the number of people who get vaccinated. Eligibility factors currently exclude approximately 25% of U.S. residents from Covid-19 vaccination. The Pfizer vaccine can be administered only to those age 16 and up; for the Moderna vaccine, it’s those 18 and up. This represents approximately 20% of the population. Furthermore, although the CDC says that pregnant people may get vaccinated, it stops short of a clear recommendation. The decision is a “personal choice” left up to individuals and their health care providers.

Excluding those currently ineligible for vaccination against SARS-CoV-2 due to age or other conditions leaves 75% of Americans with no restrictions on vaccination. Factoring in the 13% of Americans who definitely don’t want the vaccine and the 7% who would get it only if it was required means just 49.5% of Americans would have immunity in the near future. If half of those who are in a wait-and-see mode don’t get vaccinated — another 15% of the population — then we are looking at just 40% vaccine coverage of the currently eligible population, far below the 70% needed for herd immunity. And that’s even before considering that real-world vaccine effectiveness will be below clinical trial levels.

The young people who aren’t cleared to get the Moderna and Pfizer vaccines have proven to be highly efficient asymptomatic spreaders of Covid-19. Leaving this population unprotected will enable the disease to continue to spread widely.

Finally, we don’t yet know the durability of the immune response to the various vaccines. It may persist. Or it may wear off, leaving people vulnerable after they’ve been vaccinated and creating conditions for new outbreaks.

If my years of global health work on the HIV/AIDS epidemic has taught me anything, it’s that even the best laid plans can’t anticipate every challenge. To vaccinate 75% of the U.S. population, approximately 248 million people — that’s nearly 500 million doses — are needed. And it means we need to be vaccinating nearly 2 million people a day so all of them are immune by the fall of 2021. As I write this, we’re vaccinating only about 1 million people a day. At that pace, Reuters estimates it would take until April 2022 for 75% of Americans to receive at least their first vaccine dose.

And that’s only if everything goes well logistically (it won’t) and if there are no further mutations in SARS-CoV-2 that make combating it more difficult (there will be).

It’s time to stop promoting the myopic belief that the unrealistic goal of herd immunity can be achieved in 2021 and start looking to reinforcing all aspects of the health care response as we start to concede that Covid-19 will become an endemic disease that will continue to lurk in the population. For the foreseeable future, that means continued physical distancing; occupancy limits in restaurants and other retail establishments; replacement of physical menus with smart phone-based menus to prevent surface spread of the virus, and more.

We’ll also need to monitor people who have been vaccinated to gauge the durability of the immune system’s response and whether booster shots are necessary, as they are for tetanus and diphtheria. Finally, our nation’s public health infrastructure will need to be bolstered, putting in place new protocols to monitor for new variants of the virus as soon as they emerge.

Can we defeat Covid-19? We can and we will. But setting sights on a near-term goal of achieving herd immunity ignores the math that governs the spread of disease. That approach is going to take a while. To get past Covid-19, we need to use all the tools available.

Iain MacLeod is the co-founder and CEO of Aldatu Biosciences of Watertown, Mass., which develops novel viral diagnostics, including those for pathogens such as SARS-CoV-2, and a research associate at the Harvard T.H. Chan School of Public Health.

  • My piece shouldn’t be interpreted as an anti-vaccination viewpoint – that’s very much *not* the case. Vaccines are a fundamental piece in our collective response to this pandemic. I want our governments and public health authorities to acknowledge that they need to start thinking beyond vaccination programs and stop deluding the public into thinking that herd immunity is going to be achieved in the near future, be it through natural or vaccine-derived immunity. They have to acknowledge that there are still many unknowns about (whether vaccines produce sterilising immunity, and if so, what is the durability of that response), that not everyone is currently eligible for the vaccine, and even those that are, an unnervingly high proportion may choose not to get vaccinated.

    Our governments have reacted too late at every step of this crisis and with broken promises of short term goals that will end the pandemic. We need to compel them to think beyond that.

    Resurgence of COVID-19 in Manaus, Brazil, despite high seroprevalence
    “In Manaus, Brazil, a study of blood donors indicated that 76% of the population had been infected with SARS-CoV-2 by October, 2020. The estimated SARS-CoV-2 attack rate in Manaus would be above the theoretical herd immunity threshold (67%), given a basic case reproduction number (R0) of 3.

    In this context, the abrupt increase in the number of COVID-19 hospital admissions in Manaus during January, 2021 (3431 in Jan 1–19, 2021, vs 552 in Dec 1–19, 2020) is unexpected and of concern. After a large epidemic that peaked in late April, 2020, COVID-19 hospitalisations in Manaus remained stable and fairly low for 7 months from May to November, despite the relaxation of COVID-19 control measures during that period.”

    Do antibody positive healthcare workers have lower SARS-CoV-2 infection rates than antibody negative healthcare workers? Large multi-centre prospective cohort study (the SIREN study), England: June to November 2020

  • Just as Brian said, where are the ALREADY INFECTED in this calculation. The CDC back in October said we were catching 1 in 8 cases, especially since so many are completely asymptomatic (so probably about 150 million have had COVID even going by 1 in 5 standards). And, as Brian also points out, if we reach a point where almost nobody is getting hospitalized with severe symptoms, then complete herd immunity is irrelevant because we will have effective herd immunity.

    How can anyone even write this trash and tell people to “do the math”? The irony is, this author is screwing up the math completely by not addressing all the obvious variables…

  • Speaking of “ignoring the math,” where does naturally-acquired immunity fall in your equation? Because it isn’t there, rending this whole piece virtually useless. What’s more, like other commenters have said, you are basically manipulating people’s lack of understanding of the trial read outs by playing on the top-line efficacy results, ignoring the facts that this is against symptomatic disease (symptoms of which include a sniffle or a fever), that all vaccines have shown 100% efficacy against hospitalizations and death, that the JnJ trial results of 66% were a combination of three regions (results were 72% in the U.S.).

    Seriously, take this op-ed and trash it where it belongs.

    • Amen, Brian! Another clear thinker who can actually do basic math and use common sense. As I pointed out in my post, the CDC back in October said we were catching about 1 in 8 cases, so A LOT of people have already had COVID. And it’s a great point you’re making that “herd immunity” can’t just be defined as nobody ever having a positive COVID test because if almost everyone gets only a mild to moderate case, that’s effectively herd immunity (remember, some of these vaccine trials define a “moderate” case as having just ONE OR TWO symptoms of infection without serious complication).

    • Agree with Brain who saved me from similar words. Herd immunity is achieved via both vaccination and natural immunity. Some portion of society has enough natural immunity without having COVID exposure and best guesses are 30% of society has partial or strong immunity from prior infection with a 10-15% reinfection rate. Add to that non immunized kids who have a low complication rate, and by the summer hospitalizations will be very low and deaths only in the vulnerable who made to poor decision not to vaccinate. Another positive factor is the minority and other groups that are avoiding vaccination are the same groups that have a high infection rate, which increases reaching goal of herd immunity.

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