Are You Sure You Understand Herd Immunity?

A closer look at what it really means and some common misconceptions

The Covid-19 pandemic has made us familiar with terms more frequently used in infectious disease journals than in common parlance. But in conversations with patients, friends, and family, I’ve noticed that understanding of these terms is often inaccurate or incomplete. This is especially true when it comes to the concept of herd immunity.

Everyone seems to understand that herd immunity represents a crucial transition point for Covid-19, when the likelihood of getting infected drops and our ability to return to normal increases.

The questions around herd immunity have started to pick up as more Americans get vaccinated: How many more shots do we have to give before we achieve herd immunity? Are we already there? What about the idea that maybe herd immunity isn’t even achievable?

Before the headlines confuse your understanding of the term even more, let’s talk briefly about what herd immunity is and what people often misunderstand about it.

Defining herd immunity

For starters, let’s define it. Herd immunity (also known as community immunity) exists when a substantial proportion of the population is immune to an infectious disease—either through previous infection or vaccination—thus significantly limiting person-to-person spread.

What does ‘substantial’ mean? Well, it depends on the disease in question. For very contagious diseases like measles, over 90% of a population must be immune to limit transmission. For less contagious diseases, the number is much lower.

Seems straightforward, right? So what are the most common misconceptions when it comes to Covid-19 and herd immunity?

Misconception 1: The threshold for herd immunity is a fixed, known number

I often hear some version of “once we get 75% of people vaccinated, we can finally get back to normal,” as if the herd immunity threshold is set in stone or even definitively known.

But if you ask a dozen different infectious disease experts what the threshold is, you might get a dozen different answers. Sure, their answers will all be in a similar range — somewhere between 60–90%. And they’ll all agree it’s much higher than the percentage who’ve been vaccinated in the U.S. so far.

The reality is the herd immunity threshold isn’t a fixed number. It depends on multiple factors, which themselves aren’t fixed variables. That’s why it’s so hard to nail down exactly. But precision doesn’t matter—what matters are the principles driving it.

We know that continuing to practice public health measures like masking and distancing help drive the number down. That’s because they help prevent transmission as well.

A population that’s 50% vaccinated and abiding by mask mandates and indoor capacity limits is likely better at reducing transmission than one that’s 60% vaccinated but without these public health measures in place.

Also, more transmissible variants may raise the herd immunity threshold. With the more infectious B.1.1.7 variant causing the majority of new Covid-19 infections in the U.S., that will undoubtedly push the number higher.

Misconception 2: The herd immunity threshold is an on/off switch

Even if we don’t know the exact herd immunity threshold, it’s not as if something dramatically changes when we hit it. For the sake of argument, let’s say we reach herd immunity at 80%. It’s not as if everyone is vulnerable at 79% but the virus disappears once we cross the 80% threshold.

Every single vaccination gets us closer to herd immunity, because every percentage point increase in the population vaccinated gives the virus less opportunity to spread. As we’re currently witnessing, even with 27% of the U.S. population fully vaccinated, transmission can still occur. We continue to see 69,000 cases of Covid-19 every day on average in the U.S. Even so, the outbreak would undoubtedly be worse if fewer people were vaccinated.

Incremental increases in vaccination will get us closer to the herd immunity threshold, whatever it may be. But nothing magically changes once we cross it.

Misconception 3: The herd immunity estimate is a single country-wide number

When the herd immunity threshold is discussed in articles or on cable news, it’s usually presented as a single number along with a description of what it would take the whole country to get there. That’s just plain wrong.

As we’ve seen with nearly every aspect of this pandemic, herd immunity will be different at different times in different places.

Some communities, especially those that were harder hit by Covid-19 and also vaccinated a higher percentage of their population, will get to herd immunity faster than those who haven’t seen much of the virus or the vaccine.

We shouldn’t expect that all communities in the U.S.—or the world for that matter—will achieve anything close to herd immunity at the same time. Some communities will be safer much sooner than others. And some communities will still have high levels of virus transmission when others don’t. Expect smaller pockets of herd immunity that will hopefully coalesce as vaccination expands and the virus has fewer places to go.

Misconception 4: Covid-19 will disappear when we finally achieve herd immunity

Unfortunately, Covid-19 is with us for the long haul. Even if we get every single person in the U.S. vaccinated against Covid-19 (we won’t), the virus will still find opportunities to circulate. Herd immunity doesn’t mean the virus is no longer with us—it just means that its transmission is significantly slowed.

We’ll likely have recurring waves of Covid-19 for the foreseeable future. The virus could mutate enough to make our current vaccines less protective in the coming years. And for many, immunity may wane.

That’s okay. Sure, we may need to get another booster vaccine in a year or two. We may need to adapt as the virus hits certain communities in the future, and put in place measures to restrict the spread. It’s extremely unlikely Covid will ever again have the devastating impact it has already had on the U.S., but that doesn’t mean we will vaccinate it into oblivion.

Misconception 5: We’ll get to herd immunity if everyone who is eligible gets vaccinated

We definitely won’t vaccinate 100% of the U.S. population this year, or 90%. Probably not even 80%. Whatever the herd immunity threshold is, it’ll be very difficult to vaccinate enough people to reach it. There are already concerns that vaccine supply is starting to outpace demand, even though only 41% of the population has received at least one dose.

Basic arithmetic suggests getting to herd immunity through vaccination alone is unlikely. Since the vaccines are only available to people 16 and older, over 20% of the population isn’t even eligible to get vaccinated right now. Even if every eligible adult gets vaccinated (which they won’t), we would only get to 80%.

With the Pfizer vaccine likely to get FDA emergency use authorization for 12–15-year-olds in the coming months, the math might change, but it likely won’t make a substantial difference.

We know a significant proportion of the population won’t get vaccinated — recent surveys estimate up to 25% of eligible Americans will refuse their shot. Even if those with natural immunity (acquired from previous infection) help fill that gap, there will still be a significant portion of the populace vulnerable to infection with Covid.

We’re thankfully doing an incredible job with vaccination in the U.S. Every adult American is now eligible for a shot, and we’ll have enough vaccine supply for every adult by the end of May 2021. And the more we vaccinate, the closer we’ll get to herd immunity. But there’s a very good chance vaccination alone won’t get us there. Every shot makes a difference, but there’s no silver bullet.

As it has been for the last year, the best approach is still a combination of wearing masks, distancing, and doing everything we can to prevent transmission while we work to get as many people vaccinated as quickly as possible. This will dramatically reduce the amount of virus circulating in our communities, making infection much less likely even if we haven’t yet hit the herd immunity threshold. Even then, Covid will still be with us. But in this next phase, it won’t prevent us from safely seeing our friends, going out to a restaurant, or hugging our grandparents.

I’m writing weekly for Medium about my experiences as an emergency medicine doctor during the Covid-19 pandemic. You can read my previous posts on vaccine passports, why this summer will be really weird, and more, here.

NYC ER doctor | Ebola Survivor | Director of Global Health in Emergency Medicine at Columbia University | Public Health Professor | Doctors Without Borders BoD

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