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What Is Herd Immunity and How Can We Achieve It With Covid-19

Gypsyamber D'Souza
David Dowdy

A new version of this article is available. This text is presented for archival purposes only.

When the coronavirus that causes COVID-19 first started to spread, virtually nobody was immune. Meeting no resistance, the virus spread quickly across communities. Stopping it will require a significant percentage of people to be immune. But how can we get to that point?

In this Q&A, Gypsyamber D’Souza, PhD ’07, MPH, MS, and David Dowdy, MD, PhD ’08, ScM ’02, explain how the race is on to get people immune by vaccinating them before they get infected.

What is herd immunity?

When most of a population is immune to an infectious disease, this provides indirect protection—or population immunity (also called herd immunity or herd protection)—to those who are not immune to the disease.

For example, if 80% of a population is immune to a virus, four out of every five people who encounter someone with the disease won’t get sick (and won’t spread the disease any further). In this way, the spread of infectious diseases is kept under control. Depending how contagious an infection is, usually 50% to 90% of a population needs immunity before infection rates start to decline. But this percentage isn’t a “magic threshold” that we need to cross—especially for a novel virus. Both viral evolution and changes in how people interact with each other can bring this number up or down. Below any “herd immunity threshold,” immunity in the population (for example, from vaccination) can still have a positive effect. And above the threshold, infections can still occur.

The higher the level of immunity, the larger the benefit. This is why it is important to get as many people as possible vaccinated.

How have we achieved herd immunity for other infectious diseases?

Measles, mumps, polio, and chickenpox are examples of infectious diseases that were once very common but are now rare in the U.S. because vaccines helped to establish herd immunity. We sometimes see outbreaks of vaccine-preventable diseases in communities with lower vaccine coverage because they don’t have herd protection. (The 2019 measles outbreak at Disneyland is an example.)

For infections without a vaccine, even if many adults have developed immunity because of prior infection, the disease can still circulate among children and can still infect those with weakened immune systems. This was seen for many of the aforementioned diseases before vaccines were developed.

Other viruses (like the flu) mutate over time, so antibodies from a previous infection provide protection for only a short period of time. For the flu, this is less than a year. If SARS-CoV-2, the virus that causes COVID-19, is like other coronaviruses that currently infect humans, we can expect that people who get infected will be immune for months to years. For example, population-based studies in places like Denmark have shown that an initial infection by SARS-CoV-2 is protective against repeat infection for more than six months. But this level of immunity may be lower among people with weaker immune systems (such as people who are older), and it is unlikely to be lifelong. This is why we need vaccines for SARS-CoV-2 as well.

What will it take to achieve herd immunity with SARS-CoV-2?

As with any other infection, there are two ways to achieve herd immunity: A large proportion of the population either gets infected or gets a protective vaccine. What we know about coronavirus so far suggests that, if we were really to go back to a pre-pandemic lifestyle, we would need at least 70% of the population to be immune to keep the rate of infection down (“achieve herd immunity”) without restrictions on activities. But this level depends on many factors, including the infectiousness of the virus (variants can evolve that are more infectious) and how people interact with each other.

For example, when the population reduces their level of interaction (through distancing, wearing masks, etc.), infection rates slow down. But as society opens up more broadly and the virus mutates to become more contagious, infection rates will go up again. Since we are not currently at a level of protection that can allow life to return to normal without seeing another spike in cases and deaths, it is now a race between infection and injection.

What are the possibilities for how herd immunity could play out?

In the worst case (for example, if we stop distancing and mask wearing and remove limits on crowded indoor gatherings), we will continue to see additional waves of surging infection. The virus will infect—and kill—many more people before our vaccination program reaches everyone. And deaths aren’t the only problem. The more people the virus infects, the more chances it has to mutate. This can increase transmission risk, decrease the effectiveness of vaccines, and make the pandemic harder to control in the long run.

In the best case, we vaccinate people as quickly as possible while maintaining distancing and other prevention measures to keep infection levels low. This will take concerted effort on everyone’s part. But if we continue vaccinating the population at the current rate, in the U.S. we should see meaningful effects on transmission by the end of the summer of 2021. While there is not going to be a “herd immunity day” where life immediately goes back to normal, this approach gives us the best long-term chance of beating the pandemic.

The most likely outcome is somewhere in the middle of these extremes. During the spring and early summer (or longer, if efforts to vaccinate the population stall), we will likely continue to see infection rates rise and fall. When infection rates fall, we may relax distancing measures—but this can lead to a rebound in infections as people interact with each other more closely. We then may need to re-implement these measures to bring infections down again.

Will we ever get to herd immunity?

Yes—and hopefully sooner rather than later, as vaccine manufacturing and distribution are rapidly being scaled up. In the United States, current projections are that we can get more than half of all American adults fully vaccinated by the end of Summer 2021—which would take us a long way toward herd immunity, in only a few months. By the time winter comes around, hopefully enough of the population will be vaccinated to prevent another large surge like what we have seen this year. But this optimistic scenario is not guaranteed. It requires widespread vaccine uptake among all parts of the population—including all ages and races, in all cities, suburbs, and countrysides. Because the human population is so interconnected, an outbreak anywhere can lead to a resurgence everywhere.

This is a global concern as well. As long as there are unvaccinated populations in the world, SARS-CoV-2 will continue to spread and mutate, and additional variants will emerge. In the U.S. and elsewhere, booster vaccination may become necessary if variants arise that can evade the immune response provoked by current vaccines.

Prolonged effort will be required to prevent major outbreaks until vaccination is widespread. Even then, it is very unlikely that SARS-CoV-2 will be eradicated; it will still likely infect children and others who have not been vaccinated, and we will likely need to update the vaccine and provide booster doses on some regular basis. But it is also likely that the continuing waves of explosive spread that we are seeing right now will eventually die down—because in the future, enough of the population will be immune to provide herd protection.

What should we expect in the coming months?

We now have multiple effective vaccines, and the race is on to get people vaccinated before they get infected (and have the chance to spread infection to others). It is difficult to predict the future because many factors are at play—including new variants with the potential for increased transmission, changes in our own behavior as the pandemic drags on, and seasonal effects that may help to reduce transmission in the summer months. But one thing is certain: The more people who are vaccinated, the less opportunity the virus will have to spread in the population, and the closer we will be to herd immunity.

We have seen that the restrictions needed over time have varied as preventive measures have worked to drive infection rates down, but we have also seen these rates resurge as our responses have relaxed. Once we get enough people vaccinated to drive down infection rates more consistently, we should be able to gradually lift these restrictions. But until the vaccine is widely distributed and a large majority of the population is vaccinated, there will still be a risk of infection and outbreaks—and we will need to take some precautions.

In the end, though, we will build up immunity to this virus; life will be able to return to “normal” eventually. The fastest way to get to that point is for each of us to do our part in the coming months to reduce the spread of the virus—continue to wear masks, maintain distance, avoid high-risk indoor gatherings, and get vaccinated as soon as a vaccine becomes available to us.

Gypsyamber D’Souza is a professor and David Dowdy an associate professor in Epidemiology at the Bloomberg School.