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Vaccines Q & A

No Magic Number for Herd Immunity

Declaring success regarding the commonly misunderstood concept is not as simple as the public believes.

Shaun Truelove, Assistant Scientist
July 2, 2021

Public misperceptions about herd immunity have persisted through much of the pandemic, providing an additional reason for people to cite when refusing to get vaccinated. Yet the United States needs more people to get vaccinated, not fewer, because vaccination rates have stagnated just as the more highly transmissible Delta variant is spreading. Just because others are vaccinated does not protect the unvaccinated from infection. Israel outpaced the world for weeks in getting nearly 58% of its citizens fully vaccinated by July. Yet the nation and others are imposing new restrictions due to outbreaks driven by the Delta variant.

What is missing from the public’s perception of herd immunity?

‘Herd immunity’ is one of the terms that epidemiologists have used for years that the public suddenly has become aware of. The public perception is sometimes that there’s this magic threshold that we need to hit, that there’s a specific target – and that at that point we’re done, we’re safe, and life is normal again.

But that is an oversimplification.

This is something we have struggled with as epidemiologists even before the pandemic because we use a concept of herd immunity that assumes homogeneity – which essentially means we assume people behave like particles and that every single particle can bounce into every other particle in a population. We calculate a herd immunity threshold based off of what we think the transmissibility of a given pathogen is in a certain area. But that doesn’t work as cleanly as everyone wants, because people don’t really behave like particles – we have social networks, geographic restrictions, and age-contact patterns that all contribute to make disease transmission varied across populations.

“Places with high vaccination coverage will see continued declines in COVID-19 even with the variant”

The message has been convoluted in that certain populations think they’ve achieved herd immunity and that everything is fine and safe because they have “enough” vaccination or natural immunity. But in reality, we have pockets of susceptibility and pockets of immunity. And with a global pathogen, the potential for reintroduction into those susceptible pockets is high and will continue to persist for a long time.

Is herd immunity more easily calculated for smaller regions than for entire nations?

There are many different factors that can contribute to reaching a point at which enough immunity exists in a certain population to interrupt disease transmission. Is the population in a city or a rural area? What’s the vaccination level? How frequently do people interact with each other? How connected is your population to others? Some people are in contact with a lot of people. Some people are in contact with few people. Some people have more contact with the most susceptible people than others do.

We have done work in the past with measles highlighting how the degree to which susceptible individuals cluster together will quantitatively increase the herd immunity threshold.

The amount of variability in these factors across a country or even state makes calculating an accurate herd immunity threshold extremely challenging. And even for smaller populations, like in counties, prescribing a single, concise threshold target would likely be inaccurate.

Herd immunity should really be thought of as a gradient, a range of immunity at which we expect to see declines in transmission, but not where transmission risk ends completely and immediately. But we also need to remember that even this gradient can move with a changing pathogen. We are seeing this now with the increase of the Delta variant in the U.S.

What do projections about the virus tell us about herd immunity?

Some of our U.S. simulations reveal a divide between locations. For places with high vaccination coverage, we are projecting continued declines, even with the Delta variant. For other places there is the potential for increases in cases and deaths because there is sufficient immunity, particularly from vaccination.

“Outbreaks will continue in states with lower vaccination coverage”

It's even more complicated as new variants emerge that we had not captured in our projections.

The projections we conduct for the COVID-19 Scenario Modeling Hub estimate expected cases, deaths, and hospitalizations over the next six months in the United States. These projections attempt to account for what we know about the virus and the pandemic at the time at which each round of projections is produced. This includes:

  1. The increasing transmissibility coming from more infectious variants (e.g., Delta variant)
  2. Current vaccination coverage levels
  3. Expected vaccination coverage levels based on hesitancy and saturation
  4. Natural immunity from past infections
  5. Control measures and behavior from the past and what is expected in the future (i.e., non-pharmaceutical interventions).

The projections do not explicitly detail herd immunity, but they do speak to it, and the challenges of reaching it. The projections provide a glimpse into the complexity of herd immunity.

In a high transmissibility scenario, some states are expected to have substantial resurgences, others not at all. This demonstrates how some are reaching herd immunity at a broad scale. They may have continued cases, including clusters, but won’t be seeing the outbreaks that other states with lower vaccination coverage and natural immunity are expected to have.

With each new more transmissible variant, the level of immunity required to interrupt transmission and reach herd immunity increases.

What is most worrisome about the plateau in U.S. vaccination rates?

We expect to see a slowdown in vaccinations as more and more people get vaccinated. When we started, we were targeting the people who were most at risk and they were willing to get vaccinated early. Now I think we have a large portion of the population that is being very casual about getting vaccinated, saying they’ll get to it eventually. They're not necessarily against vaccination, but they're just not prioritizing it or they're waiting for full approval from the Food and Drug Administration. For a variety of reasons, others have been waiting for authorization of vaccines like Novavax, which use a different mechanism. I was a little surprised that we stagnated as early as we did with vaccinations, but hopefully, concerns about variants will compel more people to get vaccinated.

Shaun Truelove, Assistant Scientist

Infectious Disease Epidemiology, Department of International Health, Bloomberg School of Public Health