Waiting for herd immunity

Vaccinations against COVID-19 are now open to all Utahns 16 and up, with the goal of achieving “herd immunity”—a point at which enough people are immune to COVID-19 that the virus can no longer spread. But we are all a part of many herds: our families, towns and our University of Utah campus community.

So, as immunity continues to build at different rates in different places, how should we navigate amongst our herds while we wait for the nation and the world to reach full herd immunity?

We spoke with Hannah Imlay, assistant professor of infectious disease, to learn more.

Hannah Imlay, M.D., M.S.

To begin, can you give us a brief explanation of the concept of herd immunity?

Herd immunity is what happens when most of a community becomes immune to a virus or other infectious agent. Because so many people are immune the virus (for example) cannot spread from person to person anymore. When there are simply not enough susceptible people for the virus to continue to spread, it dies out and no longer circulates in that community. That is herd immunity.

Importantly, not all people in those communities have to be immune. The population dynamics of having so many people who are immune is such that infection is prevented from spreading even if not all people are immune. This is super important for infants, elderly people or immune-compromised people in the community who may not be able to develop immunity.

How does the concept apply to smaller herds or communities, like our U campus community?

To reach true population herd immunity, the vaccine needs to be offered and accepted by all communities. Since college students are younger, they are less likely to suffer life-threatening outcomes related to COVID-19. But vaccination is still very important in the college community to end the pandemic for a number of reasons:

  • There are young and healthy people who do get seriously sick or die from COVID-19; in addition, although it is not as common, many college students are older or do have chronic medical conditions putting them at high risk for bad outcomes.
  • Many young and healthy people still may have a long duration of symptoms following a COVID-19 infection.
  • The spread of COVID-19 within the college community leads to spread outside of the college community—to families of students, to faculty, to the surrounding communities, etc.—and vaccination is needed to prevent the spread of the virus rather than just disease.

Can herd immunity be achieved in some segments of the larger population before others?

Yes—a very good example of this is herd immunity to measles. Measles is one of our most contagious viruses; however, because of very high levels of herd immunity due to vaccination, we do not have circulating measles anymore. However, there are pockets of our global and national population where immunity is not as high, so we have seen outbreaks in those communities when cases of measles are introduced.

We will have to see if this is possible for COVID-19. It is possible that we’ll have immunity in some communities, but ongoing cases in others. It will all depend on how quickly and completely we are able to vaccinate.

How does the decision by an individual to be vaccinated or not affect the herd?

In a large immune herd, lack of immunity from one or two people may still maintain herd immunity. However, many people choosing to be unvaccinated may result in a lack of herd immunity. From studies done so far, we estimate that we would need 70-90% of communities to be immune to achieve herd immunity, which is a high proportion. We already know that in every community there are a portion of people who can’t achieve immunity because of compromised immune systems. We need herd immunity to protect those people.

What should U community members, regardless of vaccination status, keep in mind as they visit people and travel to places with varying levels of immunity?

While most of the population is partially vaccinated we need to keep in mind many of the same COVID concepts as before we had any vaccines. Mixing in large groups, being in unventilated spaces and going out while sick can easily overwhelm whatever defenses we have from partial vaccination.

We are all looking for community cases to go down—decreasing (or zero) community cases is how we can see the effects of herd immunity. While cases are still high, we need to maintain masking, handwashing and limiting socialization with other households.