Advance with MUSC Health

Herd Immunity and COVID-19: When will our herd be protected?

Advance With MUSC Health
July 13, 2020

As we write this column in early July, COVID-19 is rampant in the U.S. with 42 states reporting increasing new cases. We are all eager for the time when we will not be consumed with worry about getting it ourselves. In fact, everyone asks: “When will we get back to normal?” Meaning, when can we quit worrying about being infected with this virus during the current pandemic. The answer to that question is “never.” But there is another answer that is more acceptable, and that is “when the community we live in has developed ‘herd immunity.’”

What is herd immunity?

Herd community is an epidemiologic term that refers to protection from infection to an individual because those others living around the person (the herd) have developed an immunity and therefore cannot catch and spread the virus. When living among enough people who have immunity, the viral illness cannot spread because there are not sufficient susceptible people to allow spread of the virus. The figure illustrates the concept of herd immunity.

Chart showing how covid 19 spreads in communities with and without herd immunity. 
Figure 1 Two scenarios are depicted here. The left panel shows how COVID-19 spreads (arrow) from an infected person (black circle) to susceptible people (open circles.) Typically, 1 person will infect 3 others and spread the disease widely in the unprotected population. The right panel shows a population with herd immunity. The infected person cannot spread (blocked arrow) to other people because they are immune to the infection. Thus the virus cannot spread because 2/3 of the people in the community have immunity and do not allow the virus to spread to those who are unprotected. Thus the susceptible people cannot acquire the virus because the virus is not transmitted due to herd immunity.

Herd immunity with COVID-19

To achieve herd immunity to COVID-19 in a community, a sufficient number of people in the community must not be able to contract the virus and thus spread it to others. There are several factors that determine that number. First, the virus is generally considered to spread from one person to three others. To inhibit that spread there is a mathematical formula scientists have derived that predicts the number of people in the population who have immunity that is necessary to stop the spread. For COVID-19 it is believed that 67% or 2/3 of the people in a community must have immunity to keep the other 1/3 from getting infected. This is when herd immunity will be established with COVID-19. So for us to expect COVID-19 to stop spreading, 2/3 of the population must have immunity to it and thus not be able to spread it. Thus, we are in for a very long wait until we have a community in which either the majority have been vaccinated or have acquired the disease. Patience is just beginning to be exercised.

How does a population develop immunity?

There are fundamentally two ways for immunity to be acquired in a population. First is to have contracted the disease and the other is to be immunized with a specific vaccine for COVID-19. In either case, immunity exists and the virus will not spread to someone who has had the disease within the past year or more (it is unknown how long natural immunity lasts) or in people who have been vaccinated against the virus. Of course, as of this writing we have no proven vaccine against COVID-19 so the only protection from spread is to have people who have survived the disease in the population. This is the only good thing about having so many people become infected. Ultimately if enough people do become infected, then the virus will run into herd immunity by natural causes.

So many unanswered questions

Because COVID-19 is a novel coronavirus, nobody has natural immunity to it and this is the reason it has spread to millions of people around the globe. Some of the pressing questions are: If a person becomes infected how long will the immunity last? Do people who have had a mild case of COVID-19 have the same immunity as those who have had a severe case? What is the true spread of the disease, is it 1 person to 3 or is it higher or lower? The answer to that question determines exactly how many people need to be protected in a community before herd immunity exists. Will a vaccine be created that is effective? When will such a vaccine be developed? When will it be available to us (the herd)? Will the immunity from the vaccine be equivalent to the natural immunity acquired from having the disease? How long will the vaccine confer immunity?

The bottom line

COVID-19 is going to be with us for the foreseeable future. We will not be able to relax fully until herd immunity has been established in the community in which we live. The good news that goes with the bad news about so many people acquiring the disease is that this means we will reach that protective state of herd immunity sooner rather than later. We still have a long way to go, however, before 67% of our community are immune and we live in a safe herd. So in the meantime protect yourself with social distancing, hand washing, and mask wearing when in public.

MUSC Health has earned a "High-Performing Hospital" designation in geriatrics from U.S. News & World Report for 2018-2019