What is ‘herd immunity‘?


You may have heard of a term called “herd immunity” during the current pandemic. What is herd immunity?

Herd immunity is a form of indirect protection from infectious disease (such as Covid-19) that occurs when a large percentage of a population has become immune to an infection.

There are two ways this occurs: (1) through previous infections, and (2) through vaccination. When a large proportion of a population possesses immunity, such people are unlikely to contribute to disease transmission, the pathways for infection are likely to be disrupted, and the spread of the disease can slow or even stop.

The idea is that, if enough people become immune — either through prior infection or vaccination — more people within the population are less likely to contract the disease. Sometimes, herd immunity eradicates a disease almost entirely, as it did with smallpox by 1977.

In the 1930s, herd immunity was recognized as naturally occurring in the case of measles. After a significant number of children were infected, the number of new infections temporarily decreased, including among susceptible children.

The most common and accepted method of inducing herd immunity is mass vaccinations. The greatest threat to herd immunity is, in fact, opposition to vaccinations, which has posed a challenge to herd immunity and has allowed preventable diseases to persist or return to communities that have inadequate vaccination rates. Some portions of the population either cannot develop immunity after vaccination or for medical reasons cannot be vaccinated.

Here are three scenarios regarding vaccinations and herd immunity:

1. Most of the population is not immunized. The result is the contagious disease spreads throughout the population.

2. Some of the population gets immunized. The result is the contagious disease spreads through some of the population.

3. Most of the population gets immunized. The result is the spread of the contagion is contained. In this scenario, only one-fourth of the healthy unimmunized population becomes infected.

Some proponents of herd immunity call for no vaccinations and that the disease should just be allowed to run its course. This is not herd immunity when it concerns deadly diseases. Depending on the strength of the contagion, this is a recipe for total disaster.

In Central and South America, during the Spanish conquest of those regions, the Incas, Mayans, and Aztecs were nearly wiped out due to smallpox contracted from the Spaniards. In parts of North America, many Native American tribes suffered the same fate. There were no vaccinations and, thus, no “herd immunity.”

In 1347-1351, the Black Death, as the bubonic plague was known, killed up to 1/3 of the population — from 70 million up to 200 million people — from Scotland, through Europe, to North Africa, to India. There were no vaccinations and, thus, no “herd immunity.”

Some people caught the disease at night and were dead before morning. Historytoday.com calls this event the “greatest catastrophe ever.” Some towns were totally wiped out and never re-populated. Today this disease is nearly extinct and most cases can be cured by antibiotics.

No vaccine is 100% safe and some may have side effects. On the other hand, in the United States, 350 people a year are killed in bath- or shower-related incidents, 5,000 people are killed when food lodges in their windpipe, and about 50 people are struck by lightning and killed. But most people do not consider bathing, eating, or taking a walk on a rainy day as life-threatening events.

Most people my age bear the scar of a smallpox shot on their upper arm. Yet, I know of no one who has died of smallpox. Polio, a serious threat not that long ago, has been contained due to vaccines and the last case in the United States was in 1979. Those who have been in the military have been vaccinated numerous times and those who have traveled to some nations have received even more. In 1998, I traveled to Africa and was required to get seven inoculations before I could travel. To me, the benefits have outweighed the potential risks.

In summary, herd immunity is real but it depends on a significant portion of the population receiving vaccinations, assuming one is even available. If one is not available, then reasonable precautions should be taken to prevent the spread of the contagion.

Without vaccinations, large-scale herd immunity for deadly diseases does not exist. Just ask the Incas.

[David Epps is the Rector of the Cathedral of Christ the King (www.ctk.life). During the crisis, the church is live streaming at 10:00 a.m. on Sundays at http://www.facebook.com/cctksharpsburg/ He is the bishop of the Diocese of the Mid-South He may contacted at davidepps@ctk.life.]


  1. As the pandemic season draws closer, we will find ourselves more concerned about the potential dangers that a pandemic brings. One of the major concerns is the possibility of a devastating epidemic of a type that was eradicated long ago. If a new strain of the same disease comes along, it could very quickly turn out to be a catastrophe. In the worst case scenario, we could see thousands upon thousands of people dying over the span of a couple of weeks. This would bring the death toll to tens of millions of people. There would be a lot of talk about herd immunity in this situation.