Amanda Maxwell

Aug 5th 2022

What Is Herd Immunity, and How Does It work?


There’s a lot of discussion about immunity and protecting ourselves from infectious diseases these days, and with good reason — the modern marvel of vaccination has the ability to protect not just the vaccinated but also those who cannot receive an immunizing jab. Learning about disease transmission through a population can help you understand not only what herd immunity is but also how herd immunity works to provide considerable benefits to a population that has achieved this state.

It’s All In The Herd

Yes, it is quite literally all in the herd, though as Yale Medicine points out, community immunity might be a better term to use when explaining what herd immunity is.

According to The Lancet, the phrase “herd immunity” was first used by American livestock veterinarians who were seeing epidemics of spontaneous miscarriage among cattle and sheep in the early 1900s. This disease, which was called “contagious abortion,” was the leading health threat to herds at the time and caused loss of productivity and economic hardship for farmers throughout the United States, who had to destroy their animals to prevent the disease from spreading.

However, through observation, livestock veterinarians started to notice that instead of culling the herd, it was much better to build protection by naturally acquired infection. Animals that went through a miscarriage storm were ultimately protected. This led Adolph Eichhorn, writing for the Journal of the American Veterinary Medical Association in 1918, to liken the disease to a fire, noting that restricting new fuel would stem the spread. Accordingly, it was found that herd immunity could be achieved by avoiding addition of new cattle, as those that had already experienced the disease were immune.

What Is Herd Immunity?

Herd immunity is reached when a population enjoys some degree of protection from contagious disease due to a significant portion of the population developing immunity. As The Mayo Clinic describes, when this point is reached, spread declines throughout the population, conferring community protection on those who cannot gain immunity themselves.

Individuals can gain immunity from contagious disease either by catching and recovering from an infection, or through effective vaccination. Either method can give antibody protection from future infection, thereby reducing the number of individuals who would be susceptible to the disease. Disease spread then slows, which means the infection is less prevalent in the community and at-risk individuals are less likely to be exposed. This is extremely important to individuals who cannot receive vaccination, or for whom natural disease could be severely damaging or fatal.

How Does Herd Immunity Work?

For contagious disease to spread effectively through a population, a certain amount of individuals needs to be infected and carry the virus as an infected reservoir. In other words, there needs to be enough of the virus present that other individuals can catch it.

This depends somewhat on the R0 value of the disease. Healthline describes this mathematical construct, pronounced “R naught,” as a measure of the infectivity of a disease. For example, an R18 means that in a population that is completely open to infection and has no inbuilt immunity, one infected individual could pass the disease to 18 others. With infection, the disease vector multiplies and increases its spread, which may continue if immunity does not develop.

Herd immunity occurs when the proportion of individuals who are immune to the disease passes the threshold level required to maintain infection within the population. For instance, establishing herd immunity to measles, which has an extremely high R0 value, requires around 95% population immunity, according to the World Health Organization (WHO). Another example is Polio, which requires 80% immunity to “break the chain of transmission.”

What Creates Herd Immunity?

Herd immunity comes either from vaccination programs or recovery from the disease itself. Global News notes that an effective vaccination program not only controlled polio in the mid-twentieth century but also caused the extinction of the virus’s wild types 2 and 3 globally. Measles is also held in check by strong vaccination programs, though when rates fall, outbreaks can occur.

Smallpox is another vaccine success story. USA Today notes that vaccination has eradicated the disease globally, and the WHO declared it extinct in May 1980, with an 80% threshold protection achieved after the disease had caused some 300 million deaths at the beginning of the last century.

The end result of herd immunity is community protection, where people who can be vaccinated help to protect those who cannot. In the cases of smallpox and polio, a philosophy of “think global; act local” was adopted, and to good effect. We should look to these successful examples of herd immunity development as we aim to address issues of viral infection in our own time.

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