Introduction to Herd Immunity

Have you ever heard of “herd immunity”? It’s when enough people are immune to a disease, either through vaccination or past illness, that the disease can’t spread easily. This protects even those who aren’t immune, like infants or people with immune system disorders.

The Concept of Herd Immunity

So, how does herd immunity work? Picture a row of dominoes. If they’re all standing, one push can topple the entire line. But if most of those dominoes are replaced with solid blocks that can’t fall, the chain reaction stops almost immediately. Immune people function like those blocks. They break the chain of transmission, making it harder for a disease to move from one person to the next.

Why should you care about herd immunity? It allows communities to protect the most vulnerable members without everyone needing vaccination, which is great as many of those most vulnerable would not be able to be safely vaccinated. For example, even if newborns can’t be vaccinated, they stay safe because the disease can’t reach them. This indirect protection is vital for controlling outbreaks and keeping everyone healthier.

Historical Context: The Evolution of Herd Immunity

Let’s take a journey back to the late 19th century. The concept of herd immunity started in veterinary science. An American veterinary scientist, Daniel Elmer Salmon, coined the term in 1894. He used it to describe the resilience of well-fed hog herds against disease. By 1916, scientists applied it to cattle recovering from brucellosis, an infectious disease that affects livestock. They noticed that after surviving the disease, the cattle and their offspring gained immunity, slowing down future outbreaks. It should be noted that, despite being the first doctor of veterinary medicine in the United States, and heading up the Department of Agriculture’s Animal Industry division, some of his ideas didn’t exactly hold to scrutiny. For instance, he believed that Pasteur’s work on sanitation of milk products was effectively useless. Additionally, he was convinced of what is now a resurgent anti-vaccine belief that through effective nutrition and fitness, all disease could be eliminated. History doesn’t repeat itself, but certainly rhymes, it seems.

Fast forward to the 1920s, herd immunity made its way into human epidemiology. British scientists, including Sheldon F. Dudley, extended the concept to humans. They conducted experiments on mice to mimic diseases like diphtheria and influenza. These studies showed how exposure and vaccination created varying immunity levels. Dudley observed diphtheria outbreaks in a British naval school dormitory. He realized that past infections and vaccinations limited the spread, which he described as a form of herd immunity.

In the 1930s, A.W. Hedrich documented herd immunity naturally with measles in Baltimore. After many children gained immunity, new cases dropped, even among those who were still susceptible. This real-world example highlighted how a population could achieve temporary protection without everyone being immune. This phenomenon was later observed with the 1918 influenza pandemic. The spread slowed after enough people developed immunity.

The 1960s and 1970s marked a mathematical and vaccination era for herd immunity. Vaccination campaigns, like the smallpox eradication effort, relied on creating “rings” of immunity around cases. This approach, known as ring vaccination, helped stop the disease from spreading. In the 1970s, researchers Roy Anderson and Robert May formalized the concept with the herd immunity threshold (HIT) formula: [math]p_c = 1 – \frac{1}{R_0}[/math]. This formula helps determine the proportion of the population that must be immune to stop the disease from spreading.

Over time, herd immunity has become a crucial strategy in public health. It allows communities to control diseases without needing everyone to be immune. However, it requires high vaccination coverage to be effective. The journey from protecting hogs to safeguarding human populations shows the evolving understanding of this fascinating concept.

Core Principles of Herd Immunity

Let’s dive into the core principles of herd immunity. This concept is a crucial tool in controlling infectious diseases. It works by building a barrier of immune individuals. This barrier stops diseases from spreading through a population.

Understanding the Herd Immunity Threshold

One of the key concepts in herd immunity is the herd immunity threshold (HIT). This threshold tells us what percentage of a population needs to be immune to stop a disease from spreading. It is calculated using the basic reproduction number, or R₀. R₀ represents the average number of people one infected person can infect in a completely susceptible population.

Here’s the formula for the herd immunity threshold:

[math]p_c = 1 – \frac{1}{R_0}[/math]

Here’s what each part means:

• p_c = the critical proportion of the population that must be immune
• R₀ = the basic reproduction number of the disease

History of Vaccines actually has a great blog post about the underpinnings and history of HIT, and it’s worth a read.

Let’s walk through an example:

Imagine a disease like measles, which is highly contagious with an R₀ of 15. This means, on average, one person with measles can infect 15 others.

To find out the herd immunity threshold, we use the formula:

[math]p_c = 1 – \frac{1}{15}[/math]

This calculation gives us:

[math]p_c = 1 – 0.067 = 0.933[/math]

In simple terms, about 93% of the population needs to be immune to prevent measles from spreading. Therefore, if 93% of people are vaccinated or immune, the disease cannot spread easily.

Real-World Application

This principle explains why high vaccination rates are crucial. For example, measles vaccination programs aim for over 93% coverage. This ensures that the disease can’t find enough susceptible people to sustain an outbreak. Thus, the community is protected, even those who are not immune. This is part of why the United States potentially losing its Measles Elimination status is such a big deal; The efforts towards measles elimination were herculean in the first place.

Understanding the herd immunity threshold helps public health officials set targets for vaccination campaigns. It ensures that diseases are controlled and outbreaks are prevented.

Interpretation and Application of Herd Immunity

Herd immunity isn’t just a scientific concept. It’s a practical tool we use in public health to protect communities from infectious diseases, and at times even have effectively eliminated diseases with strategic use. Let’s explore how we apply this concept in the real world with some concrete examples.

When Would You Use This?

Herd immunity is crucial in vaccination programs. Public health officials rely on it to plan and execute vaccination campaigns. For instance, during the smallpox eradication campaign, herd immunity was used to determine vaccination targets. By vaccinating a large portion of the population in a strategic, measured way, the spread of smallpox was halted.

Similarly, in today’s world, we use herd immunity strategies for diseases like measles and polio. Vaccination campaigns aim to reach the critical immunity threshold, ensuring these diseases can’t spread widely.

Understanding and applying herd immunity helps save lives. It ensures that even those who can’t be vaccinated, like newborns or those with certain health conditions, are protected. Therefore, achieving the herd immunity threshold is vital for public health safety.

Strengths and Limitations of Herd Immunity

Like any tool, herd immunity has its pros and cons. Here’s the honest breakdown:

Strengths

• Indirect Protection: Herd immunity provides a safety net. It protects people who can’t be vaccinated, such as infants and those with weak immune systems. When enough people are immune, diseases can’t spread easily. This shields the vulnerable indirectly.
• Effective Disease Control: Herd immunity helps control diseases without needing to vaccinate everyone. For example, the eradication of smallpox was possible through achieving herd immunity via vaccination. This approach saves lives and resources, making it a powerful tool in public health.

Limitations

• Waning Immunity: Immunity can fade over time. This means people who were once protected might become susceptible again. Diseases like COVID-19 show how immunity can diminish, requiring booster shots to maintain protection.
• Uneven Vaccine Coverage: Not everyone gets vaccinated. This uneven coverage can lead to outbreaks. For example, low vaccination rates in certain communities can cause diseases like measles to spread easily, despite high overall immunity. This is often compounded in the fact that there are often “clumps” of populations that geographically tend to trend one way or another, either due to belief system, medical necessity, or other limiters.
• Use as an “Alternative” to Vaccination: If you grew up in the 1990s or previous, you’re probably familiar with the term “Chickenpox Parties” where a kid sick with chickenpox was introduced to a bunch of other kids, so a neighborhood/community/school could just get all the infections over with in one fell swoop. Ignoring how wild that is on its face for a moment, some groups have advocated this approach be used for everything from COVID to Measles to even more severe diseases, instead of the safer and more practical vaccination approaches to herd immunity. This often relies on misinformation about the severity of the disease being “not that bad”, and inversely inflating the risks associated with vaccination. So, if employing this method, please communicate very clearly what you mean, and don’t assume anything about the community you are speaking to.

Understanding both the strengths and limitations of herd immunity is crucial. It helps public health officials make informed decisions about vaccination strategies and disease control measures.

Conclusion

Herd immunity is a powerful tool in epidemiology that helps protect communities from infectious diseases. Now that you understand it, you can appreciate its role in public health and how it keeps vulnerable populations safe.

Here’s what we covered:

• What herd immunity actually means
• How it works (and how to use it)
• When it’s useful – and when it’s not

Come by again next week for another edition of Epi Explained!

Humanities Moment

The featured image for this article is “‘The Goose Game’ on the Vijvar at the Hague” by Hans Bol (Dutch, 1534-1593). Hans Bol was a Flemish painter, miniaturist, printmaker, and draftsman whose work exemplified the late Northern Mannerist style, marked by meticulous detail, crowded compositions, and expressive figures. He is best known for his innovative small-scale landscapes and landscape-based prints that helped transmit Flemish pictorial ideas northward and significantly influenced later Dutch landscape painting.