Herd immunity, otherwise known as community immunity or the herd effect, is the way in which an entire community can be protected from an illness by immunizing a certain percentage of individuals. If enough people in the population are immune to a disease, then it is less likely to spread, thus shielding those who are unprotected.

Illness and Immunity

Diseases occur when people are infected with pathogens like bacteria or viruses. A person’s immune system detects these intruders and responds by producing antibodies that fight the pathogen. Illnesses can be passed from person to person, and if enough people get sick it can lead to an outbreak. However, if a particular number of people in a population are immune, then the pathogen’s chain of transmission is broken and the disease cannot be passed around as easily. As a result, the disease becomes rarer over time and can even be eliminated altogether.

There are two ways people can become immune to a disease: through natural exposure or through vaccination. In both cases, the human immune system recognizes any pathogens that it has been exposed to before and is able to produce antibodies much faster to ward off sickness. This is why an adult who has already had chickenpox (varicella) as a child is unlikely to contract the disease a second time. A new type of vaccine stimulates immunity using artificial mRNA, or messenger RNA. Messenger RNA is essential to the process that reads DNA to make the building block of proteins, amino acids. The mRNA vaccine replicates part of a pathogen, which is then injected into the body, triggering the immune system to develop antibodies against it.

Natural exposure to a disease is one way of achieving immunity, but it is risky for obvious reasons: it relies on getting sick in the first place, in the hopes that the immune system is strong enough to fight the illness and recover. Vaccination, on the other hand, is a relatively safe and effective option for immunization.

Kill or Cure

Vaccination has a long history with accounts of smallpox (variola) innoculation methods being practiced by as early as the 16th century in China and India. Smallpox was a common disease that killed countless people and left those who survived with disfiguring scars. These early vaccination methods involved rubbing the pus from the scab of someone infected with smallpox into an uninfected person's arm. This techinque was popularized in the United States by Puritan minister Cotton Mather in a 1721 smallpox outbreak in Boston, Massachusetts. Mather learned the technology from Onesimus, a West African man he kept enslaved. But this method had a relatively high mortality rate.

In the 18th century, English scientist Edward Jenner enhanced this technique. Instead of innoculating people with a healthy sample of the smallpox virus, Jenner innovated using a related, and less harmful, pathogen. He noticed that milkmaids who had contracted cowpox—a similar but less aggressive disease—did not seem to get sick during smallpox outbreaks. He theorized that the two viruses were similar enough that exposure to cowpox would immunize an individual against smallpox.

Jenner decided to test his theory by inoculating a young boy with cowpox. He extracted pus from the blister of a milkmaid infected with cowpox and wiped it into a cut on the boy's arm. The boy fell ill but soon recovered. Jenner then purposely infected the boy with the deadly smallpox virus. Luckily for the boy, Jenner's theory was correct: His exposure to cowpox had made him immune, and he did not contract the disease.

Thankfully, medical science has come a long way since then. Today’s vaccines work by injecting a weakened or modified version of a pathogen, or a synthetic snippet of mRNA based on the pathogen, to stimulate an immune response without inducing sickness. If an individual is exposed to the target pathogen in the real world, that person’s immune system knows how to make the antibodies required to fight off the disease.

Vaccination is largely safe and it provides an uncomplicated way to confer herd immunity. Occasionially, vaccines can cause allergic reactions and unwanted side effects, and not everyone is a suitable candidate for every vaccine.

Safety in Numbers

For herd immunity to take effect, a certain percentage of the population must be immunized. This threshold of how many people are required to stop a disease from spreading is different for every disease and depends on many factors, including how easily it spreads and whom it infects.

One of the statistics used to calculate this threshold is the “basic reproduction number” of a pathogen, known as R0. It represents the number of people one contagious person can, on average, be expected to infect in a community of unprotected individuals. The more contagious a disease is, the higher R0 will be. A higher R0 means more people need to be immunized for herd immunity to take effect.

For example, measles (Measles morbillivirus) is a highly contagious virus that can quickly be spread through the air. It has an R0 of 12–18 and requires 95 percent of the population to be vaccinated before herd immunity kicks in. Polio, on the other hand, is less contagious and has an R0 of five to seven, meaning that only 80–85 percent of the population needs to be immunized against polio for herd immunity to work.

Why Does It Matter?

Thanks to the effects of herd immunity, vaccines protect more people than just the individuals who receive them. If enough members of the population are immunized, then people who do not receive vaccines are less likely to get sick.

For example, seasonal influenza is a contagious disease that kills 36,000 people per year in the United States alone. While having the flu is unpleasant at the best of times, it is potentially deadly for people with weakened immune systems, including children and the elderly. Flu shots are available, but their effectiveness depends on a strong immune response. As a result, the vaccine is least effective at immunizing those who need it most: people with weakened immune systems. If strong and healthy people get vaccinated, then flu outbreaks can be contained and vulnerable demographics are protected.

Herd immunity is important because it contains outbreaks and protects the most vulnerable members of society from potentially deadly diseases. It is impossible to vaccinate every single person on the planet, but if enough people are vaccinated, then herd immunity can lead to diseases being stamped out entirely. Thanks to vaccinations and the herd effect, smallpox was officially declared as eradicated in 1980—almost two centuries after Edward Jenner discovered the first vaccine. Unfortunately, the growth of the anti-vaccination movement in recent decades has led to a resurgence in deadly childhood illnesses such as measles and pertussis.