Have you ever thought about how immunity works? If so, you might have realized that immunity keeps us from becoming sick in different ways. Two types of immunity exist — active and passive:

• Active immunity occurs when our own immune system is responsible for protecting us from a pathogen.
• Passive immunity occurs when we are protected from a pathogen by immunity gained from someone else.

Both of these different types of immunity can be acquired in different ways.

A third category, community immunity, does not involve physical components of the immune system for protection but is still worth discussion in this capacity.

So, let’s take a closer look at each.

Active immunity

Individuals rely on active immunity more so than passive immunity. Active immunity is created by our own immune system when we are exposed to a potential disease-causing agent (i.e., pathogen). Most of the time, we are exposed to these potential pathogens naturally throughout the course of our day — in the air we breathe, the food we eat, and the things we touch. Luckily, most of these exposures are to agents that will not result in disease, either because they are harmless or because our immune system works to neutralize them.

In addition to “fighting off” these pathogens, active immunity is important because it lasts a long time in the form of immunologic memory. Immunologic memory consists of B and T cells that can recognize a particular pathogen (see "Adaptive immune system"). These cells circulate at low levels in our bodies and if “activated” by recognizing that pathogen in their travels, they quickly start to multiply and signal other elements of the immune system to activate as well. Memory cells are crucial for two reasons. First, they allow our immune systems to respond quickly. Second, they are specific for the pathogen, so the immune response is ready the moment the pathogen is encountered (see "Immunologic memory").

Because we don’t know about most of the work our immune system does, we often do not think about how busy it is. But, the reality is that like our hearts and lungs, our immune system is constantly working to keep us healthy. This effort is evidenced by the fact that our immune system generates grams of antibodies every single day!

Vaccines contribute to active immunity by providing us with a controlled way to create an immune response. When a vaccine is introduced, our immune system treats it like any other exposure. It works to stop the “assault” and, in the process, immunologic memory develops. Because vaccines are designed such that they do not cause illness, we gain the benefits of the exposure without the risks associated with fighting off a natural infection. In this way, vaccines offer our immune systems a chance to “train” for a future encounter and provide us with a “shortcut” to protection. We gain the immunity that follows surviving a natural infection without having to pay the price of natural infection.

Passive immunity

Passive immunity, or immunity gained in a way other than from one’s own immune system, can occur in a few ways and can be life-saving. However, passive immunity is short-lived because the antibodies are not continually replenished as they would be in an individual whose immune system is responding directly. Passive immunity can occur in a couple of ways:

Maternal antibodies

Unborn and newly born babies are protected by antibodies from the maternal immune system. These antibodies are shared in two ways: across the placenta and in breast milk.

• Placenta and circulation — When a woman is pregnant, her blood circulates through the placenta to deliver nourishment and protection to the developing fetus. As the blood circulates, so do the antibodies and immune system cells that travel in blood. Although developing fetuses are not typically exposed to any pathogens in utero, they are exposed to viruses and bacteria during and immediately after birth. The types and levels of antibodies in a baby’s blood at birth reflect those of the mother.
• Breast milk — Babies also get antibodies from breast milk, particularly from a protein-rich version of breast milk supplied in the first few days after birth known as colostrum. Colostrum, which is produced in the first three to five days after birth, contains higher levels of antibodies that protect the intestinal surface (immunoglobulin A or IgA) and lower levels of nutritional ingredients than milk produced in the weeks following birth. This transfer of antibodies from mother to child suggests its importance in the period before a baby’s immune system can generate its own protection.

Immunoglobulin treatments

In certain situations, antibodies obtained from animals, from other people, or synthesized in a laboratory can be used to treat individuals at risk of infections. For example, infants born to women infected with hepatitis B are treated with antibody preparations in addition to being vaccinated in an effort to protect them from also becoming infected with hepatitis B. In another example, people bitten by some poisonous snakes may be treated with antivenom, a mixture of antibodies against the type of snake venom to which the person was exposed.

Community immunity

Community immunity occurs when people are protected by those around them. This type of protection is indirect in that it does not involve physical components of immunity, such as antibodies, but rather results when a pathogen is less likely to infect a susceptible person because of the high numbers of protected people around them. Because this immunity is not based on “products” of the immune system, it is the least reliable. However, for some in our communities, such as those too young to be immunized or those with weakened immunity due to illness or treatment, community immunity is the only way they can be protected.

We generally talk about community immunity from two perspectives — that of the community, commonly referred to as herd immunity, and that of the individual, commonly known as cocooning. Let’s look at cocooning first.

Cocooning

This type of passive immunity is aimed at protecting a particular individual rather than focusing on the community. Ensuring that everyone around a young infant is immune to a disease like pertussis (whooping cough) is an example of this type of indirect immunity. Another example is ensuring that everyone who visits or cares for a person being treated for cancer is healthy, so that the cancer patient whose immunity is weakened by treatment is less likely to be exposed to a pathogen. While cocooning may be somewhat helpful, having one’s own immunity is always better — especially if a person is in a community that is relatively unvaccinated. This is where herd immunity comes into play.

Herd immunity

When enough people in a community have been exposed to a pathogen, it cannot spread as easily. As more people become immune, the pathogen has a smaller pool of people to infect. The result is that the community overall will have fewer outbreaks. Because not all pathogens spread with the same efficiency, the community levels of immunity necessary to benefit from herd immunity vary. For example, because measles is one of the most contagious pathogens known, a community requires almost everyone to be immune in order to stop its transmission. Or said another way, it is much more difficult for an individual to benefit from herd immunity to measles than from most other infectious agents. Importantly, herd immunity does not apply for diseases in which person-to-person spread is not a means of transmission, such as tetanus.

Factors that affect herd immunity

While the general concept of herd immunity is the same for all transmissible diseases, the specifics of herd immunity vary depending upon the disease and vaccine used to prevent it:

1. Ease of disease transmission — Diseases are not only spread by different routes, they are also not equally contagious. For example, if we compare influenza and Ebola viruses, influenza is spread fairly easily from person to person by coughs and sneezes, whereas Ebola is spread by contact with body fluids of a person who already has symptoms of disease. Because influenza is more easily spread from one person to another, the number of protected people in a community needs to be higher for a community to enjoy the effects of herd immunity against influenza as compared to Ebola.
2. Vaccine effectiveness — When we think about vaccine effectiveness, we are typically discussing how well the vaccine prevents disease in the person who received it. However, vaccine effectiveness plays a role in herd immunity as well. Because the central tenet of herd immunity revolves around disease transmission, it is probably obvious that a vaccine that is highly effective at preventing disease will strengthen herd immunity. However, a vaccine can affect herd immunity in another more subtle way — some vaccines are better than others at decreasing shedding of viruses or bacteria, which reduces spread. For example, when the rotavirus vaccine was first introduced in 2006, about 50 percent of children received it. But the vaccine caused an 80 percent reduction in diseases. This was an example of herd immunity.

When we put vaccine and disease factors together, each disease then has its own potential for the community to benefit from herd immunity. If you use the example of a ticket system, each vaccine-preventable disease, except tetanus, would be stopped in a community with only a certain limited number of “free-ride” tickets. Because some people in a community will be unable to get vaccinated for reasons such as age or health status, they will use these tickets. Likewise, people who choose not to immunize and those whose immunity is not protective will also be free-ride ticket holders. The more free-ride tickets in the community, the more likely the disease will enter the community. The diseases that can afford the fewest number of free-ride tickets before outbreaks occur are measles and pertussis. As more and more people rely on free-ride tickets, herd immunity erodes and outbreaks occur. Some authors refer to this problem as “the tragedy of the commons.”

Is herd immunity real?

Some believe that the lack of vaccine boosters given to adults provides evidence that herd immunity is a myth. So, let’s look at this notion more closely.

Adults do not require as many immunizations as children because they are often immune to the diseases of childhood. For some, it is because they are old enough to have been exposed to the disease. For others, immunity is the result of vaccinations received earlier in life. However, because children often receive booster doses, people sometimes wonder why adults do not as well. The lack of need for booster doses in adults can be for one of several reasons.

Factors affecting the need for booster doses can be divided into those related to the disease and those related to the vaccine.

Disease-related considerations

• Biology of infection — For example, measles and chickenpox require entrance and spread through the bloodstream to cause infection. Therefore, antibodies in the bloodstream can protect against subsequent infection. Typically, antibodies induced in the bloodstream after immunization are lifelong (unlike antibodies induced at mucosal surfaces), so booster doses in adulthood are not needed. In addition, these viruses do not change through time, so immune responses generated initially will remain effective years later. These types of infections tend to produce a life-long immunity. Whereas, diseases that occur at a mucosal surface (respiratory, gastrointestinal, or urogenital tracts), such as influenza and rotavirus, produce antibodies that stay at the mucosal surface and are not as long-lived in terms of the immunologic memory produced. Additionally, some pathogens change enough through time that antibodies produced at one point in time may or may not be effective during a subsequent infection, such as influenza and human immunodeficiency virus (HIV).
• Disease levels in the community — For diseases that are still common, booster doses are often not necessary because exposures are likely to occur that allow for maintenance of a protective immune response. People just don’t realize that they have been exposed. Sometimes when a vaccine first becomes available, enough of the organism is still circulating in the community that people are exposed without knowing it, leading to a natural boosting of the immune response. However, after the vaccine has been widely used, lower levels of disease may be circulating requiring boosting by other means, such as vaccination.
• Susceptibility — Some diseases are more likely to infect certain subsets of the population. In many cases, the most susceptible groups are children. Therefore, if adults are less susceptible, they would no longer need a vaccine or a booster dose.

Vaccine-related considerations

• Durability of vaccine responses — Some vaccines induce better immune responses than others. For example, infants and children less than 2 years of age do not produce strong immune responses to the original pneumococcal vaccine, known as the polysaccharide version. However, a newer version that includes a helper protein, known as the conjugate version, allows infants to develop protective immunity.
• Effectiveness of vaccine — In some cases individuals will not respond to a dose of vaccine. For example, about 94 of 100 people will be protected after one dose of the measles vaccine whereas about 99 of 100 will be protected after two doses.

Summary

In summary, various factors make the potential for herd immunity different for each pathogen. In addition, whether or not booster doses are necessary depends upon both disease- and vaccine-specific characteristics. Therefore, the fact that booster doses are not typically necessary in adults cannot be used to prove or disprove the concept of herd immunity. A good rule of thumb when evaluating statements for accuracy is that broad, general statements often overlook nuances important in understanding a particular issue. So, while it might seem to make sense at face value that the lack of adult booster doses means herd immunity is a myth, taking time to explore the different aspects of the statement is important in sorting out whether the statement may be true.

Conclusion

When thinking about herd immunity, it is important to realize that vaccines have made it easier for society to reap the benefits of this type of protection. Before vaccines, diseases continued to have susceptible pools of individuals — most often infants and young children not previously exposed to the disease. This is why childhood diseases and deaths were so common, and why no disease would ever go away without vaccinations. Vaccines give humanity the power to “short-circuit” a pathogen’s spread through communities.