Herd immunity has been a popular topic of discussion in recent years. In simple terms, it occurs when a mass population is immune to a virus or a disease. Also known as community immunity, it helps reduce the spread of infectious diseases. When it comes to herd immunity, a significant portion of the community receives some protection against disease due to vaccination or previous infection. Keep reading to learn more about herd immunity in detail.

Herd immunity
When a significant number of people belonging to a community are vaccinated, everyone, including people who are extremely sick or too young to be immunized, receives some protection against the spread of diseases.

Hence, the disease is less susceptible to transmission from one person to another because lesser germs are around to infect others. Furthermore, if a person does fall sick, the probability of an outbreak is less because most people are immune to the disease. Broadly, there are two ways to develop herd immunity.

Vaccination
It is the most effective method to develop herd immunity. It works by pausing the chain of transmission. Naturally, this method’s indirect protection level is directly proportional to the total number of vaccinated individuals in a community. In this manner, it is possible to protect individuals who cannot be immunized, such as immunocompromised patients, pregnant women, or newborns. But this indirect protection achieved through vaccine-based community immunity varies between geographic locations. It primarily depends on factors like coverage and vaccine efficacy.

However, developing vaccine-based herd immunity involves certain risks. The efficacy drops over time for some vaccines. Moreover, people who do not get a booster dose may lose the benefits. Furthermore, people who do not complete the whole course of a vaccine may stay unprotected against vaccine-preventable conditions.

In a population with a low vaccination rate, there may be outbreaks of diseases that vaccines could have prevented. One key reason behind such outbreaks is the absence of herd immunity. Furthermore, if people who do not believe in vaccinations also live in the same community, the percentage of vaccinated individuals will drastically drop.

When this percentage is below the herd immunity threshold, exposure to the infectious disease may cause the disease to spread rapidly within the community.

History of a previous infection
A history of infection can also contribute to developing herd immunity. Herein, herd immunity can be achieved when a massive amount of a population has already got the disease and recovered from it. Thus, the community develops antibodies against the disease-causing pathogen. However, developing a herd through infection is risky, especially if the antibodies formed against the pathogen do not offer long-term protection.

Herd immunity threshold
Most disease-causing microbes usually have varying infectious features. Some, like influenza and measles, transmit from one person to another more quickly than others, and some have more extreme consequences in a particular demographic group. Let’s understand this in detail with the help of the example below.

Whooping cough or pertussis symptoms might be hard for most people to manage, but they can be life-threatening for infants.

Therefore, every aspect associated with a disease, such as its severity and contagiousness, affects its threshold or the minimum percentage of immune people a community needs to avoid an outbreak. For setting this threshold, experts specializing in infectious disease transmission use a ‘basic reproduction number’ value or ‘R0.’ This number indicates how many people one infected individual can pass the infection to in an unprotected community. For example, in the case of measles, R0 was between 12 and 18, whereas for polio, R0 was between 5 and 7.

Naturally, the higher the R0 value is, the higher the immunity threshold must be for safeguarding the community. Since measles spreads through the air and is highly contagious, the immunity threshold needed to protect the community is high. On the contrary, the threshold is slightly low for diseases like polio that are less contagious.

The concept of immunity threshold is simple, but the factors involved in calculating it are usually complex. These attributes include the effectiveness of a vaccine against a disease, how long-lasting immunity is from infection and vaccination, and which populations form critical links in disease transmission. The collective differences of these aspects define the thresholds for different diseases, with R0 being a significant factor.