Herd immunity is a form of indirect protection from contagious diseases that occurs when a large percentage of a population is immune. This widespread immunity acts as a protective shield for the entire community, not just for immune individuals. When enough people cannot spread a pathogen, it becomes much harder for the disease to find susceptible individuals. This disruption in the chain of transmission helps safeguard the entire population, including those unable to become immune.
How Herd Immunity Works
The core mechanism of herd immunity is breaking the chains of infection that allow a disease to spread. Every immune person acts as a dead end for the pathogen, preventing it from moving to the next person. When there are enough of these dead ends in a population, the disease cannot propagate effectively. It is similar to a wildfire that is starved of fuel and eventually burns out.
This protective effect is important for the most vulnerable members of a community. This includes infants who are too young to be vaccinated, elderly individuals whose immune systems may be weakening, and people with medical conditions that prevent them from receiving vaccines or compromise their immune function. These individuals often cannot build their own immunity, so they rely on the immunity of those around them for protection.
The collective immunity of the community forms a barrier that reduces the likelihood of these vulnerable individuals contacting an infectious person. If an infected individual is surrounded mostly by immune people, the pathogen has very few opportunities to spread. This containment significantly lowers the risk of outbreaks and protects those who have no other way of being shielded.
The Herd Immunity Threshold
The point at which a population has enough immunity to control a disease’s spread is the herd immunity threshold. This threshold is the specific percentage of people who must be immune for a contagious disease to no longer spread widely. It is not a universal number; it changes depending on how contagious the pathogen is. This contagiousness is measured by a value called the basic reproduction number, or R0.
The R0 value represents the average number of people that one infected person will pass the disease on to in a population with no pre-existing immunity. A disease with a higher R0 is more contagious and requires a higher percentage of the population to be immune. The threshold can be calculated with the formula 1 – 1/R0. For example, if a disease has an R0 of 2, it means one sick person infects two others, and the herd immunity threshold would be 50%.
This variation is clear when comparing different diseases. Measles is extremely contagious, with an R0 that can be as high as 18, meaning its herd immunity threshold is about 95%. In contrast, seasonal influenza has a much lower R0, typically around 1.3, which corresponds to a lower threshold. For the original strain of COVID-19, the R0 was estimated to be around 3, placing its initial herd immunity threshold at 67%.
Methods of Reaching the Threshold
Achieving population-wide immunity to meet the threshold can happen in two main ways: vaccination or natural infection. Vaccination is the deliberate public health strategy designed to build immunity safely. Vaccines introduce a weakened, inactivated, or partial form of a pathogen to the body, allowing the immune system to create antibodies without causing the disease. This method has been used in global campaigns to control diseases like smallpox and polio.
Mass vaccination programs are designed to protect the individual and the community by pushing the level of immunity above the disease’s specific threshold. By doing so, they prevent widespread outbreaks and shield vulnerable populations. This approach is a part of modern public health, aiming to manage infectious diseases with minimal harm to society.
Alternatively, immunity can be acquired after recovering from a natural infection. However, relying on this method to reach the herd immunity threshold comes with severe consequences. This path requires a large number of people to get sick, which can lead to serious illness, long-term health complications, and a significant number of deaths. A widespread outbreak on this scale would place an immense burden on healthcare systems. For these reasons, building immunity through natural infection is not a safe public health strategy.