Herd immunity, or community immunity, is a form of indirect protection against infectious disease that arises when a significant percentage of a population has become immune. This collective immunity acts like a shield, making the spread of a pathogen from person to person unlikely. When the infection chain is broken, it helps safeguard those who cannot gain immunity, such as newborns or individuals with compromised immune systems. This percentage is known as the Herd Immunity Threshold (HIT), which dictates when a disease will begin to decline rather than spread exponentially.
Understanding the Herd Immunity Threshold
The Herd Immunity Threshold (HIT) is the minimum proportion of immune individuals required in a population to prevent a communicable disease from sustaining itself. Once this percentage is reached, the likelihood of an infected person encountering a susceptible person drops below a certain level. This lack of susceptible hosts means that the pathogen cannot find new victims quickly enough to keep the outbreak going. Instead of an epidemic, the disease transmission chains become isolated and fizzle out, protecting the entire community.
Reaching the HIT provides protection to the most vulnerable members of society. These individuals, who may be too young to be vaccinated or have medical conditions that prevent immunization, rely entirely on the immunity of the people around them. The threshold is calculated using mathematical models specific to the characteristics of the disease itself. It represents the point where the pathogen is effectively denied a pathway for continued, widespread transmission.
The Role of the Basic Reproduction Number (\(R_0\))
The percentage needed for herd immunity is directly determined by a measure of a disease’s contagiousness known as the Basic Reproduction Number, or \(R_0\). \(R_0\) represents the average number of new infections that one infected person will cause in a population where everyone is susceptible. If a disease has an \(R_0\) of 3, one person will typically infect three others, indicating a high potential for rapid spread.
The relationship between a disease’s contagiousness and its required immunity threshold is inverse: the higher the \(R_0\), the higher the necessary percentage of immune individuals. This is because a more transmissible pathogen requires more barriers to interrupt its spread. If an infected person is likely to infect many others, a larger percentage of the population must be immune to block those potential transmissions.
Epidemiologists use a simplified formula to calculate the Herd Immunity Threshold: \(\text{HIT} = 1 – 1/R_0\). This formula determines the proportion of the population that must be immune to reduce the effective reproduction number—the actual spread in a community—to less than one. When the effective number of new infections drops below one, the disease will eventually die out. For example, a disease with an \(R_0\) of 4 would require \(1 – 1/4\), or 75%, of the population to be immune to halt its spread.
Variability in Disease Thresholds
The wide range of \(R_0\) values across different pathogens demonstrates why the Herd Immunity Threshold is not a universal number. Measles, one of the most contagious viruses known, serves as a prime example of a disease with a very high threshold. Its \(R_0\) is estimated to be between 12 and 18, meaning one person can infect up to 18 others in a fully susceptible group. To interrupt this highly efficient transmission, the HIT for measles is calculated to be approximately 92% to 95%.
In contrast, seasonal influenza generally has a much lower \(R_0\), often estimated to be around 2. Using the formula, this lower transmissibility translates to a significantly lower HIT, typically around 50%. For the initial strain of SARS-CoV-2, which caused the COVID-19 pandemic, the \(R_0\) was estimated to be in the range of 2 to 3, suggesting a corresponding HIT of approximately 55% to 67%.
These differences illustrate that the physical characteristics of the virus, such as how it is transmitted—whether through airborne particles or through less efficient contact—directly influence the required threshold. The more easily a virus spreads, the greater the percentage of immunity needed to form a protective barrier around the community.
Achieving and Sustaining the Required Percentage
The percentage of immune individuals needed to reach the Herd Immunity Threshold can be achieved through two primary mechanisms: immunity gained from surviving a natural infection or immunity developed through vaccination. Public health efforts overwhelmingly favor vaccination, as it offers the protective benefits of immunity without the risks of severe illness, hospitalization, or death associated with natural infection.
The goal of public health vaccination campaigns is to achieve a figure known as the Critical Vaccination Coverage (CVC). CVC is the vaccination rate needed to meet or exceed the calculated Herd Immunity Threshold for a given disease. The required CVC is often higher than the HIT because vaccines are rarely 100% effective in preventing infection or transmission.
Sustaining the required percentage is an ongoing process. Continuous vaccination efforts are necessary because new susceptible individuals, like newborns, are constantly entering the population. Furthermore, immunity can wane over time, or the pathogen itself can mutate, requiring updated vaccines and booster shots to maintain the protective CVC level and prevent the resurgence of outbreaks.