Herd Immunity

Herd immunity is a form of protection against disease. In herd immunity, the immunization of a large percentage of individuals in a population makes infection of non-immunized individuals extremely unlikely. Should the general rate of immunization fall, more members of the population will become vulnerable and the chance of a disease outbreak will increase.

How Herd Immunity Works

Infectious diseases spread from one infected person to another. This means that for individuals to get a disease, they must come in contact with an infected person. Science has allowed humans a variety of ways to avoid getting sick. Vaccinations, exposure to similar viruses, or hereditary biological resistance allow select members of populations to become immune to a disease. However, it is extremely unlikely that each individual in a large population will be immune to every disease. Herd immunity occurs when immunity of a large segment of the population prevents contagious diseases from affecting those in the population who are not immune. Herd immunity prevents diseases from reemerging, even if a small percentage of a population remains vulnerable to the diseases.

When a large percentage of a community is immune to a disease, the percentage of individuals who can bring that disease into the community is reduced. If the percentage of immune individuals is high enough, vulnerable individuals may never interact with other vulnerable individuals, making them effectively immune to the disease. They are protected by herd immunity.

Herd immunity relies on a large percentage of the population being vaccinated or made immune to disease by some other means. On some occasions, individuals will assume that everyone else in the population is vaccinated, and will therefore fail to get vaccinated themselves. This reduces the percentage of immune individuals in the population, making the entire population more vulnerable to disease and decreasing the effectiveness of herd immunity. For this reason, herd immunity should never be a primary means of defense against a disease.

Threats to Herd Immunity

Potentially deadly diseases such as measles, mumps, and polio have been greatly reduced in many developed nations through vaccination and herd immunity. However, anti-vaccination movements threaten these gains. Fewer people getting vaccinated reduces the overall immunity of the population, making it more likely that a chance introduction of an otherwise controlled disease could turn into a major outbreak. In 2015, the United States experienced a measles outbreak linked to a California amusement park. Between January and April 2015, 159 people in 18 states contracted the disease, which had been declared eliminated in the United States in 2000. The majority of the cases in the 2015 outbreak were among people who were not vaccinated. Another measles outbreak, which also spread primarily among unvaccinated individuals, occurred in the United States in 2025, with the Centers for Disease Control and Prevention (CDC) reporting 2,284 confirmed measles cases and 48 outbreaks in the United States for the full year.

Measles and other contagious diseases can spread quickly when they reach a population where groups of people are not immune through vaccination or herd immunity. Even when infectious diseases have been fully or mostly eliminated from a population, vaccination remains important to protect against reintroduction of the disease.

In the case of the severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) or the novel coronavirus causing the disease COVID-19 that was first identified in China in late 2019 and began rapidly spreading around the world into 2020, leading to the declaration of a pandemic in March, the idea of accomplishing herd immunity once a large number of people had contracted the virus and recovered was debated after WHO categorized COVID-19 as a pandemic on March 11, 2020. As experts were not immediately sure whether those who had successfully battled COVID-19, thereby developing antibodies, were actually then immune to reinfection, this potential method for herd immunity was not reliable. Additionally, because populations such as that of the United States had grown so large, too many people would need to become infected with and beat the potentially deadly novel coronavirus, overwhelming hospitals, to reach herd immunity. Though the anti-vaccination movement remained a threat, researchers worked diligently on a potential vaccine as a priority, especially for potential herd immunity. As time passed, the disease proved that people could become reinfected and that contracting the virus once did not protect against future infections. While the COVID-19 vaccine does not indefinitely protect vaccinated individuals from the disease, research showed that vaccinated individuals experience milder symptoms and are at lower risk for developing long-term COVID.

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