Written by Eashan Kotha
Herd immunity is an interesting concept as it concerns public health. However, one question comes to mind. How does it relate to you? Well, herd immunity, or community immunity, occurs when a large enough part of a population is immune to a disease, greatly lowering the potential for any person to contract the illness [1, 2]. Thus, individuals not vaccinated, such as newborns, are better protected since there is less of a chance for disease to spread.
Some classic examples of childhood infections diminished by herd immunity include chicken pox, measles, mumps, and polio among others. The frequency and magnitude of these epidemics became less prevalent because the number of people susceptible to disease was kept below a certain point (via vaccination). Researchers Ken Eames, Paul Fine, and David Heymann found that incidence of disease among the elderly (who were too old to be vaccinated) dropped significantly. In fact, it led to about "one- to two-thirds of the total disease reduction" .
A problem arises, however, when the rate of immunization falls. Herd immunity collapses, resulting in an increase in cases again. This partially explains the measles and pertussis outbreaks in the UK and US, respectively. In fact, in the UK, measles outbreaks have been attributed to declining herd immunity. The same applies to pertussis in the U.S. . While herd immunity suggests that if enough people get vaccinated, diseases will be less frequent, there is still a need to keep vaccination rates high. The population is constantly growing and there are groups that can't get vaccinations such as babies, the elderly, or those with chronic illness. When the chickenpox vaccine was introduced in the U.S., the death rate from the disease plummeted by as much as 97% .
But how is the threshold itself determined? In order to set a threshold for immunity, epidemiologists need to have an "R0" value. This is known as the "basic reproduction number." The value pinpoints how many people in an "unprotected" population an infected individual could afflict. If this value is high, a higher threshold is needed to protect the community. The R0 value for measles can range as low as 12 and as high as 18. Conversely, polio's R0 value can range between 5 and 7 .
All things considered, herd immunity is certainly a beneficial aspect of immunization. However, it requires constant vigilance to ensure that people are properly informed about vaccines and are doing their part to create a safer community. The world is more connected today than it was in the past, making the spread of disease easier. Thus, it's important to maintain high immunization rates, as seen with the chickenpox vaccine.