5.9: Susceptible Hosts and Population Immunity

At the population level, we must assume that if a new communicable disease emerges, all individuals in the population are susceptible to it. For many diseases, those who are younger, older, and/or immunocompromised are at a higher risk of more severe disease. Barring any unique inherited immunity to a specific infection, the two major ways that humans develop immunity are through active immunity (“natural” exposure to the pathogen, and the subsequent immune response), or acquired immunity through vaccination. Both of these trigger an immune system response in each individual who experiences them. Some individuals experience more severe disease than others, and the most severe disease is experienced in natural exposure. In contrast, a third type of immunity is termed passive immunity, where antibodies are received from an external source. The most common examples of passive immunity are antibodies shared between a mother and fetus in utero, or via breastmilk - however some antibody treatments have also been developed for use in medicine.

Below is an oversimplified breakdown of the human immune system response.

• Humoral or innate immunity is the first line of defense. The immune system identifies the pathogen (aka antigen) and creates antibodies to “flag” this antigen and any cells that have been affected by it. White blood cells respond to destroy pathogens and infected cells. Fevers and inflammation are also used to activate chemical messengers and speed up metabolic processes that assist with the immune response. Some antibodies can linger even after the immune response has quieted down.
• Cell-mediated immunity refers to the “memory” of white blood cells (B and T cells) that allows the immune system to respond more quickly to future infections with the same antigen. This type of immunity lasts longer than the elevated levels of antibodies that occur immediately after an infection, or those provided by passive immunity.

Vaccines provide several benefits; both at the individual and population level. To the individual, a vaccine essentially primes the immune system response by triggering a much milder reaction to a dead or weakened (attenuated) virus. Multiple doses may be required to build up this immune response to levels that are protective against future infections. The individual’s body may have both specific antibodies and B and T cells stored for many years that protect them from experiencing infection (CrashCourse, 2015, CDC, 2023e).

Some pathogens evolve and/or have several hundred variants - against which it is nearly impossible to create a single vaccine. The influenza virus and SARS-CoV2 are excellent examples of this virulent evolution. Even then, acquired immunity can lessen the severity of infection with a novel strain of the virus. Thus, vaccines can reduce and prevent hospitalizations and deaths from severe disease. For public health purposes, the ideal vaccine also decreases or eliminates infection and infectiousness, and thus prevents people from both contracting and transmitting the pathogen to others.

At the population level, vaccines increase the rate at which population immunity is developed, and decrease the cost in human lives to get there. If there are few or no humans in a community who are susceptible to a particular communicable disease (because they have developed immunity to it), the pathogen is likely going to die out due to a lack of hosts. This can cause the end of an epidemic, called population immunity, community immunity, or “herd immunity”. The amount of time that it might take for this to occur naturally, along with the number of lives that would be lost, is untenable. (Jones & Helmreich, 2020). Instead, epidemiologists use the concept of population immunity to estimate the vaccination rates required in order to sufficiently protect populations from outbreaks. The more transmissible the pathogen is, the more people will need to be vaccinated to stop it. This calculation uses the estimated number of people a single infected person will likely transmit the disease to, or R0. Thus the vaccination rate would need to be 1-1/R0 in order for the population to develop immunity. If, for example, the average person infected with the flu shared it with 3 other people (R0=3), the vaccination rate would need to be at least 68% (1-⅓) in order to develop population immunity. If the average person who gets the measles infects 12-18 others, then the vaccination rate would have to be close to 100% in order to eradicate the spread (Fine et al., 2011, Guerra et al., 2017). Of course, in real life, imperfections in individual immune systems, global travel, and modern social life add much more complexity to these estimations and the possibility of population immunity. See Fig. \(\PageIndex{1}\) below.

Figure \(\PageIndex{1}\): Community Immunity (aka herd immunity). (Copyright; CC0, The National Institute of Allergy and Infectious Diseases (NIAID))