Herd immunity is the theoretical basis for mass vaccination programs. Herd immunity protects vulnerable populations. These include babies and those whose immune systems are weak and cannot develop resistance on their own. Until we better understand immunity to COVID-19, it will not be possible to know how immune a population is and how long that immunity lasts, let alone make future predictions. These challenges should rule out any plan to increase immunity within a population by allowing people to become infected. Herd immunity (or communal immunity) occurs when a high percentage of the community is immune to a disease (through vaccination and/or pre-existing conditions), making it unlikely that disease will spread from person to person. Even people who are not vaccinated (such as newborns and immunocompromised people) have some protection because the disease has few opportunities to spread within the community. Herd immunity is an important element in the balance between the host population and the micro-organism and represents the degree to which the community may or may not be susceptible to an infectious disease because members of the population have acquired active immunity either through previous infection or prophylactic immunization (see page 731). Among vaccine experts, the term «community immunity» is preferred to herd immunity, says Dr. Omer. «Population immunity» is another commonly used term.
If the sensitive proportion (S) were equal to the inverse of the baseline reproduction number of infection (1/R0), the mean number of transmissions per case (Rn) should be 1 and therefore the incidence should remain constant over time. Figure 77.5B illustrates this and brings us directly back to the herd immunity threshold (H). Since the immunized proportion only the complement of the proportion is susceptible (H = 1 − S), the spread of disease occurs when part of a population is susceptible to the disease. Herd immunity occurs when a significant portion of a population becomes immune to an infectious disease and the risk of person-to-person spread decreases; Those who are not immune are indirectly protected, as the persistent spread of the disease is very low. Herd immunity is the main driver of the proposal for universal vaccination, i.e. including boys and girls. With increasing evidence of HPV`s involvement in a relevant proportion of head and neck cancers, particularly oropharyngeal (Plummer et al., 2016), it has become clear that the burden of vaccine-preventable diseases in men is not negligible (de Martel et al., 2017). The vaccine has also been shown to be effective in precancerous lesions of the genital mucous membranes in men (Palefsky et al., 2011). Nevertheless, most models still predict that the greatest benefit of vaccinating boys is the indirect effect on cervical cancer in women due to the reduction of circulating virus and the faster establishment of herd immunity (Brisson et al., 2011). The possibilities and cost-effectiveness of vaccinating boys are still debated, with the extreme heterogeneity of policies in developed countries reflecting this uncertainty. Sources: Plans-Rubió P.
Assessment of herd immunity in the population through serological surveys and immunization coverage. Hum Immunother vaccine. 2012;8( 2):184-188. doi:10.4161/hv.18444 Similarly, the virus that causes COVID-19 mutates. While approved vaccines are effective against strains currently circulating in the U.S., researchers don`t yet know how long immunity to COVID-19 lasts after infection or vaccination. Therefore, a booster shot [an extra dose of vaccine] of some sort may well be needed, says Dr. Juthani. For herd immunity to exist, certain conditions must be met. The pathogen is limited to a single host species within which transmission occurs, and this transmission occurs relatively directly from one member of the host species to another. If we have a reservoir where the organism can exist outside the human host, herd immunity does not work because other modes of transmission may be available. In addition, infections must induce strong immunity.
If immunity is only partial, we will not accumulate a large proportion of immunized people in the community. What does that mean? Herd immunity works when the probability of an infected person encountering another individual in the population («random mixing») is equal. But if a person is infected and all of their interactions are with susceptible people (i.e., there is no random mixing of the population), they are likely to transmit the disease to other susceptible people. Herd immunity works best when populations are constantly mixing. This is a theoretical concept, because populations are obviously never mixed completely randomly. For example, we all connect more with family and friends than with strangers. However, the degree of herd immunity depends on how close the population is to random mixing.
