Experts in the UK also spoke early on of herd immunity acquired through infection as a protective force that would eventually end the epidemic. Epidemiologist Graham Medley predicted that SARS-CoV-2 would eventually become endemic, suggesting that the UK`s initial strategy should be to „manage this acquisition of herd immunity and minimise exposure to vulnerable people”. The UK`s chief scientific adviser, Patrick Vallance, explained that the aim is to flatten the curve and „build a kind of herd immunity” to „protect those who are most vulnerable to it”. Long before herd immunity was considered an elimination threshold for mass vaccination campaigns, she explained why epidemics diminish and reduce – but not eliminate – an individual`s risk of infection. Just as gravity brings an object back to Earth, herd immunity is the counterweight to sustained epidemic growth. The host`s response to a microorganism depends on its ability to resist infection. The individual may not have sufficient resistance to a particular pathogen to prevent infection when exposed to the body. Alternatively, the person may possess specific protective antibodies or cellular immunity as a result of a previous infection or immunization. However, immunity is relative and can be overwhelmed by an excessive dose of the infectious agent or if the person is infected through an unusual entrance gate; It can also be altered by immunosuppressive drug therapy, concomitant disease or the aging process. These characteristics were particularly evident during the 2009 H1N1 pandemic, when infection was observed primarily in younger individuals, as individuals over 60 years of age had acquired immunity from previous exposure. Directly from the assessment of immunity in population groups defined by antibody surveys (seroepidemiology) or skin tests; These may present „immunity gaps” and provide early warning of the population`s vulnerability. Although it can be difficult to interpret data in absolute terms of immunity and susceptibility, observations can be normalized to reveal trends and differences between different populations defined in location and time.

In accordance with the concept of pathogens to which we have sterilizing immunity, such statements were misleading. When experts – and the public – began to realise that neither a previous infection nor vaccination would lead to lasting immunity to SARS-CoV-2 infection, many became pessimistic about the possibility of herd immunity and the term was again considered irrelevant to Covid-19. „Technically, depending on how transmissibility an infectious agent is, if enough people are infected and recover – and are therefore partially immune – or if they are vaccinated, it means that it will decrease, reduce or even stop transmission of the virus in the community,” Dr. Hotez explained. It should be noted that „immunity developed by a previous infection” is how it has worked since humans have been alive. Your immune system is not designed to receive vaccines. It is designed to act in response to exposure to an infectious pathogen. Apparently, according to the WHO, this is no longer the case. In October 2020, here is their updated definition of herd immunity, which is now „a blueprint for immunization”.2 In a shocking reversal that looks like a redefinition of reality, the World Health Organization changed its definition of herd immunity. Herd immunity occurs when enough people acquire immunity to an infectious disease so that it can no longer spread widely in the community. As of June 2020, the WHO`s definition of herd immunity published on one of its COVID-19 Q&A pages matched the widely accepted concept that has been the norm for infectious diseases for decades. Here`s what he originally said, courtesy of Internet Archive`s Wayback Machine:1 We can approach this concept of herd immunity from an alternative and equally informative perspective.

If an infection persists, each infected person should, on average, transmit the pathogen to at least one other person. Otherwise, the incidence will decrease and the infection will gradually disappear from the population. The number or distribution of actual transmissions per case therefore describes the spread of infection in a population and is a function of four factors: (1) the duration of infectivity; (2) the probability of transmission by „contact” between infectious persons and susceptible persons; (3) the frequency and pattern of contact between members of the host population; and (4) the proportion of the host population. Its value is in all circumstances called the reproduction number of the infection, analogous to standard demographic measures (the average number of offspring per individual and per generation). This average number of actual transmissions should be maximum if all members of the host population are susceptible – in these circumstances it is called the baseline reproduction number (R0), formally defined as the average number of transmissions expected from a single primary case introduced into a fully susceptible population.14,15 This definition can be translated directly into the formulation of mass action (Eq. 1), by defining Ct = 1 and St = T. to represent the single case introduced into a fully sensitive population. The number of secondary cases, Ct + 1, is then by definition equivalent to the basic reproduction number (R0): In 1971, we discussed herd immunity and its relevance to vaccine practice (1), using applications from Reed Frost`s epidemic model (2) and a stochastic simulation model for a family community (3) to illustrate basic concepts. This presentation builds heavily on our previous discussion*, but will also explore how these herd immunity concepts relate to some important current immunization issues. According to a study published by the Italian Institute for International Policy Studies (ISPI) (Fig.

10.4), as of May 4, 2020, 12 countries were concentrating on the highest mortality rates, while the prevalence of COVID-19 remained quite low and certainly far from the proportion required for herd immunity (between 70% and 80%).