In the face of the appearance in China in December 2019 of a new human infectious disease called COVID-19, caused by the SARS-CoV-2 coronavirus [1,2,3,4,5], countries implemented governmental measures to control the virus dispersion among the population. The virus showed a high infection frequency in humans [6,7], and from the beginning, there was great difficulty in containing its spread through confinement [8]. Moreover, there was a high frequency of severe and lethal cases in elderly persons, mainly those with comorbidities such as diabetes and hypertension [9]. SARS-CoV-2 coronavirus propagated to several countries in February and March 2020 [10,11,12,13], and in the same year, the WHO declared a pandemic on March 11 [14].

At the start of the SARS-CoV-2 world dispersion, some governmental leaders (in countries from northern Europe, for example) proposed that its propagation could be controlled by allowing the free infection of individuals, which would induce protection by antibodies and cellular immunity in a high proportion of the population, thus achieving herd immunity (HI). That would lead to the consequent reduction or even elimination of the infectious agent [15]. Very soon, the proposition was strongly challenged [16,17,18] because achieving collective immunity through the infection dispersion could have a high cost on human lives and health complications caused by COVID-19 in elderly persons, mainly in those with comorbidities [1,9]. It was observed that although the percentage of lethality in the population was low in general, the high incidence of infection raised the absolute number of severe and lethal cases [19,20,21] so, as a better option, it was decided to control propagation through the containment of human activity to reduce the contact between individuals. Social mobility was restricted to allow only the circulation of persons dedicated to essential activities [20,21,22,23], waiting for the identification of effective antiviral drugs to treat infected individuals and the approval of efficient vaccines to achieve HI through massive vaccination [24]. In several countries, quarantine was strictly enforced, attaining a significant decrease in the number of cases during the first wave of infections [24,25,26]. Nevertheless, a few months after the pandemic’s beginning and with a still high incidence of infections and high numbers of deaths, the gradual return to essential and non-essential human activities was allowed in most countries. The decision was taken due to the population’s demand to restart their economic activities, which would be further affected by a quarantine extension [27,28,29,30,31,32]. After human activities were restarted, contagion increased again in several countries, with several waves of cases at different times and places [31,32]. For this reason, in the following months, there was a partial tolerance for the realization of economic activities, combined with partial social distancing, personal hygiene and other protection measures [33,34]. In December 2020, the massive application of different types of non-proliferative vaccines started in several countries [35,36]. Although in many developed and some developing countries, several booster shots have been administered, there are underdeveloped countries where vaccination has been delayed or is still very limited, mainly due to economic limitations, which prevented them from accessing vaccines since the first days after their approval [37,38,39,40,41]. There are still many developing countries where a sufficient proportion of the protected population has not been reached to be near the herd immunity threshold (HIT) against SARS-CoV-2 [39,40,41]. The risk of massive infections in several countries is latent, as well as the risk of the appearance and propagation of more dangerous and contagious mutant strains if the virus is still propagating in populations with low immunization rates [41]. On the other hand, in several countries, a proportion of the population opposes vaccination [42,43], which hampers the achievement of the herd immunity threshold, in addition to the fact that in many developing and underdeveloped countries, child vaccination is still low or inexistent [44,45].

After the massive application of vaccines, severe cases and mortality decreased sharply, as well as recovery time for the anew infected and the re-infected. Although there is a high proportion of immunized persons, the individual neutralizing antibody levels decline after some time, so epidemic outbreaks keep appearing due to new virus variants, some of which have been dispersed globally and others more locally. Nevertheless, as vaccinated individuals and those that recovered from infection maintain a certain degree of immunity, if they are re-infected, their symptoms are less severe, and they show lower mortality [46,47,48]. All of the above make necessary the application of boosters to help maintain protective immunity levels in the population in the face of the dispersion of new variants [49,50,51].

2. Population Immunity