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Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold.

ABSTRACT: As severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spreads, the susceptible subpopulation is depleted causing the incidence of new cases to decline. Variation in individual susceptibility or exposure to infection exacerbates this effect. Individuals that are more susceptible or more exposed tend to be infected earlier, depleting the susceptible subpopulation of those who are at higher risk of infection. This selective depletion of susceptibles intensifies the deceleration in incidence. Eventually, susceptible numbers become low enough to prevent epidemic growth or, in other words, the herd immunity threshold (HIT) is reached. Although estimates vary, simple calculations suggest that herd immunity to SARS-CoV-2 requires 60-70% of the population to be immune. By fitting epidemiological models that allow for heterogeneity to SARS-CoV-2 outbreaks across the globe, we show that variation in susceptibility or exposure to infection reduces these estimates. Accurate measurements of heterogeneity are therefore of paramount importance in controlling the COVID-19 pandemic.

SUBMITTER: Gomes MGM

PROVIDER: S-EPMC7239079 | biostudies-literature | 2020 May

REPOSITORIES: biostudies-literature

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Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold.

Gomes M Gabriela M MGM Ferreira Marcelo U MU Corder Rodrigo M RM King Jessica G JG Souto-Maior Caetano C Penha-Gonçalves Carlos C Gonçalves Guilherme G Chikina Maria M Pegden Wesley W Aguas Ricardo R

medRxiv : the preprint server for health sciences 20220214

Individual variation in susceptibility and exposure is subject to selection by natural infection, accelerating the acquisition of immunity, and reducing herd immunity thresholds and epidemic final sizes. This is a manifestation of a wider population phenomenon known as "frailty variation". Despite theoretical understanding, public health policies continue to be guided by mathematical models that leave out considerable variation and as a result inflate projected disease burdens and overestimate t ...[more]

PMID: 32511451

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Project description:SARS-CoV-2 incidence and mortality in Europe have shown wide variation. Northern Italy in particular the Lombardy region, north-eastern French regions, Switzerland and Belgium were amongst the hardest hit, while the central and southern Italian regions, all the Balkan countries from Slovenia to Greece and the Islands of Malta and Cyprus had much fewer cases and deaths per capita, and deaths per number of cases. Differences in public health measures, and health care delivery, in the author's opinion, can only partly explain the difference. The geographical distribution of Phlebotomus sand-flies and the relative distribution of arthropod borne diseases Leishmaniasis and Phlebovirus infections especially the Sicilian Sandfly fever group corresponds to most areas of low prevalence of SARS-CoV-2. A hypothesis is proposed whereby repeated arthropod or sandfly vector infection of humans by novel viruses of zoonotic origins carrying bat or mammalian RNA/DNA, such as phleboviruses may have resulted in the development of an effective evolutionary immune response to most novel zoonotic viruses such as SARS-CoV-2 by means of survival of the fittest possibly over many generations. This process probably ran in parallel and concurrent with the progressive evolution of novel coronaviruses which spread from one mammalian species to another. Other possible, but less likely mechanisms for the role of sandfly meals within a much shorter time frame may have led to, (i) previous exposure and infection of humans with the SARS-Cov-2 virus itself, or a closely related corona virus in the previous decades, or (ii) exposure of human populations to parts coronavirus protein namely either S or more likely N protein carried mechanically by arthropods, but without clinical disease causing direct immunity or (iii) by causing infection with other arthropod borne viruses which could carry bat DNA/RNA and have similar functional proteins resulting in an immediate cross-reactive immune response rather than by natural selection. The Evidence possibly supporting or disputing this hypothesis is reviewed, however the major problem with the hypothesis is that to date no coronavirus has ever been isolated from arthropods. Such a hypothesis can only be supported by research investigating the possible biological relationship of arthropods and coronaviruses where paradoxically they may be promoting immunity rather than disease.

Dataset Information

Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold.

Publications

Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold.

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