Substantial Impact of Post Vaccination Contacts on Cumulative Infections during Viral Epidemics

Abstract

BackgroundThe start of 2021 will be marked by a global vaccination campaign against the novel coronavirus SARS-CoV-2. Formulating an optimal distribution strategy under social and economic constraints is challenging. Optimal distribution is additionally constrained by the potential emergence of vaccine resistance. Analogous to chronic low-dose antibiotic exposure, recently inoculated individuals who are not yet immune play an outsized role in the emergence of resistance. Classical epidemiological modelling is well suited to explore how the behavior of the inoculated population impacts the total number of infections over the entirety of an epidemic.MethodsA deterministic model of epidemic evolution is analyzed, with 7 compartments defined by their relationship to the emergence of vaccine-resistant mutants and representing three susceptible populations, three infected populations, and one recovered population. This minimally computationally intensive design enables simulation of epidemics across a broad parameter space. The results are used to identify conditions minimizing the cumulative number of infections.ResultsWhen an escape variant is only modestly less infectious than the originating strain within a naïve population, there exists an optimal rate of vaccine distribution. Exceeding this rate increases the cumulative number of infections due to vaccine escape. Analysis of the model also demonstrates that inoculated individuals play a major role in the mitigation or exacerbation of vaccine-resistant outbreaks. Modulating the rate of host-host contact for the inoculated population by less than an order of magnitude can alter the cumulative number of infections by more than 20%.ConclusionsMathematical modeling shows that optimization of the vaccination rate and limiting post-vaccination contacts can affect the course of an epidemic. Given the relatively short window between inoculation and the acquisition of immunity, these results might merit consideration for an immediate, practical public health response.

Rates of SARS-CoV-2 transmission and vaccination impact the fate of vaccine-resistant strains

“…By contrast, a counterintuitive result of our analysis is that the highest risk of resistant strain establishment occurs when a large fraction of the population has already been vaccinated but the transmission is not controlled. Similar conclusions have been reached in a SIR model of the ongoing pandemic 56 and a model of pathogen escape from host immunity 57 . Furthermore, empirical data consistent with this result has been reported for influenza 58 .…”

Ongoing global and regional adaptive evolution of SARS-CoV-2

“…The emergence of multiple mutations associated with immune evasion during a period of the pandemic when the majority of the global population had remained naïve is striking. Such adaptations are generally expected to emerge among host populations where many individuals have acquired immunity either through prior exposure or vaccination ( 55 – 58 ). Furthermore, this pattern of, most likely independent, emergence of persisting variants among both human and mink populations suggests the possibility that these mutations represent nonspecific adaptations acquired shortly after zoonosis.…”

Evolution of human respiratory virus epidemics

“…This is important when assessing the impact of novel or imperfect vaccination which can lead to counterintuitive results. 15,39,40 Diversification related to immune evasion commonly enables the maintenance of large virus populations over long time scales, as is the case for influenza. [41][42][43] In the case of SARS-CoV-2, immune evasion, diversification, and host adaptation of SARS-CoV-2 have already been demonstrated.…”