An interesting article published in the Canadian Medical Association Journal by Fisman and others dated 25th April 2022 answers this question.

The speed of vaccine development has been a singular achievement during the COVID-19 pandemic, although uptake has not been universal.

Vaccine opponents often frame their opposition in terms of the rights of the unvaccinated. This article sought to explore the impact of mixing of vaccinated and unvaccinated populations on the risk of SARS-CoV-2 infection amongst the vaccinated people.

They constructed a simple susceptible infectious recovered compartmental model of a respiratory infectious disease with two connected subpopulations; those who were vaccinated and those who were unvaccinated.

They simulated a spectrum of patterns of mixing between vaccinated and unvaccinated groups that ranged from random mixing to complete like-with-like mixing, in which people have contact exclusively with others with the same vaccination status.

They developed the dynamics of an epidemic within each subgroup in the population as a whole.

They found that the risk of infection was markedly higher among unvaccinated people over vaccinated people under all mixing assumptions.

The contact-adjusted contribution of unvaccinated people to infection risk was disproportionate, with unvaccinated people contributing to infections among those who were vaccinated at a rate higher than would have been expected based on contact numbers alone.

They found that as like-with-like mixing increased, attack rates amongst vaccinated people decreased from 15% to 10% and increased from 62% to 79% among unvaccinated people, but the contact-adjusted contribution to risk amongst vaccinated people derived from contact with unvaccinated people increased.

Although the risk associated with avoiding vaccination during a virulent pandemic accrues chiefly to people who are unvaccinated, their choices affect risk of viral infection amongst those who are vaccinated in a manner that is disproportionate to the proportion of unvaccinated people in the population.

The remarkable speed of vaccine development, production and administration during the COVID-19 pandemic is a singular human achievement. Whilst the ability to vaccinate to herd immunity has been held back by the increasing transmissibility of novel SARS-CoV-2 variants of concern such as the Delta and Omicron variants, and global distribution of vaccines is inequitable, the effectiveness of SARS-CoV-2 vaccines in reducing severity of disease and disrupting onward transmission even when breakthrough infections occur is likely to have saved many lives.

The emergence of the immune-evasive Omicron variant may undermine some of these gains, although provisions of booster vaccine doses may restore vaccination to a high level of potency, and vaccines developed specifically to enhance immunity to the Omicron variant may emerge.

However, anti-vaccine sentiment fuelled in part by organised disinformation efforts, has resulted in suboptimal uptake of readily available vaccines in many countries, with adverse health and economic consequences.

Although the decision not to receive vaccination is often framed in terms of the rights of individuals to opt out, such arguments neglect the potential harms to the wider community that derive from poor vaccine uptake.

Non-vaccination is expected to result in amplification of disease transmission in unvaccinated subpopulations, but the communicable nature of infectious diseases means that this also heightens risk for vaccinated populations, when vaccines confer imperfect immunity.

Although assortative like-with-like mixing is characteristic of many communicable disease systems this may be expected to limit interactions between vaccinated and unvaccinated subpopulations to some degree, the normal functioning of society means that complete like-with-like mixing is not observed in reality.

Furthermore, the airborne spread of SARS-CoV-2 means that close-range physical mixing of people from vaccinated and unvaccinated groups is not necessary for between-group disease transmission.

Historically, behaviours that create health risks for the community as well as individuals have been subject to public health regulation.

This is true of communicable infectious diseases but also applies to public health statutes that limit indoor cigarette smoking and legal restrictions on driving under the influence of alcohol and other intoxicants.

Simple mathematical models can often provide insights into the behaviour of complex communicable disease systems.

Risk amongst unvaccinated people cannot be considered as self-regarding, and considerations around equity and justice for people who do choose to be vaccinated, as well as those who choose not to be, need to be considered in the formulation of vaccination policies.

It is unlikely that SARS-CoV-2 will be eliminated and these findings are likely to be relevant to future seasonal SARS-CoV-2 epidemics or in the face of emerging variants.

Dr Paul Ettlinger

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