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Research

Impact of population mixing between vaccinated and unvaccinated subpopulations on infectious disease dynamics: implications for SARS-CoV-2 transmission

David N. Fisman, Afia Amoako and Ashleigh R. Tuite
CMAJ April 25, 2022 194 (16) E573-E580; DOI: https://doi.org/10.1503/cmaj.212105
David N. Fisman
Dalla Lana School of Public Health (Fisman, Amoako, Tuite), University of Toronto, Toronto, Ont.; Centre for Immunization Readiness (Tuite), Public Health Agency of Canada, Ottawa, Ont.
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Afia Amoako
Dalla Lana School of Public Health (Fisman, Amoako, Tuite), University of Toronto, Toronto, Ont.; Centre for Immunization Readiness (Tuite), Public Health Agency of Canada, Ottawa, Ont.
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Ashleigh R. Tuite
Dalla Lana School of Public Health (Fisman, Amoako, Tuite), University of Toronto, Toronto, Ont.; Centre for Immunization Readiness (Tuite), Public Health Agency of Canada, Ottawa, Ont.
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    Figure 1:

    Simulated epidemics for different levels of mixing between vaccinated and unvaccinated populations. (A, C, E) Incident cases and (B, D, F) population-adjusted incidence per 100 population in unvaccinated, vaccinated and overall modelled populations. The degree of like-with-like mixing (assortativity, η) varies from (A, B) random mixing (η = 0) to (C, D) intermediate like-with-like mixing (η = 0.5) to (E, F) near exclusive mixing with people of the same vaccination status (η = 0.9). As like-with-like mixing increases, epidemic size among the vaccinated subpopulation is smaller in absolute terms than among the unvaccinated subpopulation and also has a different contour. (G) Increasing like-with-like mixing increased cumulative attack rates among unvaccinated people and decreased cumulative attack rates among vaccinated people. The highest overall attack rates were seen with intermediate levels of like-with-like mixing.

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    Figure 2:

    Impact of mixing between vaccinated and unvaccinated subpopulations on contribution to risk and final epidemic size for (A) varying reproduction numbers and (B) vaccine effectiveness. Both panels show the impact of increasing like-with-like mixing on outbreak size among the vaccinated subpopulation and contact-adjusted contribution to risk of infection in vaccinated people by unvaccinated people (ψ). As like-with-like mixing (η) increases, the attack rate among vaccinated people decreases, but ψ increases. This relation is seen across a range of (A) initial reproduction numbers and (B) vaccine effectiveness. These effects are more pronounced at lower reproduction numbers and are attenuated as vaccines become less effective. We used a base case estimate of 6 for the reproduction number in the sensitivity analysis on vaccine effectiveness and a base case estimate for vaccine effectiveness of 0.8 in the sensitivity analysis for R.

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    Figure 3:

    Impact of mixing between vaccinated and unvaccinated subpopulations on contribution to risk and final epidemic size with increasing population vaccination coverage. Increasing population vaccination coverage decreases the attack rate among vaccinated individuals and further increases the relative contribution to risk in vaccinated individuals by the unvaccinated at any level of like-with-like mixing. For levels of vaccination coverage that were evaluated, increasing like-with-like mixing decreases the attack rate among the vaccinated but increases the relative contribution to risk in vaccinated individuals by the unvaccinated.

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    Table 1:

    Model parameters

    Parameter descriptionSymbolValuePlausible rangeReference
    Probability of transmission per contact multiplied by contacts per yearβ437164–728Calculated
    Rate of recovery from infection (per yr)γ7341–91Wolfel et al.31
    Basic reproduction numberR064–8UK Health Security Agency,3 Hogan et al.,7 Xia et al.28
    Mixing between subpopulations (0 = random, 1 = assortative)η0.50–0.9Assumption (approach based on Garnett and Anderson13)
    Proportion vaccinatedPv0.80.6–0.99Little32
    Vaccine effectivenessVE0.80.4–0.8UK Health Security Agency,3 Hogan et al.,7 Higdon et al.33
    Approximate adult population of OntarioN10 000 000—Statistics Canada34
    Baseline immunity in unvaccinated people0.2—Assumption
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Canadian Medical Association Journal: 194 (16)
CMAJ
Vol. 194, Issue 16
25 Apr 2022
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Impact of population mixing between vaccinated and unvaccinated subpopulations on infectious disease dynamics: implications for SARS-CoV-2 transmission
David N. Fisman, Afia Amoako, Ashleigh R. Tuite
CMAJ Apr 2022, 194 (16) E573-E580; DOI: 10.1503/cmaj.212105

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Impact of population mixing between vaccinated and unvaccinated subpopulations on infectious disease dynamics: implications for SARS-CoV-2 transmission
David N. Fisman, Afia Amoako, Ashleigh R. Tuite
CMAJ Apr 2022, 194 (16) E573-E580; DOI: 10.1503/cmaj.212105
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