Comparing the impact of vaccination strategies on the spread of COVID-19, including a novel household-targeted vaccination strategy

Abstract

With limited availability of vaccines, an efficient use of the limited supply of vaccines in order to achieve herd immunity will be an important tool to combat the wide-spread prevalence of COVID-19. Here, we compare a selection of strategies for vaccine distribution, including a novel targeted vaccination approach (EHR) that provides a noticeable increase in vaccine impact on disease spread compared to age-prioritized and random selection vaccination schemes. Using high-fidelity individual-based computer simulations with Oslo, Norway as an example, we find that for a community reproductive number in a setting where the base pre-vaccination reproduction number R = 2.1 without population immunity, the EHR method reaches herd immunity at 48% of the population vaccinated with 90% efficiency, whereas the common age-prioritized approach needs 89%, and a population-wide random selection approach requires 61%. We find that age-based strategies have a substantially weaker impact on epidemic spread and struggle to achieve herd immunity under the majority of conditions. Furthermore, the vaccination of minors is essential to achieving herd immunity, even for ideal vaccines providing 100% protection.

Fig 1. Effect of vaccine deployment on the reproductive number, according to strategy and variant.

Epidemic reproduction number as a function of vaccinated fraction of population and vaccine efficiency for each of the four presented strategies, assuming a base reproduction number of R₀ = 2.5 for the COVID-19 Alpha variant and R₀ = 3 for Delta. Each data point (mean and standard error bar) is based on 20 duplicate simulations.

Fig 2. Impact of different vaccination strategies on the reproductive number R according to varying pre-vaccination R₀.

R₀ is adjusted by varying the amount of contact in the random layer of the model, while other layers (schools, workplaces, households) are kept constant. Dashed lines indicate the fraction of population which needs to be vaccinated in order to achieve herd immunity R < 1 depending on R₀ for an unvaccinated population. Herd immunity thresholds are also shown for random and age-dependent vaccination for comparison purposes. Grey denotes infeasible vaccination fractions (>80%) for the 18+ strategy. The depicted R values are averages of 20 duplicate simulations for each data point.

Fig 3. Herd immunity thresholds according to R₀ and choice of strategy.

Fraction of population requiring vaccination in order to reach herd immunity (R < 1) according to each strategy and R₀ for an unvaccinated population. Each data point and corresponding errorbar represent the mean R and standard deviation of 20 duplicate simulations, respectively.