Return to normal pre-COVID-19 life is delayed by inequitable vaccine allocation and SARS-CoV-2 variants

Abstract

As a result of the COVID-19 pandemic, whether and when the world can reach herd immunity and return to normal life and a strategy for accelerating vaccination programmes constitute major concerns. We employed Metropolis-Hastings sampling and an epidemic model to design experiments based on the current vaccinations administered and a more equitable vaccine allocation scenario. The results show that most high-income countries can reach herd immunity in less than 1 year, whereas low-income countries should reach this state after more than 3 years. With a more equitable vaccine allocation strategy, global herd immunity can be reached in 2021. However, the spread of SARS-CoV-2 variants means that an additional 83 days will be needed to reach global herd immunity and that the number of cumulative cases will increase by 113.37% in 2021. With the more equitable vaccine allocation scenario, the number of cumulative cases will increase by only 5.70% without additional vaccine doses. As SARS-CoV-2 variants arise, herd immunity could be delayed to the point that a return to normal life is theoretically impossible in 2021. Nevertheless, a more equitable global vaccine allocation strategy, such as providing rapid vaccine assistance to low-income countries/regions, can improve the prevention of COVID-19 infection even though the virus could mutate.

Keywords: Administered vaccinations; COVID-19; delta variant; herd immunity; vaccine effectiveness.

Fig. 1.

Five compartments of the SIRV model and their relationships with the parameters.

Fig. 2.

Flowchart of the implementation of epidemic forecasting based on the SIRV model and Metropolis–Hastings sampling. Parameter estimation is driven by the daily confirmed cases and the numbers of vaccinations administered at 2-week intervals. The optimal parameters are directly incorporated into the SIRV model to predict the dynamics of the infected and vaccinated populations.

Fig. 3.

Timetable for herd immunity in 191 countries/regions. The timetable based on the current vaccinations administered (a) demonstrates that 58 countries/regions may face more than 3 years of vaccine shortages, and that based on a more equitable vaccine allocation strategy (b) demonstrates that all countries/regions could reach herd immunity by the end of 2021. Figure (c) shows the time span between (a) and (b).

Fig. 4.

Polar plots based on (a) the current vaccinations administered and (b) a more equitable vaccine allocation strategy. The plots include the date of herd immunity (the radial axis) and the number of required doses of vaccines (the angular axis, billions). The dots represent countries that have four different levels of gross national income per capita. The poles of the plots correspond to the start date of 12 September 2021.

Fig. 5.

(a) Cumulative case-to-vaccine dose ratio grouped by income based on the current vaccinations administered. Ratios close to 1 indicate that the corresponding countries have small infected populations, whereas ratios far from 1 indicate that the countries have severe pandemic situations and large infected populations. (b) Days to herd immunity and daily confirmed cases based on different vaccine efficacy levels. This experiment assumes that 1% of the population is vaccinated each day. The time required to achieve herd immunity increases from 18 to 36, 48 and 49 days as the vaccine efficacy gradually decreases from 90% to 80%, 70%, 60% and 50%, respectively. The results may vary depending on the parameters in Eq. (1).