Effects of implemented vaccine deployment strategies on the spread of SARS-CoV-2, the ensuing global burden of disease, and the emergence of new variants

In a recent study published in natural medicineresearchers conducted a retrospective analysis to investigate the impact of vaccine sharing strategies on the heterogeneous global distribution of coronavirus disease 2019 (COVID-19) vaccines.

Study: Retrospective modeling of the effects of increased global vaccine sharing on the COVID-19 pandemic. Image Credit: M-Foto/Shutterstock


During the early stages of the COVID-19 pandemic in early 2020, most countries relied on non-pharmaceutical interventions such as social distancing and lockdowns to reduce the spread of severe acute respiratory syndrome coronavirus 2 ( SARS-CoV-2). However, these mitigation strategies were socially and economically detrimental.

Unprecedented growth in vaccine technology has resulted in nearly 49% of the world’s population having received two primary doses of one of many COVID-19 vaccines by January 2022. However, global distribution coverage has been uneven, with nearly 90% of adults in many high-income countries being fully immunized, while middle- and low-income countries have significantly lower coverage.

Most low- and middle-income countries depend on vaccines donated by high-income countries or provided by World Health Organization (WHO) vaccine-sharing programs. However, having achieved nearly complete primary immunization coverage, most high-income countries are faced with the decision to extend their immunization program to the younger population and provide booster doses or donate excess vaccines to countries that need it.

While high-income countries have larger older populations, low-income countries lack the medical and economic resources to manage increased morbidity due to COVID-19 or apply non-pharmaceutical mitigation measures. Although some countries have succeeded in limiting the disease within their borders, the global spread and emergence of new variants can only be limited by reducing the number of global infections.

About the study

In the current study, researchers used a global model based on national-level SARS-CoV-2 vaccination and COVID-19 data from 152 countries to perform a retrospective evaluation to understand the effect of a shared increase in vaccines on the spread of the pandemic. The 152 countries have been divided into four income groups according to World Bank classifications. The model also accounted for the age-related dynamics of COVID-19 by incorporating each country’s demographic data, age-related susceptibility, symptoms and disease severity into the model.

The model simulated five vaccine sharing scenarios. The first was the current vaccine sharing scenario. Scenarios two, three, and four were where vaccines were shared after countries with vaccines had given two doses to 100% of their population, all individuals over 40, and the entire population over the age of 40. 65 years, respectively. The final scenario simulated the full sharing of vaccines to achieve global immunization of equal portions of each country’s population or the entire global elderly population.

The study also looked at two models of non-pharmaceutical interventions – one where compliance with mitigation protocols was not dependent on infection levels, and another where countries that donated more vaccines relaxed social restrictions more slowly to compensate for lower vaccination coverage. In addition, the proportion of each variant calculated from data from the Global Initiative on Sharing Avian Influenza Data (GISAID) database was incorporated into the model.


The results indicated that increased vaccine sharing without adaptive non-pharmaceutical interventions would have reduced global deaths by 1.3 million during the COVID-19 pandemic, particularly in low-income countries. The simulations also revealed that additional measures, such as a slower easing of mitigation strategies by countries that donated more vaccines, could have further reduced the death rate.

Higher vaccine sharing would have reduced COVID-19 infections in low-, middle-, and high-income countries by 25.9%, 12.6%, and 15%, respectively, by mid-2021. The authors believe that increased sharing of vaccines during the early stages of the pandemic would have been more beneficial because the rapid spread of infection in countries without vaccine coverage would have generated population immunity, making delayed sharing of vaccines less efficient. Nevertheless, the sharing of vaccines remains necessary with the decrease in the effectiveness of vaccines and the emergence of variants evading immunity.


To summarize, in this retrospective study, researchers used a global model based on data on COVID-19 vaccination coverage and SARS-CoV-2 infections in 152 countries to determine the effect of various scenarios of sharing vaccines on the spread of COVID-19, planned between early 2020 and late 2021.

Overall, the results indicated that increased vaccine sharing by vaccine-rich countries combined with adaptive mitigation measures where non-pharmaceutical interventions were slowly relaxed in countries that donated more vaccines would have significantly reduced COVID-19-related mortality by mid-2021. The authors proposed that for future pandemics, a vaccine sharing strategy based on need, not wealth, would benefit all countries.

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