Background
The coronavirus disease 2019 (COVID-19) pandemic is still raging globally, with variants such as Omicron emerging and spreading widely, and with unprecedented impact on societies and economies [
1]. Several non-pharmaceutical interventions (NPIs) have been effective to reduce virus transmission [
2,
3], but it is unrealistic to continue many of these NPIs for long-term maintenance. Rapid and successful development of efficacious and effective vaccines against severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) and its variants is making it possible to manage the pandemic [
4].
General public health use of COVID-19 vaccines started in December 2020 and has accelerated globally at an unprecedented rate. However, inequitable country-level access to vaccines may unbalance global immunity and will likely impede global reopening. People may not get vaccinated for a variety of reasons, including supply-and-demand challenges, country purchase capacity, not being a member of a country-specific vaccine target population, lack of perceived risk, inconvenience, and personal hesitancy to receive vaccines [
5‐
10]. Most high-income countries have purchased vaccine directly using advanced purchase agreements with vaccine developers and manufacturers [
5]. Low-income countries were less able to purchase/receive vaccines, although global efforts like COVID-19 Vaccine Global Access (COVAX) increase access [
11].
Disparities in COVID-19 vaccination progress have been observed between and within countries. Several dashboards use official data sources to track vaccination progress, but there has been insufficient focus on variation in countries’ vaccination policies [
1,
12]. One study explored the correlation between gross domestic product (GDP) per capita and vaccine coverage in 138 countries and showed disparities in vaccine rollout among countries with different income levels [
13]. Country-level spatiotemporal disparities in vaccination were also seen [
14,
15]. Descriptive analyses of vaccination campaigns across countries, especially of vaccination policy and policy-specific demand, have not been realized to the extent necessary for upcoming policy formulation.
In this study, we aimed to describe the global landscape of COVID-19 vaccination policy by authorized vaccines, primary/booster vaccination, and target population. We constructed metrics for vaccine coverage to explore the extent and variation in global and regional vaccination progress, and we estimated specific demand for vaccine.
Discussion
Our study provided insight into the global landscape of COVID-19 vaccination policy, vaccine coverage, and demand of vaccines at this phase of vaccine rollout. Most countries have made COVID-19 vaccination a priority, and more than 10 billion doses of COVID-19 vaccines administered around the world represents a significant milestone in the response to the COVID-19 pandemic. With increases in vaccine production, the main target populations globally were groups aged 12 years and older, with some countries extending vaccination to children as young as 2 years old. However, we showed vast differences in vaccine coverage across countries, in which doses administered per capita in high-income countries was 11.9 times that of low-income countries. As a result, a huge imbalance in demand raises concerns of inequitable access to vaccines, even as production capacity increases.
Vaccination has shaped the epidemic curve of COVID-19, especially for severe outcomes [
1]. However, we found a pattern similar to the dilemma of vaccine distribution during the 2009 H1N1 influenza pandemic in which high-income countries procured most vaccines, and access to vaccines remained inequitable [
39,
40]. Although vulnerable groups have not been effectively protected in some resource-poor countries, countries with high vaccine coverage have initiated booster programs or even fourth doses based on the evidence that current COVID-19 vaccines provide sustained protection against severe outcomes and even variants of concern, but antibody levels have waned [
41]. This may further widen the gap of population immunity between countries, as supply exceeded demand very early in high-income countries, with relatively little transfer of vaccines to LMICs. Disparities in coverage among countries may impede the global effort of building herd immunity to stop the pandemic. Slow vaccine rollouts in some low-income countries left individuals vulnerable to emerging variants, and the expanding epidemic may further increase risk of emergence of new SARS-CoV-2 variants [
42]. Optimal allocation for limited supplies of vaccines to people at high risk or who have no immunity may save more lives and help contain the pandemic by building more immunity. Although COVAX planned to procure and deliver at least 2 billion doses by the end of 2021 [
43], only 367 million doses have been successfully allocated as of August 2021 [
31].
Scaling up production capacity of current vaccines and developing more effective vaccines remain top priorities, especially since concerns over waning immunity and SARS-CoV-2 variants have led some countries to deploy extra vaccine doses. It may be feasible to use a booster dose with less antigen as a dose-sparing strategy that still provides adequate immune response [
44,
45]. With limited supplies of several vaccines, heterologous prime-boost regimens also should be considered because they appear to induce strong immune responses [
46]. Research and development of one-dose, variant-specific, and broad-spectrum vaccines could curb the pandemic more effectively and efficiently. It is also vital to strengthen international coordination of development, manufacturing, and deployment—for example, by sharing knowledge and expertise, and providing useful guidance to build or improve production facility layouts and production lines [
47], enabling more countries to make vaccines.
In addition to challenges of production, affordability, and allocation, vaccine hesitancy is a key barrier for some countries with sufficient supplies to reach the expected vaccine coverage levels among target populations (Additional file
1: Fig. S9) [
48]. Since vaccine hesitancy is not a singular problem, interventions should be implemented that build and sustain vaccine confidence through joint efforts by vaccine manufacturers, governments, and other parties to ensure safety and effectiveness of vaccines and provide timely disclosure of relevant information to the public [
49]. Although the varying degree of vaccine hesitancy among countries might have impacted our association analyses and estimates of demand, we believe the impact was relatively small since acceptance of COVID-19 vaccines will likely change over time as more robust evidence and monetary incentive policies emerge [
50].
Our univariate analysis showed that socioeconomic and health system-related factors might be predictors of vaccine coverage. Countries with a higher SDI and HAQ score were more likely to have more vaccine doses administered than those with lower scores. This finding may reflect that healthcare systems play a key role in vaccine rollout, because health-related resources and capabilities are necessary for vaccination campaigns. For example, inadequate equipment for temperature control and time lags between shipments and deployment of vaccines may impede vaccination progress in low-income countries. However, the associated factors identified in our univariate analysis cannot be always interpreted as being naturally causal and the interaction of those factors are not addressed since these are complex processes that affect vaccination rollout and merit further investigations [
6]. Arguably, widespread vaccine coverage needs to fit multiple efforts simultaneously—at global, national, and sub-national levels, including accessibility of vaccine, individual acceptance, and healthcare system requirements for vaccination [
5,
51].
Planning COVID-19 vaccination programs is complex and arduous. Some concerns were raised during vaccination campaigns. Immunization strategy for individuals previously infected with SARS-CoV-2 remains unclear, while the number of confirmed cases worldwide has reached 400 million (as of 8 February 2022), accounting for 5.3% of the total global population [
1]. Interim evidence suggests that antibody levels in naturally-infected people persist for over 1 year [
52,
53]. Vaccine-induced immune responses in individuals with mild previous infection are generally higher [
54], as previously infected individuals who were given one dose of a COVID-19 vaccine have higher responses than full-schedule vaccination of people who had not been previously infected [
55,
56]. Therefore, a one-dose schedule for previously infected individuals might be acceptable to achieve the dual purpose of protecting populations and saving supplies. Certainly, more evidence is needed to guide future policies.
The emergence and global dominance of Omicron variant further highlights the importance of rapidly addressing the global vaccine disparities and inequality. COVAX would benefit from increasing cooperative actions facilitating access to more pre-qualified vaccines for participating countries. For LMIC populations that have not benefited from COVAX vaccines, the World Bank, wealthier countries, and other capable groups could provide more financial support for vaccine purchase or direct donation of vaccines. With these approaches, some countries have achieved relatively high vaccine coverage, including Indonesia and Bangladesh [
57,
58]. The delivery of COVID-19 vaccines requires much preparation—for example establishing vaccination rollout platforms and infrastructure in LMICs [
59]. Low-income countries can construct regional networks, apart from bilateral partnerships, to support ultracold chain requirements of mRNA COVID-19 vaccines and train healthcare system personnel to identify target population [
60]. Most importantly, transferring vaccine technologies to LMIC-based manufacturers would be a long-term benefit to global equity. Such transfer needs waiver of intellectual property protections for COVID-19 vaccines and strengthening of regulatory capacity [
60,
61].
There are several limitations to our study. First, all data in our study were obtained from public sources. Therefore untimely, opaque, and language-restricted data disclosure limited data completeness and prediction of subpopulation immunity. Standardizing reporting of COVID-19 vaccination and increasing data sharing and transparency could promote progress of the global vaccination campaign. Second, estimates of population sizes with contraindications and immunosuppressing conditions were constrained because it is difficult to determine what proportions of these populations could be vaccinated. Some countries use vaccines off label, further complicating determination of the eligible population. Third, country vaccination policies will change with more real-world evidence and experience; subnational units may make local vaccination policy independent of national recommendations. Vaccine demand in our study reflected only national level policies. Fourth, to provide a comprehensive landscape to the end of the study period, we included country data within two weeks of the end of the study period, introducing a small amount of error from missing end-period values. Fifth, our association analyses make it impossible to accurately measure the contribution of each factor, thereby further studies are needed to identify the explicit drivers of vaccination.
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