Several distinct factors were considered important in assessing national needs for COVID-19 vaccines across countries, including factors related to disease burden, disease control, health system capacity, geography, society, demography, economics, and readiness.
On average, participants considered the proportion of the overall population and of the high-risk population not fully vaccinated to be the most important factors. These two factors were four to five times more important than the least important factor included in the DCE survey: the existence of a national vaccine deployment and prioritisation plan. Factors not routinely used in real-world global vaccine allocation [3, 7], but deemed among the most important in this study, included the proportion of the high-risk population not vaccinated, the economic impact of lockdowns, variants of concern, COVID-19 deaths, and the existence of a national vaccine deployment and prioritisation plan.
Relevance to the ongoing COVID-19 pandemic
Existing vaccines are less effective in preventing symptomatic disease and transmission with the Omicron variant compared to previous variants, with booster doses providing additional protection against severe illness . The global shift towards booster doses in response to Omicron is exacerbating global vaccine inequity . Boosters can only be given after a primary course of vaccination, with several countries recently shortening the interval at which individuals are given a booster .
Though the World Health Organization (WHO) has been advocating for countries to vaccinate 70% of their populations (with a primary course) by the middle of 2022, most countries missed this target . Our findings will support the fair allocation of primary courses of vaccines across countries, which in turn may limit the inequity caused by the disproportional procurement of booster doses by wealthy countries . Given the possibility for regular COVID-19 vaccine boosters, and the threat of novel variants requiring a further tweaking of vaccines, this scoring tool represents an important step towards equity in long-term pandemic response.
Assessing vaccine needs across countries
Needs assessment for COVID-19 vaccines at the country-level is complex but the COVAX Facility aims to deliver vaccines to cover 20% of all populations, including the elderly, those with co-morbidities and healthcare workers. After this, countries receive doses based on need, determined by: the effective reproduction number (R number) and its trend, hemisphere location, universal health coverage (UHC) service coverage index, health system saturation, and the size of groups at a high-risk of severe disease or death, supported by a qualitative assessment . In addition, up to 5% of vaccine doses are reserved as part of a humanitarian buffer for populations such as refugees or asylum seekers. All of these factors were identified in our Delphi survey, but not all were among the most frequently reported factors.
The Fair Priority Model is an alternative tool , which uses reproduction numbers, years of life lost, and national economic indicators to help consider vaccine needs, with needs assessment changing over time with sequential vaccine programme objectives. A third tool, proposed by researchers at Vanderbilt University , suggests allocating vaccines to countries based on their ability to distribute vaccines and capacities to provide care, which we identified as one of the most important factors relating to vaccine needs.
Despite a range of proposed tools to allocate scarce vaccines across populations, all involve the use of a relatively narrow set of metrics and fail to assess or account for the relative importance of the various indicators included. In order of decreasing importance, the factors not considered by COVAX  which were deemed among the most important by participants in our study included the proportion of the high-risk population not vaccinated, the economic impact of lockdowns, variants of concern, COVID-19 deaths, and the existence of a national vaccine deployment and prioritisation plan.
Our previously proposed framework (COVID-NEEDS)  included six of the nine most important factors identified in this study. The added value of the proposed scoring tool will likely be in expanding discussions to better consider horizontal equity, where country needs are deemed to be similar according to existing risk assessments based on a less comprehensive set of indicators. Many rich countries have used vaccines as a form of foreign aid  tied to diplomatic or economic objectives, often under the auspices of equity. By making national vaccine needs assessment more transparent, explicit, and objective, this study makes the concept of equity less abstract and open to political manoeuvring.
Operationalising the prioritisation scoring system
To consolidate and regularly update data on the identified factors in a single tool for all countries will be challenging. Nevertheless, this is possible given the wealth of COVID-19 data available in the public domain and the ability of international institutions such as the WHO to access further real-time information at the country-level. The assessment of some factors (e.g. size of the clinically vulnerable population) will be based upon estimates, which though available across countries , may vary in quality. An assessment of capacity to purchase vaccines can broadly be considered by country income-group status, with low-income countries having a very limited capacity to compete with wealthier ones with respect to ability-to-pay for vaccines from manufacturers .
The available quantitative data (including an understanding of variation in quality) must be considered alongside qualitative information from stakeholders within countries and familiar with real-time on-the-ground realities. This may be particularly important to support some quantitative metrics such as on COVID-19 variants, due to the risk of under-prioritising countries with limited genomic surveillance capacities . Current COVAX plans propose using both qualitative and quantitative data for the same purpose , meaning the current scoring tool will not require any additional ancillary inputs above those within existing WHO processes.
For the domestic allocation of COVID-19 vaccines, most countries have opted for relatively simple methods [25, 26]. This has increased the speed at which populations have been immunised. For international allocation, speed and logistics have been similarly important as reflected in COVAX plans. Further research is required to understand how the use of this scoring tool may affect the efficiency of real-world vaccine allocation. As with many health interventions, there may be a trade-off between equity and efficiency . But because most of the factors in our scoring tool are part of existing international risk assessments, it is unlikely to have a substantial impact on the efficiency of international vaccine allocation.
Although we were able to include a wide range of experts in this study, our findings may not necessarily be representative of all experts in the field. Having said this, the tool improves on existing priority-setting mechanisms. Factors were only included if reported by multiple experts working across several countries and institutions, and determining their weights was performed using a choice-based exercise (the DCE) instead of more traditional questionnaire or ranking methods.
In addition, the study was limited by a response rate of approximately 50% of the experts we invited. Although all participants initially agreed to participate, most of them were actively contributing to the pandemic response, leading to significant time pressures, meaning that many were unable to complete the DCE. The mean weights for some factors may have been different with additional participants, although large changes would be required for the ranking of the factors to change substantially.
The study also started before booster doses were widely used, meaning that the factors and weights identified may differ for the exclusive allocation of booster doses, given differences in epidemiological utility for disease control and severity compared to primary course vaccination [17, 28, 29].
Finally, we were limited in the amount of contextual detail we were able to provide in the DCE survey. There may have been further factors, beyond the two outlined for each country, that influenced decisions about which country was in greater need for COVID-19 vaccines. But by assuming everything else remained constant, we were able to isolate the quantitative importance of specific factors, which can support more qualitative country-specific information.
Implications for research
To further develop the evidence-base, future studies must aim to compare needs for vaccines as assessed by experts to those of decision-makers and members of the public. Given the vast range of impacts of the COVID-19 pandemic, it is difficult for any single group of individuals (including experts) to provide a comprehensive understanding of the range of factors involved in assessing needs for vaccines. Previous research suggests there is broad support for some level of donation of vaccines, from high-income to less wealthy settings , but how exactly the public perceive needs for vaccines is less well-understood. Although several technical issues may be better understood and analysed by experts, the value judgements of all stakeholders involved (including the public who play an important role in shaping political decisions) must be considered, to develop robust, inclusive and sustainable priority-setting processes [31, 32].
Disease X (as it has been coined) represents the knowledge that a serious international pandemic could be caused by a pathogen currently unknown to cause human disease . Monkeypox has recently been declared a public health emergency of international concern. The global case count continues to increase and vaccines are a being used for high-risk contacts, to control the spread of disease and limit severity. The United States has already signed a deal to buy 2.5 million doses from a company producing the vaccine . Vast inequities in vaccine availability are likely to re-emerge globally as the Monkeypox situation develops. Although our findings cannot be directly extrapolated to health emergencies of different diseases, they highlight the complexity of international vaccine needs assessment. Several of the factors identified here may prove to be important considerations for Monkeypox, and by making them explicit, our study will aid international discussions in the early phases of response to this and future crises.