Out of 4257 articles found, finally 57 articles were included in the study. These articles examined successful interventions in 40 countries on the path to UHC. Most studies were conducted in Asia and the least in Europe.
The results of the reviewing the time trend of study publication revealed that most studies (except 2) were published after announcing the UHC as a national and international policy for countries by World Health Organization in 2008 [3, 114]. Also, the time trend of articles publication indicates the importance and great attention of countries to international health-related policies. Considering the many efforts being made in this area, again the high importance of this issue and the repeated mention of it by the WHO and the commitment of countries to achieve UHC by 2030, demonstrates the feasibility of conducting interventional studies and publication of very important and modeling articles for other countries. Therefore, published articles in this field need to address the performed intervention in more detail so that other countries do not make repeated and costly mistakes when using model countries' experiences [3, 6, 115, 116].
Most of the interventions were done in lower-middle and upper-middle income countries. Few studies were conducted in high or low-income countries. One of the possible reasons could be that the high-income countries are fully achieved UHC and need for no major interventions. In low-income countries, also, the low interventions could be due to their low income and their inability to finance the structural and basic interventions and to develop major infrastructure. However, evidence suggests that low-income countries have also taken substantial measures and successful interventions to achieve UHC, given the importance of health and its impact on their economies and sustainable development. However, Valuable interventions in countries such as Nepal [82, 90], Ugunda [117, 118] Rwanda , Tanzania [50, 77], Ethiopia , Afghanistan  and Madagascar  have been conducted and positive results have been reported. In addition to published studies in this area, it should be noted that many successful interventions may have taken place prior to the announcement of this global policy in 2008, which has not been published with the aim of UHC, and these studies are likely to be lost.
Each country, taking into account its own needs and specific circumstances, undertakes specific interventions to achieve UHC. Considering the economic conditions of countries, the results of the present study show that most of the interventions have performed in the field of financing or financial protection functions. Thus, most interventions in these countries have focused on the insurance system and targeted the poor people or specific groups of society [67, 75, 78, 80–82, 120]. Also one of the issues that mostly reformed by countries is the payment and premium systems [60, 65, 99, 121]. Similarly, a study by Elio Borgonovi and Emilia Compagni (2013) indicates that social, economic, and political sustainability are key drivers of health interventions and reforms in achieving UHC . Also in many studies, social health insurance , premium [44, 45, 124], cost containment , national health insurance system [126, 127], tax revenue [128, 129], risk-pooling mechanisms [4, 128], strategic purchasing  and such as are crucial factors in the financial protection function of achieving UHC, that positive reform interventions in these areas can draw countries one step closer to achieving their original goal. That is why financing function or the financial protection dimension and interventions in them can be considered as an essential component of achieving UHC. The results of the study showed that successful interventions in the countries under study have positive results such as, reduction in the intensity of care and decrease in length of hospital stay [60, 131], elimination of costs of services that cause overuse of health services , allocating a large percentage of income to health , reducing out-of-pocket payments , and so on.
Interventions in service coverage show that most interventions is in areas such as modifying or creating service packages based on people's needs at all levels of service provision or for different age groups and specific diseases and also, implementing fundamental interventions in primary health care or community-based health care. Interventions in this area have been in line with the findings of studies that have identified service packages as a major and very effective factor in achieving UHC [14, 125, 133–135].
The results show that most of the interventions in the field of service coverage are targeted specific groups such as children, adolescents, women as well as specific patients, which could be due to the high vulnerability of women and children under 5 years old and the financial inability of these groups in low- and middle-income countries. Also designing dedicated and fully customized service packages can be much more efficient and effective than comprehensive service packages designed for the population of a country without considering the specific needs of different groups. With regard to specific diseases, special conditions such as chronicity, being erodible, and cost consuming can lead to catastrophic expenditure and get patient poor, for this reason, people with specific illnesses are covered by free services.
The study showed that most of the interventions in the field of population coverage were related to the coverage expansion in specific groups or diseases [59, 63, 65, 71, 77, 79–82, 92, 136]. Countries are also trying to cover poor households and individuals by implementing mandatory or voluntary insurance programs in order to cover more population [14, 80, 120, 137, 138]. Thailand, for example, with the aim of cover its population considering socio-economic conditions, by implementing a 30 Bahat health plan, could increase its insured population from 40–95% within four years, which is also considered one of the most successful interventions in this field . China has also been able to cover its entire population by implementing urban and rural health insurance plans [67, 78].
The results of the present study show that quality interventions with 18 cases had the least report. While studies on UHC have identified the quality of care and regulatory mechanisms for quality as one of the most influential factors in achieving UHC [33, 49, 96, 97, 140]. One of the reasons for low interventions in this dimension could be the over-emphasis of countries on the quantity of services provided and coverage of the majority of the population. It can also be attributed to the late introduction of this dimension and its recognition as the hidden dimension of UHC. While implementing programs related to other dimensions of public health coverage, countries need to take into account the quality dimension and monitoring it, so that the interventions can be more efficient and effective. However, given the importance of quality in health, countries have taken important interventions such as training and using experienced nurses , using information technology to reduce medical errors and filling the gap in access to health services , maintaining quality of service by licensing qualified individuals and hospitals accreditation [15, 76] and improving patient safety . However, the results of interventions in the quality dimension indicate maintaining and enhancing the quality of health care for service receivers through the implementation of quality standards and increasing users' satisfaction with services and eliminating the inequality in accessing quality health services in some are countries [99, 101, 141].
Study the results of interventions in different countries showed that most of the interventions targeted financial protection and most of the published results also were in this area. This could be for reasons such as the importance of financing and reimbursement in the health system and financial protection of citizens against illness, which interventions in this area can reflect an early impact and can be better monitored and evaluated. However, results from the quality dimension are less reported. One of the reasons may be that interventions in this field are new and on the other hand the long-term impact of quality improvement interventions can be effective in publishing less studies in this area. Delays in the considering of the quality of health services by the World Health Organization as the fourth dimension could also be another reason for the low publication of quality interventions and their effects on UHC.
The present study shows the comprehensive and clear view of successful interventions performed in most countries at different income levels, which seek to achieve UHC, which summarizing and reporting these successful interventions can be a model and guide for other countries to avoid the costs and recurring mistakes. But, one of the limitations of this study was the use of only two English and Farsi languages to search and collect studies and documentation due to the authors' familiarity with these two languages. While reports and documentation of successful interventions in countries may have been published and documented in other than Farsi and English language that has not been reviewed in this study. It should be noted that the present study examined only successful interventions that had good and significant results for countries. The reader should note that also some countries on the path to achieving UHC have had unsuccessful and costly interventions that can be used as a model to learn from failures and only focus on successful patterns can’t be effective.