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Abstract Governments have developed and implemented various policies and interventions to fight the COVID-19 pandemic. COVID-19 vaccines are now being produced and distributed globally. This study investigated the role of good governance and government effectiveness indicators in the acquisition and administration of COVID-19 vaccines at the population level. Data on six World Bank good governance indicators for 172 countries for 2019 and machine-learning methods (K-Means Method and Principal Component Analysis) were used to cluster countries based on these indicators and COVID-19 vaccination rates. XGBoost was used to classify countries based on their vaccination status and identify the relative contribution of each governance indicator to the vaccination rollout in each country. Countries with the highest COVID-19 vaccination rates (e.g., Israel, United Arab Emirates, United States) also have higher effective governance indicators. Regulatory Quality is the most important indicator in predicting COVID-19 vaccination status in a country, followed by Voice and Accountability, and Government Effectiveness. Our findings suggest that coordinated global efforts led by the World Health Organization and wealthier nations may be necessary to assist in the supply and distribution of vaccines to those countries that have less effective governance.

For more than a century, the causal relationship between government size and economic growth has been a challenging issue in the public sector economics. However, there is no theoretical or empirical consensus among economists on this issue. Accordingly, it seems that the best way to resolve these theoretical and empirical contradictions is experimentally investigation the causal relationship between government size and economic growth in each country. Therefore, this paper investigates the causal relationship between government size and economic growth using the Markov-Switching Granger Causality Approach in Iran over the period 1967-2017. The findings confirmed the existence of a non-linear causal relationship between government size and economic growth and showed that government size had a significant negative effect on economic growth in the form of a two-regime structure (regime Zero: 1966-2002 and regime one:1983-1987), although this negative effect was greater in regime one than in regime zero. This larger negative effect can be rooted in the fact that the share of current expenditure in total government expenditure was significantly larger in the years related to regime one (compared to regime zero). Finally, contrary to Wagner's law, Findings did not confirm the positive and significant effect of economic growth on government size in Iranian economy.

Financial crises, along with the negative and destructive effects of the debt stocks on the economy of countries with the national debt, have caused the "economic effects of the public debt stocks problem," and has become a controversial issue in the public sector economics literature. Using a Smooth Transition Regression (STR) model, this paper investigates the asymmetric impact of the size of government debt - the ratio of government debt to the central bank to GDP - on economic growth in Iran during 1973-2017. The findings showed that the size of government debt to the central bank in a two-regime structure, with two thresholds, affected economic growth by 4.40% and 28.98%. At low levels of debt (years that the size of government debt to central bank is less than 4.4%) and high levels of debt (years that the size of government debt to central bank is greater than 4.40% and less than 28.98%), government borrowing from the central bank has had a negative and positive effect on economic growth, respectively. Finally, contrary to the expectations, during the period 1980-1991 (years that the size of government debt to the central bank is greater than 28.98%), the amount of government debt to the central bank has positively affected economic growth. This positive impact can be due to the specific features of the revolution and war periods in Iran, such as reducing crowding-out effects, the significant gap between real and potential production, and the more efficient cost management during years.

One of the most challenging issues in tax policy debates is the trade-off between direct and indirect taxes. The main question is that should policy-maker focus on increasing direct taxes or indirect taxes? Accordingly, this paper investigates the issue of trade-off between direct and indirect taxes using a self- exciting threshold autoregressive (SETAR) model in Iran for the period 1971-2017. The findings showed direct and indirect taxes had affected on economic growth in a tree-regime structure. In the first regime (years that economic growth is less than -6.41%), the effect of direct and indirect taxes (% GDP) on economic growth was not significant. In the second regime (years that economic growth is greater than -6.41% and less than 9.47%), this effect was positive and significant for indirect taxes and insignificant for direct taxes. Finally, in the third regime (years that the economic growth is greater than 9.47%), direct and indirect taxation (% GDP) has had a significant negative and significant positive effect on economic growth, respectively. Therefore, it can be inferred that in the range of economic growth greater than -6.41%, a trade-off can be assumed to increase indirect taxes rather than direct taxes, so that has a positive effect on economic growth.

The annual data on economic growth and energy consumption in Iran during the period 1352-1386 is used to study the effect of energy use on the growth of Iranian economy. To that end, a smooth transition regression model is used. The selected model has two regimes and two thresholds. Regime one starts from 1353 and ends in 1362; while the second regime covers the period 1363-1386. The results show that the energy use had a negative effect on the economic growth in both regimes, and that the effect is larger in the first regime. Therefore, based on these results the growth hypothesis does not hold in the Iranian economy. This could be considered as a sign of inefficient use of energy in Iran.