The sharp increase in the number of new COVID-19 cases in late 2021 and early 2022, which occurred even in countries with high vaccination levels, has raised the question about the possibility of overcoming the pandemic only through vaccination. Averaged daily numbers of new COVID-19 cases and deaths per capita and their ratio were used to find links with the percentages of fully vaccinated people and boosters and the tests per case ratio. The datasets for European and some other countries and regions corresponding to February 01, 2022 were used. Simple linear and non-linear statistical methods showed that vaccinations can significantly reduce the likelihood of deaths and the increase in the tests per case ratio diminishes the number of infections. These conclusions are consistent with the previous study based on the data from September 01, 2021 (before the advent of the Omicron). However, the numbers of new daily Omicron cases per capita increase with the increase of the percentage of fully vaccinated people and boosters. It looks that removal of restrictions on the movement of vaccinated persons and the abolition of many quarantine restrictions have led to a sharp increase in the number of infections. To overcome the pandemic, the increase of the tests per case ratio and extension of quarantine restrictions are necessary.

Large differences in the number of registered SARS-CoV-2 cases per capita in different countries encourage research into the causes of this phenomenon. In particular, a strong linear correlation was revealed between the gross domestic product per capita (GDP) and the numbers of cases per capita (CC) registered in African countries as of February 1, 2022. A non-linear correlation and datasets for African and European countries corresponding to August 1, 2022 show that CC values increase for richer countries probably be due to higher levels of testing. The much lower values of registered accumulated numbers of cases and deaths per capita (DC) in Africa are most clearly explained by the low testing level than by the influence of other factors. In some countries, the number of undetected COVID-19 cases may be tens or even hundreds of times higher than the number of registered ones, which increases the probability of the appearance of new dangerous SARS-CoV-2 strains and reduces the prospects for a quick end to the pandemic. Many unreported cases make it difficult to analyze the impact of vaccinations and other factors. In particular, the decline of DC values and the case fatality risk (DC/CC) with increasing vaccination rates in Europe requires additional research.

A simple statistical analysis of the accumulated and daily numbers of new COVID-19 cases and deaths per capita was performed with the use of recent datasets for European and some other countries and regions in order to find correlations with the testing and vaccination levels. It was shown that vaccination can significantly reduce the likelihood of deaths. However, existing vaccines do not prevent new infections. It looks that vaccinated individuals can spread the infection as intensely as unvaccinated ones and it is too early to lift quarantine restrictions in Europe and most other countries. The constant appearance of new cases due to re-infection increases the likelihood of new coronavirus strains, including very dangerous. As existing vaccines are not able to prevent this, it remains to increase the number of tests per registered case. If the critical value of the tests per case ratio (around 520) is exceeded, one can hope to stop the occurrence of new cases.

INTRODUCTION:The number of COVID-19 cases per capita (accumulated (CC) and daily (DCC)) are important characteristics of the pandemic dynamics indicating the effectiveness of quarantine, testing, and vaccination. They also indicate the appearance of new waves (e.g., caused by new coronavirus strains) and may be the result of various demographic and seasonal factors. OBJECTIVES: We investigate the influence of the volume and the density of population and the urbanisation level on the CC values accumulated in European countries and regions of Ukraine at the end of June 2021 and the impact of seasonal factors on the DCC values by comparing their dynamics in the spring and summer of 2020 and 2021 for northern and southern regions. METHODS: The influence of demographic factors on CC values was investigated with the use of linear regression. Since DCC values are very random and demonstrate some weekly period, we have used the 7-days smoothing proposed before. The second year of the pandemic allows us to compare its dynamics in the spring and the summer of 2020 with the same period in 2021 and investigate the influence of seasonal factors. We have chosen some northern countries and regions: Ukraine, EU, the UK, USA and some countries located in the tropical zone and Southern Hemisphere: India, Brazil, South Africa and Argentina. The dynamics in these regions was compared with the global one. RESULTS: The accumulated number of cases per capita CC does not depend on the demographic factors used for analysis, although it may differ by about 4 times for different regions of Ukraine and more than 9 times for different European countries. The number of COVID-19 per capita registered in Ukraine is comparable with the same characteristic in other European countries but much higher than in China, South Korea and Japan. Some seasonal similarities are visible for global dynamics, EU and South Africa. Before July 2020, the southern countries demonstrated exponential growth, but northern regions showed some stabilization trends. CONCLUSION: The CC values in Europe do not show any visible dependence on the volume of population, its density and the urbanization level. More or less similar seasonal behaviour of DCC values are visible for global dynamics in July and August. Unfortunately, we cannot conclude where the quarantine restrictions were the most effective since the dynamics of the pandemic are influenced by many other factors not considered in this study, in particular, the emergence of new strains and the large number of unreported cases.

The relative accumulated and daily characteristics of the COVID-19 pandemic dynamics in Africa were used to find links with the gross domestic product per capita (GDP), percentages of fully vaccinated people and daily numbers of tests per case. A simple statistical analysis of datasets corresponding to February 1, 2022 showed that accumulated and daily numbers of cases per capita, daily numbers of deaths per capita and vaccination levels increase with the increase of GDP. As in the case of Europe, the smoothed daily numbers of new cases per capita in Africa increase with the increasing of the vaccination level. But the increase of the accumulated numbers of cases and daily number of deaths with increasing the vaccination level was revealed in Africa. In comparison with Europe, no significant correlation was revealed between the vaccination level and the number of deaths per case. As in the case of Europe, African countries demonstrate no statistically significant links between the pandemic dynamics characteristics and the daily number of tests per case. It looks that countries with very small GDP are less affected by the COVID-19 pandemic. The cause of this phenomenon requires further research, but it is possible that low incomes limit the mobility of the population and reduce the number of contacts with infected people. In order to overcome the pandemic, quarantine measures and social distance should not be neglected (this also applies to countries with a high level of income and vaccination).