The daily new confirmed cases of COVID-19 of 10 mostly affected countries and South Korea from the date with more than 100 cases to the date with less than 100 cases or by May 4, 2020, were collected and illustrated in Fig. 1. Only USA and Russia showed the peak of daily cases higher than 10,000. The countries with the peak of daily cases higher than 5,000 but less than 10,000 were Spain, Italy, France, Germany, UK and Turkey. China and Iran showed the peaks of daily cases higher than 3,000 cases but less than 4,000 cases. The peak of daily cases in South Korea was less than 1,000.
Only China and South Korea whose daily new cases dropped down to less than 100 cases. The intervals between > 100 to < 100 cases were 46 and 42 days in China and South Korea, respectively, with the peaks of 3704 cases on the day 16 in China and 815 cases on the day 12 in South Korea (Fig. 1, Table 1). The declining intervals from top to < 100 cases in China and Korea were both 30 days, which were markedly longer than the increasing intervals from > 100 cases to top (16 and 12 days, respectively). By May 4, the daily cases in Germany, France and Iran reduced to less than 1,000 after around 60 days, while that of Italy, Spain and Turkey were less than 3,000 and that of the USA, UK and Russia were still higher than 3,000. As shown in Table 1, the increasing intervals from > 100 to > 1,000 cases among the countries were from 8 (China and Spain) to 14 (Russia) days. The intervals from > 1,000 to > 3,000 cases differed, with the shortest of the USA (2 days), the longest of Iran (22 days) and 6–7 days of the majority of the European countries and China. The days from > 3,000 cases to the top day were closely related with the highest case umbers of every countries. China and Iran had the highest daily cases of 3704 and 3186, respectively, whose intervals were 1 and 2 days. The interval of the USA was remarkably long (36 days), with the highest daily case number of 44,638, while that of Russia was 18 days with the highest daily case number of 10,632. The intervals in the 7 European countries were 6–8 days, with the daily peaks in the zone of 5,000–10,000.
Table 1
Duration times and Rt values in the increase and decrease period based on the daily case number
Country | > 100 cases | > 1000 cases | > 3000 cases | Top | < 3000 cases | < 1000 cases | < 100 cases | By May 4 |
day | day | Rt | day | Rt | day | Rt | day | Rt | day | Rt | day | Rt | day |
China | 1 | 8 | 2.94 | 15 | 1.50 | 16 | 1.40 | 22 | 0.53 | 30 | 0.23 | 46 | 0.27 | - |
South Korea | 1 | - | - | - | - | 12 | 0.46 | - | - | - | - | 42 | 0.37 | - |
France | 1 | 12 | 2.85 | 19 | 2.23 | 27 | 1.10 | 46 | 0.33 | 60 | 0.78 | NA | NA | 61 |
USA | 1 | 11 | 7.26 | 13 | 5.59 | 49 | 0.95 | NA | NA | NA | NA | NA | NA | 60 |
Germany | 1 | 9 | 5.45 | 16 | 1.92 | 23 | 1.06 | 39 | 0.58 | 59 | 0.50 | NA | NA | 61 |
Italy | 1 | 11 | 4.14 | 18 | 2.02 | 25 | 0.98 | 60 | 0.70 | NA | NA | NA | NA | 69 |
Spain | 1 | 8 | 5.37 | 14 | 2.50 | 21 | 0.95 | 59 | 0.34 | NA | NA | NA | NA | 61 |
UK | 1 | 10 | 3.51 | 20 | 1.55 | 30 | 0.99 | NA | NA | NA | NA | NA | NA | 54 |
Iran | 1 | 9 | 1.86 | 31 | 0.94 | 33 | 0.78 | 35 | 0.70 | 66 | 1.10 | NA | NA | 68 |
Turkey | 1 | 9 | 3.79 | 18 | 1.50 | 25 | 0.91 | 39 | 0.58 | NA | NA | NA | NA | 48 |
Russia | 1 | 14 | 3.30 | 22 | 1.93 | 40 | 1.73 | NA | NA | NA | NA | NA | NA | 41 |
Further, we took the data of the highest daily reported as 100% and then calculated the intervals from the emerging of the 1st cases to the increases of > 100 cases, approximately 25%, 50%, 75%, 100% and decreases of -75%, -50%, -25% and > 100 cases. As shown in Fig. 2 and Table 2, the intervals from the 1st case to > 100 cases in the majority of the enrolled countries ranged from 25 to 45 days. Russia showed the longest interval with 54 days, whereas Iran and Turkey were much short with 7 and 8 days, respectively. Only the daily cases in China (March 12) and South Korea (April 2) declined to less than 100 cases during the epidemic, with the similar alterative patterns. Calculation of the intervals from > 100 cases to top and from top to < 100 cases revealed a short increasing time and a long decreasing time. The portions of increasing and decreasing times were 34.8% and 65.2% in China, and 28.6% and 71.4% in South Korea. Compared with increasing times of China (16 days) and South Korea (12 days), those of Germany (23 days), Italy (25 days), Spain (21 days), France (27 days), Iran (33 days), UK (30 days), Turkey (25 days) Russia (40 days) and USA (49 days) were obviously longer, particularly in the zone of > 100 cases to 50% (Fig. 2, Table 2).
Table 2
Duration times and Rt values in the increase and decrease period based on the daily case ratio
Country | > 100 cases | Appr 25% | Appr 50% | Appr 75% | 100% | Appr − 75% | Appr − 50% | Appr − 25% | < 100 cases | By May 4 |
% | day | % | day | Rt | % | day | Rt | % | day | Rt | % | day | Rt | % | day | Rt | % | day | Rt | % | day | Rt | % | day | Rt | Rt |
China | 2.8% | 1 | 25.0% | 6 | 4.18 | 50.7% | 11 | 2.03 | 74.6% | 14 | 1.60 | 100% | 16 | 1.40 | 78.3% | 22 | 0.53 | 55.6% | 25 | 0.25 | 21.8% | 32 | 0.25 | 2.7% | 46 | 0.27 | - |
South Korea | 12.3% | 1 | 28.1% | 2 | 6.59 | 62.0% | 7 | 1.68 | - | - | - | 100% | 12 | 0.46 | - | - | - | 55.0% | 16 | 0.33 | 27.5% | 21 | 0.49 | 10.9% | 42 | 0.37 | - |
France | 1.8% | 1 | 24.6% | 15 | 2.59 | 50.6% | 19 | 2.24 | NA | NA | 1.55 | 100% | 27 | 1.10 | 76.4% | 40 | 0.32 | 50.5% | 45 | 0.32 | 24.7% | 49 | 0.40 | NA | NA | NA | 0.84 |
USA | 0.3% | 1 | 27.4% | 19 | 2.69 | 54.1% | 25 | 1.51 | 74.3% | 29 | 1.14 | 100% | 49 | 0.95 | 75.5% | 50 | 0.85 | NA | NA | NA | NA | NA | NA | NA | NA | NA | 0.82 |
Germany | 3.2% | 1 | 21.0% | 12 | 3.33 | 47.7% | 15 | 2.30 | 62.1% | 21 | 1.31 | 100% | 23 | 1.06 | 71.2% | 36 | 0.54 | 47.3% | 38 | 0.56 | 16.9% | 51 | 0.56 | NA | NA | NA | 0.55 |
Italy | 2.0% | 1 | 27.4% | 13 | 3.25 | 53.3% | 18 | 2.01 | 81.2% | 23 | 1.14 | 100% | 25 | 0.98 | 72.9% | 36 | 0.80 | 46.7% | 56 | 0.75 | 21.3% | 68 | 0.44 | NA | NA | NA | 0.43 |
Spain | 1.7% | 1 | 25.4% | 11 | 3.62 | 48.2% | 16 | 1.95 | 79.2% | 18 | 1.50 | 100% | 21 | 0.95 | 75.9% | 34 | 0.81 | 48.3% | 51 | 0.65 | 18.8% | 60 | 0.33 | NA | NA | NA | 0.33 |
UK | 1.6% | 1 | 24.5% | 13 | 2.81 | 49.8% | 21 | 1.55 | 68.0% | 25 | 1.23 | 100% | 30 | 0.99 | 67.4% | 39 | 0.92 | 46.0% | 54 | 0.95 | NA | NA | NA | NA | NA | NA | 0.95 |
Iran | 3.3% | 1 | 26.2% | 7 | 2.20 | 55.3% | 27 | 1.30 | 75.0% | 29 | 1.04 | 100% | 33 | 0.78 | 71.4% | 40 | 0.66 | 49.4% | 48 | 0.71 | 25.9% | 66 | 1.09 | NA | NA | NA | 1.16 |
Turkey | 1.8% | 1 | 23.2% | 9 | 3.79 | 47.1% | 16 | 1.73 | 75.1% | 21 | 1.13 | 100% | 25 | 0.91 | 70.4% | 37 | 0.58 | 50.9% | 44 | 0.57 | NA | NA | NA | NA | NA | NA | 0.61 |
Russia | 2.7% | 1 | 26.1% | 22 | 1.92 | 26.1% | 26 | 1.66 | 74.6% | 38 | 1.77 | 100% | 41 | 1.73 | NA | NA | NA | NA | NA | NA | NA | NA | NA | NA | NA | NA | 1.71 |
Average Rt | | | | | 3.36 | | | 1.81 | | | 1.34 | | | 1.03 | | | 0.67 | | | 0.57 | | | 0.51 | | | | |
The intervals from top to < 100 cases in China and South Korea were both 30 days. The days from top to -50%, from − 50–25%, and from − 25% to < 100 cases were 9, 6 and 15 in China, 4, 5 and 21 in South Korea, respectively, in which South Korea revealed relatively long decreasing time from − 25% to < 100 cases than China. By May 4, the daily cases in Germany, Italy, Spain, France and Iran dropped to approximate − 25%, UK and Turkey to roughly − 50%, while that of Russia and USA were still at the high plateau. Calculating the intervals from top to -50% and to -25% revealed markedly long times in Germany (15 and 28), Iran (15 and 33), France (18 and 22), more than 30 days in Spain (30 and 39), and Italy (31 and 43), respectively. The intervals from top to -50% in UK and Turkey were 24 and 19 days, respectively (Fig. 2).
Further, the time of the high plateau (75% to -75%) of the daily cases each country was analyzed. As shown in Fig. 2, China, with the peak of 3,704 cases, lasted its high plateau for only 8 days, while Iran, with the peak of 3,186 cases, lasted for 17 days. The times of the high plateau in the countries with the peaks of daily cases ranging from 5,000 to 10,000 cases were 21 days in France (peak of 7,578 cases), 16 days in Spain (peak of 8,271 cases), 13 days in Italy (peak of 6,557 cases), 15 days in Germany (peak of 6,993 cases), 14 days in UK (peak of 8,681 cases), 16 days in Turkey (peak of 5,138 cases), respectively. The high plateau of USA lasted for 28 days with the peak of 44,638 cases, while Russia was still at the stage of increase by May 10.
The Rt value of every day since the daily cases exceeding 100 cases each country was measured and illustrated in Fig. 1 together with the daily cases. On the day < 100 cases, the Rt values of China and Korea were 0.2655 and 0.3672. By May 4, the Rt values of France, Germany, Italy, Spain, UK, Iran, Turkey and USA were below 1.0 but mostly still higher than 0.5, while that of Russia (1.7096) was still higher than 1.0 (Table 2).
The Rt values of the days of 25%, 50%, 75%, 100%, -75%, -50% and − 25% each country were further collected and summarized in Table 2. Generally, the Rt values of all enrolled countries declined along with the decreases of the daily new cases. Most of the enrolled countries showed the Rt values around 1.0 on the day of the top. The Rt values of three time-points in the increase period in every country were all higher than 1.0, with the average of 3.36 on the day of 25%, 1.81 on that of 50% and 1.34 at that 75%. On the contrary, the Rt values of three time-points in the decrease period were lower than 1.0, with the average of 0.67 on the day of -75%, 0.57 on that of -50% and 0.51 on that of -25%. The Rt values of 11 countries at every time-point seemed to be quite similar that varied in a limited range.