3.2 Epidemic trend and progression
All four countries had distinct disease progression since the report of their first case. South Korea reported the highest number of cases (10,237), with only 7.2% imported. Singapore confirmed 1,309 cases, 42.2% of which were imported; Hong Kong confirmed 891 cases, 56.8% were imported; and Taiwan confirmed 363 cases, 86% were imported. (Table 1). All countries confirmed their first imported infections from Wuhan, China between January 20 and 23, 2020. Considering differences in population sizes, incidence of imported cases in the countries (per 100,000 population) were 9.68, 1.32, 6.74 and 1.43 for Singapore, Taiwan, Hong Kong and South Korea respectively. Despite South Korea recording the greatest absolute number of imported cases as of April 5, Singapore had the highest imported incidence rate due to its small population size (Fig. 6).
3.3 Country specific case trend and travel restriction progression
i. Singapore (Table 1 & Fig. 1)
Singapore first implemented its travel restriction against China by ceasing inbound flights from Wuhan on January 23, 2020. Over the next few days, measures were tightened with the introduction of 14-day quarantine, visa issuance suspension and banned entry/transit of travellers from Hubei. Despite having extended the travel ban to the entire China by February 1, Singapore recorded its first local case on February 3. New local cases were detected daily and declined after peaking on February 13, while imported cases were sporadically detected at low numbers. Local cases rose again during the period when travel restrictions were imposed on Daegu and Cheongdo, Iran and Italy. Imported cases re-surfaced on March 4 since the last case on February 21 and were consistently reported from March 8 onwards. Daily cases spiked on March 17, a day after Singapore imposed its first travel restriction against Europe. Between March 10 and 25, imported cases constituted most daily new cases. Local cases predominated after the implementation of Singapore’s ban on entry and transit of all short-term visitors.
ii. Taiwan (Table 1 & Fig. 2)
Taiwan had its first imported case on January 21 and introduced its first travel restrictions – banning entry of Hubei residents/Chinese nationals and suspending entry permits for Chinese nationals – on January 25. The first local case was reported 3 days later on January 28 and subsequently on January 30. Within the next week, Taiwan banned entry of Chinese nationals and introduced 14-days quarantine for returning citizens from China, Hong Kong and Macau. Notably, there were no local cases reported between January 30 and February 15. The subsequent period whereby 14-day quarantine extended to South Korea, Italy and Iran was marked by sporadic local cases. On March 14, the daily cases started to increase as travel restriction expanded to Europe and other regions, and with increased intensity from quarantine to banned entry and transit suspension. Local case count remained low during this period.
iii. Hong Kong (Table 1 & Fig. 3)
Hong Kong had its first imported case on January 23 and first banned entry of Hubei residents on January 27. The restriction was extended to include suspension of entry permit for mainland Chinese before the first local case was reported on January 30. Subsequently, border crossings between mainland China and Hong Kong were suspended and a 14-day quarantine was required of travellers. Local cases continued to occur in small clusters. Between February 25 and 29, Hong Kong banned entry of non-residents travelling from South Korea and introduced 14-day quarantine for Iran, parts of northern Italy. Local cases remained low and sporadic during this period. From March 14, the quarantine expanded to Europe and other countries as local cases started to increase. On March 25, a travel ban was placed on all travellers except those from China, Taiwan and Macau with no travel history in the past 14 days. A 14-day quarantine was, however, required for this group. After the tightening of travel restriction, local cases increased for approximately 2 days before declining.
iv. South Korea (Table 1 & Fig. 4)
Korea encountered its first imported and local case on January 20 and 30 respectively. Between the detection of the index case and the introduction of its first travel restriction against China on February 4, there were sporadic and relatively low numbers of COVID-19 cases. However, the daily cases surged from February 19 with local outbreaks erupting in Daegu and Cheongdo. On March 9, quarantine and visa suspension were introduced for travellers originating from Japan, and by then, daily cases had generally decreased since end-February. Breakdown of daily local and imported cases were not reported through March 16. Daily cases started to rise from March 17 following infusion of imported cases, while local cases remain high. This was despite introducing a 14-day quarantine for travellers from Europe, US and eventually all travellers. By April 5, the number of imported cases constituted approximately half of the total daily cases reported.
Overall, COVID-19 was the best contained in Taiwan, with daily reported cases not exceeding 30, and local cases (both absolute and incidence) consistently maintained at the minimum. The local situation in Taiwan, Hong Kong and Korea went under control as cases headed for a downward trend. However, Korea was seeing a resurgence of imported cases, contrary to the other three countries, while Singapore was experiencing increased local transmission.
3.4 Comparison of countries’ response against different waves of epidemic centres & impact on imported cases
i. China (Wave 1)
Singapore was the only country to implement a travel restriction (against Wuhan) on the day the first imported case was reported (on January 23). However, it was not till January 29 that a full entry and transit ban on travellers who have visited Hubei within the past 14 days was implemented. Taiwan, Hong Kong and Korea recorded 3, 8 and 8 imported cases respectively before implementing their first travel restriction against Hubei Province; 4, 4, and 15 days after reporting their first imported cases respectively. Korea implemented its first travel restriction on February 4, more than a week later than the last of the three countries. Notably, only South Korea reported local transmission (7 cases) before any travel restriction was implemented.
ii. South Korea/Iran/Italy (Waves 2 and 3)
Singapore, Taiwan and Hong Kong were relatively similar in the pace of implementing restrictions against each wave of epidemic. However, Singapore lagged slightly after the first wave of actions taken against China. In the second wave of response against South Korea’s epidemic, Singapore implemented its first restriction two days after a state of emergency was declared in Korea, while Taiwan and Hong Kong both took one day. In the third wave of response against epidemics in Iran and Italy, Singapore took 12 and 8 days to enact any form of restriction for travellers from the respective regions; Taiwan took 8 and 4 days, while Hong Kong took 9 and 5 days respectively. Additionally, Singapore responded to the local epidemics in Korea by only banning and quarantining travellers who have visited Daegu & Cheongdo, while Taiwan and Hong Kong’s restrictions applied to travellers from the entire Korea. It was only in early March that Singapore extended its ban to all South Korea orginated travellers, together with the restrictions circumventing the third wave of epidemics in Iran and Italy. Following the third wave, imported and local cases in Singapore consistently surpassed that of Taiwan and Hong Kong. South Korea did not implement any restrictions on Iran and Italy.
Interestingly, neither Singapore, Taiwan nor Hong Kong reported any imported cases from Korea and Iran. Singapore confirmed its first imported case from outside China on March 4, in a traveller who had been to Europe (France, Portugal and the United Kingdom). Since then until before the first Europe-related travel restriction was implemented, Singapore recorded 56 imported cases – 21 were epidemiologically linked to travel solely in Europe, and 4 others were linked to multiple countries including Europe. Taiwan confirmed its equivalent on February 6, in a traveller who had been to Hong Kong and Italy. From then until before its first Europe-related restriction, Taiwan recorded 12 imported cases, only 2 were epidemiologically linked exclusively to Europe, and 5 others were linked to multiple countries including Europe. Hong Kong’s first non-China imported case, apart from those disembarked from cruise, was confirmed on March 4 in a traveller who had been to India. Before its first Europe-related restriction, Hong Kong received 22 imported cases – 4 were epidemiologically linked exclusively to Europe, and 1 was linked to multiple countries including Europe. Notably, imported cases from the United States surfaced even before travel restrictions targeted Europe’s emergence as the epicenter. In the same period, 10 of 56 imported cases in Singapore were linked to the United States. There was 1 case in Hong Kong who had travelled to US amongst other countries, while Taiwan recorded none.
iii. Europe, excluding Italy (Wave 4)
As the COVID-19 epicenter shifted to Europe in mid-March, Taiwan and Hong Kong swiftly implemented 14-day quarantines on the day Europe was declared an epicenter. Taiwan’s quarantine was more extensive and applied to all European travellers from the Schengen border-free zone. Singapore followed 2 days after, implementing a ban on inbound visitors from 4 European countries. South Korea once again, implemented its restriction – a 14-day quarantine – almost a week belatedly, on March 22.
Following the implementation of Europe-targeted restrictions, cases continued in an upward trend in all four countries for at least 7–15 days before peaking, with Singapore recording its maximum imported cases first within 7 days (Figs. 1–4). Moving average of daily imported cases decreased towards late March for Singapore, Hong Kong and Taiwan (Fig. 7). A comparison of weekly imported cases revealed that cases peaked at week 12 for Singapore, and week 13 for Taiwan, Hong Kong and South Korea. Although South Korea recorded the most imported cases, incidence per 100,000 stood at a mere 0.62, compared to 3.66 cases per 100,000 during Singapore’s peak (Fig. 5).
Europe was the greatest source of imported cases for all four countries (Fig. 6). All countries detected cases from Europe (week 9–10) even before travel restrictions were implemented against the region. The proportion of cases traced to Europe peaked in week 12, shortly after travel restrictions targeted the region. Notably, Taiwan received much less imported cases from Europe compared to the other countries, despite relatively similar proportions imported from other regions. Cases returning from the Americas were evidently on the rise starting from week 12, before dropping in week 14 for Singapore and Taiwan, and plateauing for Hong Kong.
iv. All countries (Wave 5)
The pattern in speed of issuing travel restrictions by all four countries were consistent across all waves. In terms of a sweeping ban against inbound travellers, Taiwan acted as early as March 18, way ahead of Singapore and Hong Kong. Singapore, Taiwan and Hong Kong took actions against all international travellers 80, 77, and 78 days respectively, after the outbreak began in Wuhan. By March 25, entry of most international travellers was prohibited with few exceptions. Korea only implemented its equivalent, a 14-day quarantine on all inbound travellers in early April – 91 days after outbreak began in Wuhan.