Due to transport convenience, travelers may be exposed to infectious diseases or become ill overseas and may spread the disease from one country to another, causing a transnational pandemic. Thus, international travelers are of great significance in the epidemiology of infectious diseases [29]. For example, in the past decade, international travelers have faced threats such as Ebola, Zika, and chikungunya. Infectious disease surveillance networks that constantly evolve with time or environmental changes (e.g., the Global TravEpiNet, an alliance formed by health clinics across the United States, and Taiwan CDC’s travelers’ health surveillance network) have contributed to meeting the continuous demand for international traveler surveillance. In addition, the risks of travel-related diseases vary by destination and traveler characteristics. To assess the actual risks of international travelers, this study collected and analyzed correlations between imported cases of COVID-19 (i.e., international travelers) at a specific period and domestic cases in the same period.
Based on the chronological order of COVID-19 infection, the results indicate that increased numbers of domestic cases may have been caused by international travelers transmitting SARS-CoV-2 in Taiwan in March 2020. Moreover, the majority of imported cases fell within the age group of 20–29 years, which was similar to findings reported elsewhere [30]; however, the age group of 50–59 years had the highest proportion of domestic cases. This indicates that young people infected with COVID-19 may spread the disease to Taiwan through international travel, leading to a sharp rise in the infection rate among low-immunity populations (middle-aged and older adults) in the destination country (Taiwan), in turn causing health threats and a health care burden. Furthermore, according to studies elsewhere, most confirmed COVID-19 cases are concentrated in metropolitan areas [31], which concurs with the findings of this study. This study revealed that with the concentration of imported cases in metropolitan areas of northern Taiwan (particularly Taipei City), domestic cases also rose significantly, resulting in increased risk for local residents. This study inferred that the incidence and spread of COVID-19 is geographically concentrated and limited. Accordingly, the SARS-CoV-2 infections imported through overseas travel are likely to be the major risk factor for the COVID-19 pandemic in Taiwan. These findings may serve as the basis for governments or public health experts to formulate pandemic prevention policies and health resource allocation decisions.
According to the literature, no difference has been observed in the sex of patients with COVID-19 [32], which is consistent with the results from the number of confirmed COVID-19 cases (including imported and domestic cases) in this study. However, this study also discovered that patients differed by sex in their risk of infection according to month of infection and area of residence, with the highest risk observed in men infected in April 2020 or men living in southern Taiwan. Thus, individuals of different sex may have been exposed to different levels of health threats due to the month of infection or area of residence when they were infected with COVID-19. Second, according to another study [33], patients with COVID-19 are mainly adults, and most cases of childhood infections are related to contacts with or family gatherings involving adults with confirmed infections. The results of this study revealed that the majority of patients with COVID-19 in Taiwan are adults (aged 20 years or older, 94.4%, 418/443) and that the source of childhood infection was family gatherings, similar to the findings of the aforementioned study. However, this study also determined that risks to individuals of different ages varied by month of infection and area of residence. Middle-aged and older adults (aged 50–59 years and 70 years or older) had a higher risk of infection in January and February, whereas young people were at higher risk during March–May. Therefore, the medical and health care burden for different age groups varies in different months, which causes unbalanced consumption of medical resources. In addition, older adults (aged 60–69 years) living in northern Taiwan also faced a higher infection risk. This indicates that when individuals of different age groups are infected with COVID-19, they, similarly to the aforementioned comparison by sex, are exposed to different health hazard risks due to the month of infection or area of residence. Furthermore, this study determined that the risk of local residents being infected with COVID-19 differs according to month of infection and place of residence. Another possible reason is the disproportionate attention to pandemic prevention by patients with COVID-19 in different months of infection and the varying effectiveness of pandemic prevention in different areas of residence, which led to the rise in the risk of COVID-19 infection. The aforementioned results can serve as a reference for the Taiwanese government in formulating pandemic prevention policies, preventive medicine, and clinical care.
Regarding the epidemiological trends of COVID-19, the results indicated that compared with other age groups, patients aged 20–39 years (including imported and domestic cases) accounted for the highest proportion (percentage) regardless of sex, age, or area of residence, which concurs with findings reported elsewhere [33]. The possible reason is that young people have milder COVID-19 symptoms and higher infectivity; thus, the number of confirmed cases for this group was high. In addition, the results also indicate that incidence of COVID-19 in March 2020 was highest regardless of sex, age, or region, which was inferred to be related to the major outbreak of the pandemic in Europe and the United States in March 2020. Moreover, this study discovered that patients with COVID-19 were concentrated in the densely populated metropolitan areas of Taipei City and New Taipei City in northern Taiwan (47%, 209/443), which was inferred to be related to the characteristic of COVID-19 being transmitted rapidly in crowded and confined spaces [34]. Taiwan’s epidemiological characteristics of COVID-19 in human, temporal, and geographical dimensions are recommended to be incorporated into the planning and strategy implementation of public health measures and pandemic prevention.
According to the findings, the mortality of patients with COVID-19 in Taiwan was 1.6%; men accounted for most of these deaths (85.7%, 6/7). Deaths were also concentrated in the age group of 40–80 years. These findings concur with those of other studies [35-37]. Most COVID-19 cases involving children exhibit mild symptoms, with sporadic deaths [38]; no children died in Taiwan. A noteworthy aspect is that the sources of infection for patients with COVID-19 in Taiwan (including those resulting in death) were contact infection, family gatherings, and hospital infection; no community spread was recorded. A possible reason is the decisive and accurate pandemic prevention policies by the government and the joint efforts of the public and the government to combat the pandemic.
This study has the following two limitations. First, the infectious disease data published by the Taiwan CDC on the Internet platform provide only the basic epidemiological data of patients with COVID-19; they do not contain clinical data or detailed experimental procedures. However, the authenticity of the positive confirmed cases announced by Taiwan’s CDC is convincing. In addition, differences or trends in patients’ clinical data or symptoms could not be compared. Second, the data disclosed on the platform also do not contain any information related to the SARS-CoV-2 genotype; thus, the study is incapable of determining (1) the genotype of SARS-CoV-2 currently prevalent in Taiwan or (2) the genetic relationship between the SARS-CoV-2 genotypes in Taiwan and those in other countries. However, this study has one advantage, which is the instant and accurate data disclosed by Taiwan’s public sector on the online public platform. In addition, the platform has maintained an abundance of information over a long period, enabling researchers and institutions to describe or employ statistical methods on the monitored infectious disease data to produce academic value, promote effective disease control, and maintain the health of the Taiwanese public.