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On May 5, 2023, the World Health Organization (WHO) announced that coronavirus disease 2019 (COVID-19) no longer constituted a public health emergency of international concern (PHEIC) (1). However, cross-border travel remains a key driver of the global spread of infections (2), and the changes in risks of imported travel-related infectious diseases and their impact on global health quarantine still warrant significant concern (3–4). Therefore, it is crucial to enhance the capacity of frontier public health administration to effectively detect and respond to potential inbound infected individuals. We conducted a study of frontier public health administration, assessing the potential for cross-border transmission of infectious diseases before and after the pandemic via commercial air travel and the efficiency of quarantine measures implemented at Shanghai international airports. Our study indicated that the pattern of changes in the positive detection rate by Shanghai Customs District among inbound travelers was generally in line with the disease incidence in countries from which travel originated. Our analyses showed that accurate risk assessment at points of entry would offer significant efficiency for screening out arriving individuals who had travel histories from affected countries, histories of infection, or infectious contact, especially in the early stages of epidemic control and prevention when available data are inadequate.
We analyzed epidemiological surveillance data for all travel-related infectious diseases at entry points in Shanghai and compared the distributions and contributing factors of these diseases in 2019 (pre-pandemic) and 2023 (post-pandemic). We calculated descriptive statistics for all variables and summarized continuous variables as median and range. We estimated the incidence of various infectious diseases among inbound travelers at Shanghai’s 2 international airports. We calculated proportions according to various categories and constructed graphs to show the distribution patterns of proportions among different departure or exposure countries.
China Customs implemented entry quarantine measures at ports of entry as appropriate in response to suspected imported infectious diseases. These measures included health declaration verification, fever screening, epidemiological investigation, and nucleic acid testing. In 2023, 10.1 million international travelers arrived in Shanghai and underwent entry quarantine inspections, representing a 57.5% decrease from the pre-pandemic (2019) level. A total of 5,252 suspected infections were identified: 5,198 (98.97%) respiratory, 45 (0.86%) gastrointestinal, and 9 (0.17%) vector-borne. This amounted to an overall incidence of 5.2 per 10,000 travelers, a 2.5-fold increase from the pre-pandemic level. The proportion of respiratory infections increased by 10 percentage points, while gastrointestinal and vector-borne infections decreased by 8 and 3 percentage points, respectively (Table 1).
Variable Post-epidemic (n=5,252) Pre-epidemic (n=1,503) P* Sex† 0.014 Male 2,949 (56.2) 898 (59.7) Female 2,300 (43.8) 605 (40.3) Age, years† <0.001 ≤9 193 (3.7) 196 (13.0) 10–19 299 (5.7) 245 (16.3) 20–39 2,610 (50.0) 551 (36.7) 40–59 1,590 (30.5) 334 (22.2) ≥60 529 (10.1) 177 (11.8) Nationality† <0.001 Chinese 4,030 (76.8)§ 1,153 (76.7)¶ Foreign 1,220 (23.2) 350 (23.3) Occupation <0.001 Laborer 1,596 (30.4) 3 (0.2) Student 607 (11.5) 27 (1.8) Crew 20 (0.4) 2 (0.1) Sailor 128 (2.4) 1 (0.1) Diplomatic Personnel and Entourage 28 (0.5) 0 (0.0) Business 1,013 (19.3) 127 (8.4) Child 98 (1.9) 28 (1.9) Retiree 184 (3.5) 15 (1.0) Others 1,578 (30.0) 1,300 (86.5) Country of departure†, ** <0.001 China†† 845 (16.4) 212 (16.0) East Asian countries 1,235 (23.9) 402 (30.4) South-East Asian & South Asian countries 1,719 (33.3) 444 (33.5) Middle Eastern countries 334 (6.5) 42 (3.2) European countries 736 (14.2) 97 (7.3) African countries 47 (0.9) 24 (1.8) North American countries 157 (3.0) 67 (5.1) South American countries 21 (0.4) 4 (0.3) Oceanian countries 71 (1.4) 32 (2.4) Symptom Upon Entry <0.001 Asymptomatic 3,434 (65.4) 0 (0.0) Fever 864 (16.4) 1,367 (91.0) Other symptoms 954 (18.2) 136 (9.0) Clinic Diagnosis <0.001 Respiratory 5,198 (99.0) 1,331 (88.6) Gastrointestinal 45 (0.8) 128 (8.5) Insect-borne 9 (0.2) 44 (2.9) Note: Data are reported as n (%).
Abbreviation: SAR=Special Administrative division.
* P calculated using χ2 test.
† Data have missing values for post-epidemic.
§ Data included 3,657 (69.7%) of Chinese (mainland) and 373 (7.1%) of Chinese (Hong Kong SAR, Macao SAR, and Taiwan, China).
¶ Data included 1,084 (72.1%) of Chinese (mainland) and 69 (4.6%) of Chinese (Hong Kong SAR, Macao SAR, and Taiwan, China).
** Data have missing values for pre-epidemic.
†† Data included Hong Kong SAR, Macao SAR, and Taiwan, China.Table 1. Characteristics of the cases detected through entry quarantine before & after the epidemic.
In early 2019, Shanghai Customs District began employing the Customs Surveillance and Warning System (hereinafter referred to as “the System”) at its entry points, which became the major approach to detecting suspected cases instead of fever screening. In 2023, the system detected a total of 3,217 imported cases (61.25%) at entry screening, of which 1,116 (21.25%) were detected by medical inspection, 675 (12.85%) by health declaration, 202 (3.85%) by fever screening, and 41 (0.78%) by declaration of conveyance operators. However, in 2019 (pre-pandemic), 1,367 imported cases (90.95%) were detected by fever screening at entry screening, and 85 (5.66%) by declaration of conveyance operators. These results show that accurate risk assessment at entry-exit ports provides significant efficiency for screening out arriving individuals with travel histories of affected countries and histories of infection or infectious contact. According to our data, the incidence of high-risk arriving individuals targeted by accurate risk assessment at entry screening was 65 times that of low-risk individuals. For mega entry-exit ports, accurate risk assessment is beneficial for maximizing epidemic prevention effects with minimal input.
The occupational composition of imported cases differed significantly between 2019 (pre-pandemic) and 2023 (post-pandemic). In 2023, the three largest traveler groups were laborers [1,596 (30.39%)], businesspeople [1,013 (19.29%)], and students [607 (11.55%)], collectively representing 61.23% of imported cases). Compared to the pre-pandemic period, 2023 saw a sharp decline in the proportion of tourists, constituting only 5.0%. The sociodemographic characteristics of travelers from different countries also contributed to the risk of imported infections.
The number of imported cases varied significantly by point of departure. In 2023, most were from nonstop Shanghai-bound flights originating from 88 countries and regions, including 10 American, 27 European, 9 Middle Eastern, 17 African, 4 Oceanian, and 21 Asian countries. Most inbound travelers arrived from Southeast Asia and South Asia [1,719 (33.3%)], East Asia [1,235 (23.9%)] while most identified cases were imported from Thailand, Japan, and South Korea. The geographic distribution of imported cases at points of departure closely resembled that of the pre-pandemic period. In 2019, most imported cases originated near Chinese mainland, including South Asia and Southeast Asia [444 (33.53%)], and East Asia [402 (30.36%)]. The incidence of imported infections was a key contributing factor to entry screening positive detection rates. This influence was more significant for vector-borne infectious diseases, which are closely related to seasonal factors.
The positive detection rate of imported diseases at entry points was significantly influenced by disease incidence in travelers’ countries/regions of origin. In 2023, peaks in the weekly positive detection rate of entry screening in Shanghai mirrored incidence trends in several countries and regions (Figure 1). This correlation was particularly strong for travelers from Thailand, Singapore, Vietnam, Iran, Russia, and Italy, although changes in the positive detection rate lagged slightly behind incidence rate changes, reflecting the time lag between outbreak and cross-border transmission. However, due to variations in surveillance techniques, healthcare capacity, information transparency, and data accessibility among countries, the incidence rate in some countries, such as the UAE, declined sharply. Nonetheless, in the early stages, changes in the positive detection rate generally aligned with disease incidence in travelers’ countries of origin.
Figure 1.Cross-border infection peaks by the weekly positive detection rate in Shanghai in Year 2023. (A) Thailand; (B) Japan; (C) Singapore; (D) Vietnam; (E) Malaysia; (F) Britain; (G) United Arab Emirates; (H) Iran; (I) Russia; (J) Germany; (K) United States; (L) Italy; (M) Philippines; (N) France; and (O) Netherlands.
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