The RIDs have huge impacts on people’s health globally, and with the progress of globalization, the social, economic, cultural, and population exchanges of countries around the world have become more frequent, which poses a challenge to the prevention and control of cross-border transmission of RIDs[20, 21]. This study described the epidemiological characteristics of imported RIDs in China during 2014–2018, and reported the correlation on the number of imported RIDs cases with inbound population volume, and the number of influenza cases in the source countries. Influenza accounted for the majority of imported RIDs with obvious seasonality. The spatial distribution of imported RIDs was different between Chinese and foreigners. Among inbound foreigners, the incidence of imported RIDs was comparable high in male, 0–14 years old group, and Oceania. With the increase of inbound travel volume and number of influenza cases in source countries, the number of imported RIDs cases increased. We hope the findings can help to improve the surveillance and early warning of imported RIDs, promote the construction of joint prevention and control of infectious diseases among countries with high burden of RIDs, and protecting the health and safety of people around the world.
There are differences in the incidence of travel-related infectious diseases in different genders and age groups[22, 23]. The differences are relative to the type of infectious disease, the population, and the destination of travel. In this study, males and the 0–14 age group have a higher risk of importation of RIDs, which is consistent with previous studies[24–27]. Males, accounting for the major of travelers, may be more susceptible to RIDs infection due to some risk behaviors and habits during the travel. Children may be more susceptible to respiratory infections due to the lack of proper protection against infectious diseases and lower immunity compared with adults[24–27]. Most children have been infected with at least one influenza virus by the age of 6 years old[29, 30]. Children infected with influenza during travel will increase the risk of infection to their parents and other relatives, which can be reduced by vaccination before travel.
The seasonal fluctuation of imported RIDs are observed in our study, which may be affected by several factors, e.g. international travel on holidays, and seasonality of RIDs in original areas.The number of population traveling abroad increase during holidays, such as Chinese traditional spring festival from January to February and summer vacation for Chinese students from July to August, which promote the cross-border spread of RIDs. In addition, the seasonality of the influenza imported to China was similar to influenza in Asian countries, which accounted for the majority of imported RIDs. The import peak in January-March 2018 was different from that in 2014–2017, which might be mainly affected by the 2018 spring influenza pandemic in northern hemisphere countries.
Travel volume is an important factor influencing the number of RIDs cases imported[33, 34]. Passengers who contracted infectious diseases before or during travel spread the infectious diseases to another country by cross-border travel. Generally, the risk of imported infectious diseases increases with the increase of passenger volume. A previous study on the cross-border transmission of H1N1 revealed that the risk of importing H1N1 into countries that received more than 1,400 passengers from the endemic countries has increased significantly. Travel volumes, especially the air travel data, are often used as an important variable for estimating the risk of cases importation at certain conditions. Therefore, when emerging and reemerging infectious diseases outbreak, the cross-border spread of infectious diseases can be constrained effectively by scientific travel restriction, which has been proved in the practice of prevention and control of RIDs in recent years[37–40].
The number of imported cases is associated with the number of reported cases in source countries, which may be affected by prevalence of the disease and population of that country. In general, the higher prevalence of infectious diseases in the source country, the higher risk of importation into neighboring countries. However, to estimate the risk of imported cases, the source countries’ RIDs prevalence needs to be analyzed comprehensively with some other factors, such as travel restriction, cultural customs, social environment, travel distance, travel transportation and purpose. This study found that the average number of influenza case reports in the source countries of Asia is smaller than that in the Americas and European countries, but the number of importation cases from Asia countries is the highest. The travel volume variable can explain part of that variation, but other significant variables influencing on the importation of infectious diseases should be further explored.
In order to assess and predict the risk of importation, previous studies have developed some statistical models based on the data related to importation, such as international flights information, the epidemiology of selected diseases and demographic information of the source countries[42, 43]. Given the prerequisites of these models, it is often assumed that all residents have the same chance of infection, and all infected persons have the same chance of boarding the flights abroad. However, this assumption is difficult to come real, which is also the main reason for the difference between the predicted results of the model and the true results. Therefore, some other factors that can affect imported infectious diseases should be further studied and included in the analysis of the predictive models.
By the way, we think that there are additional factors that affect the epidemiological characteristics of imported RIDs, which need to be further explored. Vaccination status is one of the factors that influence imported RIDs. Also, environmental changes, e.g. temperature, humidity, air pollution, and sun exposure may also influence the RIDs spread. Travel duration is often considered as another important factor of imported RIDs. Besides, due to the data mainly from EESNC and NNIDRS, it is possible to lose some information in our study, and might lead to inexactly calculated incidence of imported RIDs.