Discussion
Our analysis showed a high risk of transmission of 2019-nCoV through air flights from four Chinese cities to neighbouring Asian countries. The risk of 2019-nCoV (SARS-COV-2) transmission was relatively low in Africa and South America. Several countries in both North America and Oceania showed high risk with these countries reporting at least one case of 2019-nCoV. Our risk index showed a very high correlation with the WHO's reported COVID-19 cases.
China has four times as many air passengers now than it had during SARS outbreaks in 2003. A large number of workers now travel internationally where China is heavily investing in infrastructure development in Africa, parts of Asia and Latin America. A significant and mobile Chinese population live in Europe and North America alongside an increasing amount of Chinese tourism. This travel poses a high risk of 2019-nCoV travelling across international borders. Although acquiring a case is low for these countries, the consequences are likely to be higher because of the country's capacity to control such situations [15].
Based on our model, the countries with the highest risk index but have not reported any case of 2019-nCoV as yet are Indonesia, Pakistan, Turkey, Qatar and Ethiopia. These countries are at risk and they should be the priorities for investment in case detection and airport screening. Compared to the SARS outbreak of 2003, the situation in 2020 differs due to the increased frequency and volume of international air travel.
During these early stages of the epidemic, case numbers have doubled on average every 7.4 days with an estimated basic reproduction number (R0) of 2.2 (1.4–3.9) [1]. Although the data so far suggest that the disease is mild in most cases and that the case fatality rate is currently reported to be lower than SARS or MERS, the situation is likely to go on for months and could cause severe disruption in countries that are not well prepared. Hence countries ranked as high risk in our model (4th and 3rd quantiles) should take all steps necessary to ensure prompt detection of cases and the capacity to manage these cases to prevent ongoing spread. International investment needs to be directed especially to countries with limited healthcare and public health surveillance capacity to enable the detection of cases and disease control [16, 17]
Our estimation showed a lower risk of transmission in Africa and South America. Nevertheless, low and middle countries on these continents are more likely to see the ongoing spread and major disruption from the introduction of a single case, even if the risk of importation is lower. Direct flights between Chinese cities and African countries are few which has contributed to a lower estimated risk of 2019-nCoV transmission. As of 5 February 2020, five cases have been reported from the United Arab Emirates (UAE) which acts as an important travel hub for onward journeys to the African continent. Implementation of mildly symptomatic passenger screening in the UAE may reduce the potential for 2019-nCoV to enter Africa. Screening and diagnostic capacity in Africa has been supported by a rapid grant from the Bill and Melinda Gates Foundation to the African CDC, mitigating the consequences of an importation.
The current situation is extremely dynamic and since then some countries have instigated flight restrictions and closed borders (e.g. Russia). These decisions were relevant for these locations but not based on probabilities. WHO has not recommended a cessation of transportation to free countries but suggested preventive measures. This would seem appropriate for Africa and South America with the caveat that only one case is needed to initiate a local epidemic without proper biosecurity and quarantine measures, whilst other regions will need to decide on a case-by-case basis through appropriate risk assessment.
Our study has several limitations. We considered flights from four cities of China, three of which (Beijing, Shanghai and Guangzhou) are ranked among top five busiest airports (based on the number of flights) in China and Wuhan as the site of origin of outbreaks. While including further cities in our analysis would have added further information, Beijing and Shanghai cover most of the international destinations to which other airports are connected. Further, we have adjusted for the number of reported cases in each departure. When developing the model, we initially explored using only Wuhan as the departure airport, the rank of top 10 at-risk countries remained the same. Thus our findings are still representative of the total risk posed by other airports or cities. We did not consider the risk associated with the travel route through water and land which might have an impact in the spread of 2019-nCoV. Another limitation is that the model does not account for travel patterns in other affected countries. For example, some cases have started acquiring the disease outside of China: the third case notified in the UK acquired the disease in Singapore. However overall, the risk compared with the risk of acquisition in China is very low, therefore it probably would not change the order of countries.