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We would like to thank the reader for taking interest in our work. We used 2018 international arrival data, as these were the latest published figures at the time of the study. Based on yearly trends in international arrivals before 2018, we have no reason to believe that these figures and more importantly, the between-country differences, would differ from early 2020 figures. We therefore assumed that the 2020 figures recorded prior to the awareness of the spread of SARS-CoV-2 would have been the same as previous years, despite subsequent decreases in international flights occurring after January 2020 as observed by Dr Cairns.
We used 2018 international data as a proxy for countries’ global connections and have therefore found that countries which were better connected globally had significantly higher increases in COVID-19 related mortality during the first wave of the pandemic. A plausible mechanistic link for our findings may therefore be that the virus would have spread significantly out of China before February 2020, after which community transmission would have become predominant in each individual country. Therefore, our findings support the hypothesis that very early travel restrictions should be considered to control the spread of SARS-CoV-2 during future waves of the current and future pandemics. This is of particular importance currently, especially as the world is facing the emergence and international spread of new SARS-CoV-2 strains.
"We however found that the main determinant was the total number of international arrivals in the country (2018 figures), signifying transmission of the infection through travel. Although the data were from 2018, there is no reason to believe that international travel figures between countries would be different in early 2020."
The study period from December 2019 to June 2020 covered a period in which international passenger capacity reduced by 8% in February 2020, by 46% in March 2020 and by 90% in April 2020 year-on-year compared to 2019 (ICAO data). It is therefore demonstrably incorrect to assert that there is no reason to believe travel figures would be different.