ABSTRACT
BACKGROUND: Influenza circulated at historically-low levels during 2020 and 2021 due to COVID-19 pandemic travel restrictions. In Australia, international arrivals to Australia were required to undertake 14 days hotel quarantine to limit new introduction of SARS-CoV-2 virus. METHODS: We used routine testing data for travellers arriving on repatriation flights to Darwin, Australia from 3 January to 11 October 2021 to identify importations of influenza virus into Australia and used this information to estimate the risk of a case exiting quarantine while still infectious. Influenza-positive samples were sequenced and cases were followed-up to identify transmission clusters. Data on the number of cases and total passengers was used to infer the risk of influenza cases existing quarantine while infectious. RESULTS: Despite very low circulation of influenza globally, 42 cases were identified among 15,026 returned travellers, of which 30 were A(H3N2), two were A(H1N1)pdm09 and 10 were B/Victoria. Virus sequencing data identified potential in-flight transmission, as well as independent infections prior to travel. Under the quarantine strategy in place at the time, the probability that these cases could initiate influenza outbreaks in Australia neared 0. However, this probability rose as quarantine requirements relaxed. CONCLUSIONS: Detection of influenza virus infections in repatriated travellers provided a source of influenza viruses otherwise unavailable and enabled development of the A(H3N2) vaccine seed viruses included in the 2022 Southern Hemisphere influenza vaccine. Failing to test quarantined returned travellers for influenza, represents a missed opportunity for enhanced surveillance to better inform public health preparedness.