As in past pandemics, co-circulating pathogens may play a role in the epidemiology of coronavirus disease 2019 (COVID-19), caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Here we hypothesized that influenza interacted with SARS-CoV-2 during the early 2020 epidemic of COVID-19 in Europe. We developed a population-based model of SARS-CoV-2 transmission, combined with mortality incidence data in four European countries, to test a range of assumptions about the impact of influenza. We found consistent evidence for a 2-2.5-fold population-level increase in SARS-CoV-2 transmission associated with influenza during the period of co-circulation. These results suggest the need to increase vaccination against influenza, not only to reduce the burden due to influenza viruses, but also to counteract their facilitatory impact on SARS-CoV-2.
Influenza may facilitate the spread of SARS-CoV-2