Impacts of the COVID-19 pandemic on future seasonal influenza epidemic

Seasonal influenza viruses typically cause annual epidemics worldwide infecting 5-15% of the human population. However, during the first two years of the COVID-19 pandemic, seasonal influenza virus circulation was unprecedentedly low with very few reported infections. The lack of immune stimulation to influenza viruses during this time, combined with waning antibody titres to previous influenza virus infections, could lead to increased susceptibility to influenza in the coming seasons and to larger and more severe epidemics when infection prevention measures against COVID-19 are relaxed. Here, based on serum samples from 165 adults collected longitudinally before and during the pandemic, we show that the waning of antibody titres against seasonal influenza viruses during the first two years of the pandemic is likely to be negligible. Using historical influenza virus epidemiological data from 2003-2019, we also show that low country-level prevalence of each influenza subtype over one or more years has only small impacts on subsequent epidemic size. These results suggest that the risks posed by seasonal influenza viruses remained largely unchanged during the first two years of the COVID-19 pandemic and that the sizes of future seasonal influenza virus epidemics will likely be similar to those observed before the pandemic.