Influenza control during the COVID-19 pandemic.

As the northern hemisphere influenza season begins, challenges loom for health systems bracing to manage a simultaneous rise in cases of COVID-19 and influenza. Successive winters have taught us that the burden of influenza is high in ordinary times, and a COVID-19 pandemic caused by a virus with shared symptomatology, but with protracted hospital admissions and a higher risk of mortality, could potentially make the forthcoming northern hemisphere influenza season a public health catastrophe.

COVID-19 spread through the southern hemisphere just as the influenza season began, yet the experience this autumn and winter has been remarkable for the near absence of influenza. Following on from weekly surveillance data from Australia and New Zealand, which showed historically low levels of influenza infections during the 2020 influenza season, we reviewed data from the WHO Global Influenza Surveillance and Response System shared on FluNet. Across countries in the temperate southern hemisphere, the number of specimens positive by subtype from WHO sentinel surveillance sites corroborates little southern hemisphere influenza activity since mid April, 2020 (appendix). Although testing might have been focused away from influenza and onto severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in some settings, this was not the case in Australia, for example, where more influenza tests were done in 2020 than in previous years, with few positive results.

Observational data cannot determine causation, but these early findings are consistent with the hypothesis that the non-pharmaceutical interventions (NPIs) put in place to control the spread of COVID-19 could have dramatically reduced the burden of influenza and prevented winter epidemics. If this were the case, it would not be consistent with prevailing dogma that specific NPIs prominent in the management of COVID-19 (eg, widespread mask use, school and workplace closures, physical distancing, travel restrictions, and limits on gathering sizes) would have limited efficacy for influenza control, due to the characteristics and transmission dynamics of influenza virus and experience in previous influenza pandemics.2, 3

As restrictions are reinstated in Europe to control increasing COVID-19 case numbers, the southern hemisphere experience suggests consideration must be given to whether these NPIs could affect other transmissible infections—particularly influenza, with its high morbidity, mortality, and health-care costs—and how this off-target effect on viruses other than SARS-CoV-2 could protect health system capacity. As evidence on both the benefits and costs of NPIs in the COVID-19 pandemic accrues, their role in the management of future influenza pandemics can be carefully considered.