The Incoming Influenza Season - China, the United Kingdom, and the United States, 2021-2022

Abstract

Introduction: Seasonal influenza activity has declined globally since the widespread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission. There has been scarce information to understand the future dynamics of influenza - and under different hypothesis on relaxation of non-pharmaceutical interventions (NPIs) in particular - after the disruptions to seasonal patterns.

Methods: We collected data from public sources in China, the United Kingdom, and the United States, and forecasted the influenza dynamics in the incoming 2021-2022 season under different NPIs. We considered Northern China and Southern China separately, due to the sharp difference in the patterns of seasonal influenza. For the United Kingdom, data were collected for England only.

Results: Compared to the epidemics in 2017-2019, longer and blunter influenza outbreaks could occur should NPIs be fully lifted, with percent positivity varying from 10.5 to 18.6 in the studying regions. The rebounds would be smaller if the mask-wearing intervention continued or the international mobility stayed low, but sharper if the mask-wearing intervention was lifted in the middle of influenza season. Further, influenza activity could stay low under a much less stringent mask-wearing intervention coordinated with influenza vaccination.

Conclusions: The results added to our understandings of future influenza dynamics after the global decline during the coronavirus disease 2019 (COVID-19) pandemic. In light of the uncertainty on the incoming circulation strains and the relatively low negative impacts of mask wearing on society, our findings suggested that wearing mask could be considered as an accompanying mitigation measure in influenza prevention and control, especially for seasons after long periods of low-exposure to influenza viruses.

Keywords: COVID-19; Influenza; Mask-wearing intervention; Non-pharmaceutical interventions.

Figure 1

Predicted influenza activities in 2021–2022 season under no NPI and varying NPIs. Weekly percent positivity under mask-wearing intervention for (A) Northern China, (B) Southern China, (C) England and (D) the US. Weekly percent positivity under international mobility mitigation being reduced by 50% for (E) Northern China, (F) Southern China, (G) England and (H) the US. Weekly percent positivity under domestic mobility mitigation being reduced by 50% for (I) Northern China, (J) Southern China, (K) England and (L) the US. Shaded area refer to 95% CI.

Figure 2

Predicted influenza activities in 2021–2022 season under NPIs with alternative assumptions. Weekly percent positivity under no interventions and three timings of mask-wearing intervention, implemented during the full influenza season, the first half of the season the second half of the season, for (A) Northern China, (B) Southern China, (C) England and (D) the US. Weekly percent positivity under international mobility mitigation measures, assuming the international mobility reduced by 30%, 50% or 70%, for (E) Northern China, (F) Southern China, (G) England and (H) the US.

Figure 3

Predicted influenza activities in 2021–2022 season under alternative mask-wearing interventions and combined NPIs. Weekly percent positivity under no intervention and the differential magnitude of mask-wearing intervention, for (A) Northern China, (B) Southern China, (C) England and (D) the US. Weekly percent positivity under alternative scenarios considering a mask-wearing intervention with intensity 70% less than (i.e., 30% of) that during the COVID-19 period coordinated with a vaccination program where an extra 20% population vaccinated at 60% vaccine efficacy, for (E) Northern China, (F) Southern China, (G) England and (H) the US. Weekly percent positivity under combined NPIs, mask-wearing and 50% reduction on international mobility as well as 50% reduction on both domestic and international mobility for (I) Northern China, (J) Southern China, (K) England and (L) the US.