Seasonal influenza activity declines globally during the coronavirus disease 2019 (COVID-19) pandemic (1-4). For instance, in China, influenza activity, as measured by percentage of submitted specimens testing positive, dropped from 11.8% to 2.0% in 2020–2021 influenza season, compared to the past 5 years (5). The long-period of low-exposure to influenza viruses adds great uncertainty on preparedness for the incoming 2021–2022 influenza season. Influenza vaccination is one of the most effective measures in seasonal influenza prevention and control, but with only a few influenza viruses circulating, it could be difficult to determine the targeted strains for vaccination.
In this context, it is of primary importance to identify alternative mitigation measures for the incoming 2021–2022 influenza season, the first season after long periods of virtually no influenza outbreaks worldwide. Using data from China, the United Kingdom, and the United States, we forecasted the influenza activity in the incoming 2021–2022 influenza season under hypothetical scenarios without non-pharmaceutical interventions (NPIs) and with different assumptions on mask-wearing and mobility levels.
Under the scenario without COVID-19 mitigation measures, we predicted that influenza percent positivity would be 18.6 [95% confidence interval (CI): 13.1, 24.2] and 16.9 (95% CI: 12.7, 21.6) for Northern China and Southern China, and 10.5 (95% CI: 6.4, 14.1) and 13.4 (95% CI: 9.3, 17.4) for England and the US, respectively. In Southern China, the rebound could continue until the summer with a secondary peak, a pattern more similar to that in the years before 2018 than in recent years (Figure 1B).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. Abbreviations: NPIs=non pharmaceutical interventions; CI=confidence intervals.
Influenza activity was projected to stay at a low level with percent positivity below 10.0 if the mask-wearing could continue throughout the 2021–2022 season. Late-season rebounds were observed in Southern China if the mask intervention were relaxed. For all regions, if the intervention were relaxed in the mid of influenza season, a sharper rebound could occur (Figure 2). When implemented in the full 2021–2022 season, mask-wearing alone could reduce 7.0–16.8 influenza activity in the four regions (Table 1).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. Note: Weekly percent positivity under domestic mobility mitigation measures, assuming domestic mobility reduced by 30%, 50% or 70%, for (A) Northern China, (B) Southern China, (C) England and (D) the US. Abbreviation: NPIs=non pharmaceutical interventions.
NPIs Northern China Southern China England United States Mean 95% CI Mean 95% CI Mean 95% CI Mean 95% CI Mask-wearing alone 16.8 (11.5, 22.2) 15.9 (11.8, 20.6) 7.0 (4.2, 9.5) 9.3 (6.2, 12.4) International mobility alone (Reduced by 50%) 7.2 (3.8, 10.7) 3.2 (1.0, 5.4) 3.7 (1.5, 5.7) 4.6 (1.9, 7.3) Domestic mobility alone (Reduced by 50%) 3.0 (−2.6, 11.9) 4.7 (−4.2, 12.9) 1.2 (0, 8.9) 3.3 (0, 14.1) Abbreviation: NPIs=non pharmaceutical interventions.
Table 1. Predicted reductions on percent positivity under alternative NPIs (relative to no NPIs) in 2021–2022 season.
Our projected estimates for the mask-wearing intervention relied on the actual acceptance of mask-wearing measures during the COVID-19 period. Should a mask-wearing measure with a magnitude 70% less than that during the COVID-19 period be implemented, the incoming winter could still have a modestly large influenza outbreak (Figure 3E–H). Nevertheless, when coordinated with an appropriate vaccination program, a much less stringent mask-wearing measure was capable of keeping the influenza activity at low levels. For example, if an extra of 20% population were vaccinated with influenza vaccines [considering 60% efficacy at all age groups (8)] before the influenza season starts, a winter mask-wearing intervention with only 30% magnitude of that in the COVID-19 period for about two months, was able to reduce influenza activity to low levels (Figure 3I–L).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. Abbreviations: NPIs=non pharmaceutical interventions; COVID-19=coronavirus disease 2019.
Finally, the rebound would also be smaller if international mobility mitigation measures continued only, but the decline depended on the magnitude of the mitigation as well as the past seasonal patterns. Only in regions with the influenza profile exhibiting a single winter-peak outbreak, e.g., Northern China, England, and the US, and with mobility reduced by 50% or higher from normal levels, influenza activity could be deflected substantially (Figures 1 and 2). As expected, simultaneously mitigating both international mobility and domestic mobility could flatten the influenza activity (Figure 3I–L). We estimated that reducing 50% of the international mobility, relative to normal mobility prior to the COVID-19 pandemic, could reduce 3.2–7.2 positivity in the 4 study regions. Domestic mobility mitigation was likely to have a smaller impact than international mobility except in Southern China, where reducing domestic mobility during the influenza season by half could maintain influenza activity at markedly lower levels (Figure 1J and Figure 2). We estimated that reducing 50% of the domestic mobility could reduce 4.7 (95% CI: –4.2, 12.9) influenza activity in Southern China and 1.2–3.3 in the other three regions (Table 1).