Estimating the Impact of Low Influenza Activity in 2020 on Population Immunity and Future Influenza Seasons in the United States

Background Influenza activity in the 2020–2021 season was remarkably low, likely due to implementation of public health preventive measures such as social distancing, mask wearing, and school closure. With waning immunity, the impact of low influenza activity in the 2020–2021 season on the following season is unknown. Methods We built a multistrain compartmental model that captures immunity over multiple influenza seasons in the United States. Compared with the counterfactual case, where influenza activity remained at the normal level in 2020–2021, we estimated the change in the number of hospitalizations when the transmission rate was decreased by 20% in 2020–2021. We varied the level of vaccine uptake and effectiveness in 2021–2022. We measured the change in population immunity over time by varying the number of seasons with lowered influenza activity. Results With the lowered influenza activity in 2020–2021, the model estimated 102 000 (95% CI, 57 000–152 000) additional hospitalizations in 2021–2022, without changes in vaccine uptake and effectiveness. The estimated changes in hospitalizations varied depending on the level of vaccine uptake and effectiveness in the following year. Achieving a 50% increase in vaccine coverage was necessary to avert the expected increase in hospitalization in the next influenza season. If the low influenza activity were to continue over several seasons, population immunity would remain low during those seasons, with 48% of the population susceptible to influenza infection. Conclusions Our study projected a large compensatory influenza season in 2021–2022 due to a light season in 2020–2021. However, higher influenza vaccine uptake would reduce this projected increase in influenza.