Impact of Low Rates of Influenza on Next-Season Influenza Infections

INTRODUCTION: Interventions to curb the spread of COVID-19 during the 2020–2021 influenza season essentially eliminated influenza during that season. Given waning antibody titers over time, future residual population immunity against influenza will be reduced. The implication for the subsequent 2021–2022 influenza season is unknown. METHODS: An agent-based model of influenza implemented in the Framework for Reconstructing Epidemiological Dynamics simulation platform was used to estimate cases and hospitalizations over 2 successive influenza seasons. The impact of reduced residual immunity owing to protective measures in the first season was estimated over varying levels of similarity (cross-immunity) between influenza strains over the seasons. RESULTS: When cross-immunity between first- and second-season strains was low, a decreased first season had limited impact on second-season cases. High levels of cross-immunity resulted in a greater impact on the second season. This impact was modified by the transmissibility of strains in the 2 seasons. The model estimated a possible increase of 13.52%–46.95% in cases relative to that in a normal season when strains have the same transmissibility and 40%–50% cross-immunity in a season after a very low one. CONCLUSIONS: Given the light 2020–2021 influenza season, cases may increase by as much as 50% in 2021–2022, although the increase could be much less, depending on cross-immunity from past infection and transmissibility of strains. Enhanced vaccine coverage or continued interventions to reduce transmission could reduce this high season. Young children may have a higher risk in 2021–2022 owing to limited exposure to infection in the previous year.