Postpandemic immunity debt of influenza in the USA and England: an interrupted time series study

OBJECTIVES: The concept of "immunity debt" has gained attention in the public sphere, and some have argued that the recent out-of-season resurgence of respiratory syncytial virus demonstrates the presence of immunity debt. This study investigates the existence of immunity debt in the context of influenza. STUDY DESIGN: Interrupted time series analysis. METHODS: The positivity rate of influenza in the USA and England was gathered from the Centers for Disease Control and Prevention and the UK Health Security Agency. A time series model with an autoregressive approach was used to model the dynamics of positivity rate. Binary indicator variables were included in the model to account for the impact of non-pharmaceutical interventions (NPIs) and immunity debt. RESULTS: The impact of NPIs and immunity debt on the positivity rate of influenza was found to be statistically significant. CONCLUSIONS: This present work provides evidence supporting the existence of immunity debt in influenza in both the USA and England in the immediate month following the removal of NPIs such as lockdowns and facemask mandates.