Increased population susceptibility to seasonal influenza during the COVID‐19 pandemic in China and the United States

Abstract

To the best of our knowledge, no previous study has quantitatively estimated the dynamics and cumulative susceptibility to influenza infections after the widespread lifting of COVID‐19 public health measures.We constructed an imitated stochastic susceptible‐infected‐removed model using particle‐filtered Markov Chain Monte Carlo sampling to estimate the time‐dependent reproduction number of influenza based on influenza surveillance data in southern China, northern China, and the United States during the 2022–2023 season. We compared these estimates to those from 2011 to 2019 seasons without strong social distancing interventions to determine cumulative susceptibility during COVID‐19 restrictions.Compared to the 2011–2019 seasons without a strong intervention with social measures, the 2022–2023 influenza season length was 45.0%, 47.1%, and 57.1% shorter in southern China, northern China, and the United States, respectively, corresponding to an 140.1%, 74.8%, and 50.9% increase in scale of influenza infections, and a 60.3%, 72.9%, and 45.1% increase in population susceptibility to influenza.Large and high‐intensity influenza epidemics occurred in China and the United States in 2022–2023. Population susceptibility increased in 2019–2022, especially in China. We recommend promoting influenza vaccination, taking personal prevention actions on at‐risk populations, and monitoring changes in the dynamic levels of influenza and other respiratory infections to prevent potential outbreaks in the coming influenza season.

Temporal shifts in 24 notifiable infectious diseases in China before and during the COVID-19 pandemic

“…However, this period of restricted transmission was followed by increased reported cases of diseases such as HFMD, AHC and pertussis. Other studies have also reported a 74.8–140.1% increase in influenza infections during the 2022–2023 season 39 . This phenomenon, referred to as the “immune gap,” highlights the “broad-spectrum” effectiveness of PHSMs.…”

Heterogeneity of influenza infection at precise scale in Yinchuan, Northwest China, 2012–2022: evidence from Joinpoint regression and spatiotemporal analysis

“…Due to the prolonged absence of exposure to the pathogenic microorganism, there is an increasing number of individuals lacking immunity to the influenza virus 49 , 50 . Wang et al 51 quantified the population susceptibility to influenza viruses has increased by 45.1–72.9% after the COVID-19 pandemic. Thus, once pandemic prevention measures are lifted, there is a significant surge.…”

Resurgence of seasonal influenza driven by A/H3N2 and B/Victoria in succession during the 2023–2024 season in Beijing showing increased population susceptibility

“…The weekly positive rate was calculated by dividing the number of positive cases by the total number of cases tested in a given week 10 . The flu epidemic season was deemed to have commenced when the weekly positive rate exceeded 10% and remained at above 10% for the subsequent 2 weeks 18 . The conclusion of the flu epidemic season was determined by the weekly positive rate dropping below 10% and maintaining this level for a minimum of two consecutive weeks 18 .…”

“…10 The flu epidemic season was deemed to have commenced when the weekly positive rate exceeded 10% and remained at above 10% for the subsequent 2 weeks. 18 The conclusion of the flu epidemic season was determined by the weekly positive rate dropping below 10% and maintaining this level for a minimum of two consecutive weeks. 18 All analyses in this study were based on natural weeks.…”

“…18 The conclusion of the flu epidemic season was determined by the weekly positive rate dropping below 10% and maintaining this level for a minimum of two consecutive weeks. 18 All analyses in this study were based on natural weeks.…”