An Intense Out-of-Season Rebound of Influenza Activity After the Relaxation of Coronavirus Disease 2019 Restrictions in Beijing, China

Li Zhang; Wei Duan; Chunna Ma; Jiaojiao Zhang; Ying Sun; Jiaxin Ma; Yingying Wang; Daitao Zhang; Quanyi Wang; Jue Liu; Min Liu

doi:10.1093/ofid/ofae163

An Intense Out-of-Season Rebound of Influenza Activity After the Relaxation of Coronavirus Disease 2019 Restrictions in Beijing, China

Open Forum Infect Dis. 2024 Mar 20;11(4):ofae163. doi: 10.1093/ofid/ofae163. eCollection 2024 Apr.

Authors

Li Zhang^{1

2}, Wei Duan², Chunna Ma², Jiaojiao Zhang², Ying Sun², Jiaxin Ma², Yingying Wang², Daitao Zhang², Quanyi Wang^{3

4}, Jue Liu¹, Min Liu¹

Affiliations

¹ Department of Epidemiology and Biostatistics, School of Public Health, Peking University, Beijing, China.
² Institute for Infectious Disease and Endemic Disease Control, Beijing Center for Disease Prevention and Control, Beijing, China.
³ Center Office, Beijing Center for Disease Prevention and Control, Beijing, China.
⁴ Beijing Research Center for Respiratory Infectious Diseases, Beijing, China.

Abstract

Background: The aim of this study was to investigate the changes of epidemic characteristics of influenza activity pre- and post-coronavirus disease 2019 (COVID-19) in Beijing, China.

Methods: Epidemiologic data were collected from the influenza surveillance system in Beijing. We compared epidemic intensity, epidemic onset and duration, and influenza transmissibility during the 2022-2023 season with pre-COVID-19 seasons from 2014 to 2020.

Results: The overall incidence rate of influenza in the 2022-2023 season was significantly higher than that of the pre-COVID-19 period, with the record-high level of epidemic intensity in Beijing. The onset and duration of the influenza epidemic period in 2022-2023 season was notably later and shorter than that of the 2014-2020 seasons. Maximum daily instantaneous reproduction number (R_t) of the 2022-2023 season (R_t = 2.31) was much higher than that of the pre-COVID-19 period (R_t = 1.49). The incidence of influenza A(H1N1) and A(H3N2) were the highest among children aged 0-4 years and 5-14 years, respectively, in the 2022-2023 season.

Conclusions: A late, intense, and short-term peak influenza activity was observed in the 2022-2023 season in Beijing. Children <15 years old were impacted the most by the interruption of influenza circulation during the COVID-19 pandemic. Maintaining continuous surveillance and developing targeted public health strategies of influenza is necessary.

Keywords: COVID-19; incidence; influenza; instantaneous reproduction number; surveillance.