Surveillance of seasonal influenza viruses during the COVID-19 pandemic
in Tokyo, Japan, 2018--2023, a single-center study
Abstract
Introduction: COVID-19 pandemic led to significant reductions in
influenza detection worldwide, making influenza trends challenging to
monitor. The number of influenza cases decreased significantly in Japan,
raising concerns about the potential risk of decreased immunity to
influenza in the population. Our single-center study aimed to
investigate influenza trends before and during the COVID-19 pandemic in
Tokyo, Japan. Materials and Methods: This cross-sectional study included
patients of all ages who visited Tokyo Shinagawa Hospital between April
1, 2018, and March 31, 2023. Influenza and COVID-19 tests were conducted
using Quick Navi-Flu2 and polymerase chain reaction (PCR). We analyzed
data from before and during the COVID-19 epidemic, based on patient
background, hospitalization, and deaths, collected from medical records.
Results: A total of 12,577 influenza tests were conducted, with
approximately 100 tests consistently performed each month even in the
influenza off-season. Throughout the observation period, 962 positive
cases were identified. However, no cases were observed for 27 months
between March 2020 and November 2022. Cases of influenza A were observed
again in December 2022, and cases of influenza B were observed again in
March 2023, similar to the influenza incidence reports from Tokyo. The
positivity rate during the 2022–2023 winter season was lower than
before the COVID-19 epidemic and decreased in elderly patients, with no
hospitalizations or deaths observed. Conclusion: This single-center
study provided actual trend data for influenza patients before and
during COVID-19 outbreaks in Tokyo, which could offer insights into the
potential impact and likelihood of influenza virus infection in Japan.