Decreased influenza prevalence may be associated with long-term public health measures against COVID-19 in China, according to a study.

The research, published in the journal Health Data Science, provides preliminary evidence of the long-term relationship between prevention and control measures and influenza transmission in China during the COVID-19 pandemic.

"Since December 2019, various nonpharmaceutical interventions (NPIs) and COVID-19 vaccination have been used to prevent and control the community transmission of COVID-19," said Zirui Guo from Peking University School of Public Health.

"However, the relationship between the changing influenza epidemic and COVID-19 prevention and control was unclear," Guo said.

Influenza is an acute respiratory infectious disease that can impose a high burden and lead to severe seasonal epidemics or even pandemics.

However, a marked decrease in influenza activity was registered in Japan and the US, among other regions in the Northern Hemisphere, in early 2020, the researchers said.

Australia, Chile, and other areas in the Southern Hemisphere shared this observation during their influenza season in 2020, they said.

"The COVID-19 outbreak changed the epidemic trend and characteristics of influenza, according to our analysis," said Professor Min Liu, author of the study.

A significant decrease in influenza activity in China was observed during the 2020-2022 COVID-19 pandemic, particularly in winter and spring. In addition, influenza seasonality was evident from 2010 to 2019 but absent during the 2020/2021 season across China, whether north or south.

The researchers analysed the epidemiology and seasonal patterns of influenza based on the timeline of COVID-19 NPIs in place in China.

They found that the reduction of influenza viral infection might be related to everyday COVID-19 public health interventions in the country. Respiratory diseases frequently occur in winter and spring when COVID-19 and influenza tend to circulate easily.

The resurgence of other respiratory viruses once suppressed under COVID-19 NPIs in 2020-2021 was recognised worldwide, the researchers said. The entire population has missed the opportunity for enhanced immunity against influenza after a prolonged low flu season during 2020-2022, they said.

As a result, high-risk populations, such as young children and elderly individuals, are more vulnerable to widespread and severe illness from influenza as time goes by, according to the researchers.

To protect these vulnerable populations, besides increasing influenza vaccine coverage, efforts should be paid to strengthen influenza surveillance and establish a comprehensive surveillance system for influenza virus and SARS-CoV-2, they said. "Given the limitations in our analysis, further studies should be performed to confirm our results," Liu said.

"More indicators, such as influenza-positive rate and the number of influenza report cases, might supplement and verify our study," the scientist added.