The impact of the SARS-CoV-2 pandemic on global influenza surveillance: Insights from 18 National Influenza Centers based on a survey conducted between November 2021 and March 2022
The emergence of a new respiratory infection (COVID-19) in early 2020 has had a profound impact on the activity and surveillance of other respiratory infections including influenza. Influenza causes seasonal epidemics in most parts of the world and is responsible for a high burden of disease. In temperate climates, influenza activity generally peaks in colder, winter months. However, during the Northern Hemisphere's winter of 2019/2020, seasonal influenza activity was dramatically interrupted with limited to no activity reported in most countries in both hemispheres. In contrast, during the Southern Hemisphere winter of 2021 and Northern Hemisphere winter of 2021/2022, many countries saw co-circulation of both SARS-CoV-2 and influenza. However, the levels of influenza activity during winter remained far lower compared with pre-COVID years. Yet, several countries (e.g., France and South Africa) experienced a second wave of influenza activity outside of the typical influenza season. High levels of influenza activity returned for several Southern Hemisphere countries in 2022 with Australia experiencing higher weekly case counts compared with the 5-year average. A variety of factors are thought to have contributed to these unusual patterns of influenza activity. These include the introduction and/or relaxation of non-pharmaceutical interventions (NPIs), such as travel regulations, mask wearing, viral interference as well as high (er) influenza vaccination rates, and changes in care-seeking behavior or patient pathways.