Global patterns and drivers of influenza decline during the COVID-19 pandemic

Abstract

Influenza circulation declined during the COVID-19 pandemic. The timing and extent of decline and its association with interventions against COVID-19 were described for some regions. Here, we provide a global analysis of the influenza decline between March 2020 and September 2021 and investigate its potential drivers. We computed influenza change by country and trimester relative to the 2014-2019 period using the number of samples in the FluNet database. We used random forests to determine important predictors in a list of 20 covariates including demography, weather, pandemic preparedness, COVID-19 incidence, and COVID-19 pandemic response. With a regression tree we then classified observations according to these predictors. We found that influenza circulation decreased globally, with COVID-19 incidence and pandemic preparedness being the two most important predictors of this decrease. The regression tree showed interpretable groups of observations by country and trimester: Europe and North America clustered together in spring 2020, with limited influenza decline despite strong COVID-19 restrictions; in the period afterwards countries of temperate regions, with high pandemic preparedness, high COVID-19 incidence and stringent social restrictions grouped together having strong influenza decline. Conversely, countries in the tropics, with altogether low pandemic preparedness, low reported COVID-19 incidence and low strength of COVID-19 response showed low influenza decline overall. A final group singled out four "zero-Covid" countries, with the lowest residual influenza levels. The spatiotemporal decline of influenza during the COVID-19 pandemic was global, yet heterogeneous. The sociodemographic context and stage of the COVID-19 pandemic showed non-linear associations with this decline. Zero-Covid countries maintained the lowest levels of reduction with strict border controls and despite close-to-normal social activity. These results suggest that the resurgence of influenza could take equally diverse paths. It also emphasizes the importance of influenza reseeding in driving countries' seasonal influenza epidemics.

Scenario modelling for diminished influenza seasons during 2020/2021 and 2021/2022 in England

“…Public health interventions against SARS-CoV-2 have had a profound impact on the worldwide circulation of other respiratory viruses, including seasonal influenza [17]. As a result of non-pharmaceutical interventions in place for COVID-19 (such as mask-wearing, reduced social interactions and international travel) influenza activity levels were extremely low globally in 2020 to 2021 [18]. In the temperate Northern Hemisphere, during the warmer summer months of 2022 when rates of respiratory viral infections would normally be very low or absent, some countries saw unusual seasonal changes in the prevalence of influenza [19], respiratory syncytial virus (RSV) [20] and parainfluenza [21]; these observations coincided with a reduction in COVID-19 mitigation measures.…”