What have been the drivers of influenza decline through the COVID-19 pandemic?
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In a current research posted to the medRxiv* preprint server, researchers investigated the potential drivers of a world decline in influenza virus circulation through the coronavirus illness 2019 (COVID-19) pandemic.

Study: Global patterns and drivers of influenza decline through the COVID-19 pandemic. Image Credit: fizkes/Shutterstock


Since 2020, with the worldwide unfold of extreme acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the research confirmed an obvious shortening of the flu season in tropical and a few northern hemisphere international locations. The pattern continued in 2021 and expanded to international locations within the southern hemisphere like Australia and New Zealand within the following 18 months.

Since each influenza virus and SARS-CoV-2 have an identical transmission route, social restriction measures imposed through the pandemic and non-pharmaceutical interventions (NPIs) in opposition to COVID-19 additionally hindered influenza circulation globally. Studies have investigated this phenomenon for particular international locations however not globally. A worldwide evaluation of the noticed decline in influenza circulation through the COVID-19 pandemic may assist assess future viral circulation and interpret the present transmission traits.

About the research

In the current research, researchers used information from the FluNet influenza database to compute influenza change for 166 international locations and all 13 weeks-long trimesters between March 2020 and September 2021 relative to a pre-pandemic interval between 2014 and 2019. There have been 20 trimesters between winter 2014 to 2015 and autumn 2019 (pre-pandemic interval) and 6 trimesters between spring 2020 to summer season 2021 (pandemic interval).

Next, the researchers computed the proportion of influenza-positive instances for six pandemic trimesters. Additionally, they decided the ratio between the proportion of influenza-positive instances throughout pandemic trimesters and the imply share of optimistic instances within the corresponding pre-pandemic trimesters.

The group used random forests (RFs)-based VSURF algorithm to determine influenza decline predictors of a big worth from a listing of 20 covariates (e.g., COVID-19 incidence, COVID-19 pandemic response, COVID-19 pandemic preparedness, demography, climate) from public sources and the International Air Transport Association (IATA). Finally, they retained covariates with minimal variables, having a prediction error lower than the minimal prediction error plus one commonplace deviation (SD). As per the predictors of influenza decline, the group categorized comparable observations in time and house utilizing a regression tree. Last, they carried out a robustness evaluation to verify for stochastic fluctuations within the dataset analyzed utilizing the regression tree and the standards for together with the FluNet information.

Study findings

The FluNet information confirmed larger peaks in seasonal influenza instances in northern international locations and decrease peaks for southern international locations through the pre-pandemic interval. The authors noticed a pointy drop within the share of influenza-positive checks through the COVID-19 pandemic interval to a minimal of 0.04% throughout July and August 2020.

The authors made 376 trimester-country observations from 112 international locations through the pandemic. Compared to 2 orders of magnitude through the pre-pandemic interval, the proportion of influenza optimistic checks diversified over 5 orders of magnitude throughout international locations and trimesters through the pandemic interval. Contrary to the expectations, the proportion of optimistic influenza checks was greater than 100 instances smaller for 135/376 trimester-country observations. Notably, Japan reported zero optimistic checks out of 26114 processed checks in spring 2021. Conversely, the proportion of optimistic checks was 15% in Haiti throughout Winter 2020-21 in comparison with barely 2.2% earlier than the pandemic.

During spring 2020, the decline in influenza circulation was lower than 90% from the pre-pandemic interval for 46/65 international locations. In the next trimesters, this decline turned extra pronounced for Europe, Mexico, North America, and Japan through the winter 2020-21 and spring 2021. Likewise, the decline turned pronounced for a number of Southern-hemisphere international locations throughout summer season 2020 and summer season 2021.

The regression tree analyzed information from 93 international locations and 330 trimesters. While North America and Europe had a low influenza decline regardless of strict restrictions through the COVID-19 pandemic within the spring of 2020, tropical international locations confirmed a lesser lower in influenza instances regardless of poor pandemic preparedness. In the tropical international locations, the COVID-19 case incidence and the energy of COVID-19 response have been additionally decrease in comparison with the European and American nations.

After spring 2022, international locations of temperate areas grouped to indicate a steep decline in influenza circulation. These nations had excessive COVID-19 incidence and stringent social restrictions and have been extremely ready for a pandemic. The evaluation additionally recognized 4 international locations with ‘no COVID-19’ instances, with a low influenza decline total. Intriguingly, these international locations had not imposed strict social restrictions however solely strict border controls to take care of the bottom discount ranges.


The research confirmed a decline in influenza circulation globally, and of the 11 predictors of the log relative influenza stage, recognized COVID-19 day by day instances and infectious illness vulnerability index (IDVI) as the 2 key covariates accountable for this decline, following Breiman’s rule. More importantly, the research evaluation pointed at a heterogeneous spatiotemporal decline in influenza circulation through the COVID-19 pandemic.

Further, the authors noticed a non-linear affiliation between the noticed decline in influenza circulation and the sociodemographic stage of the COVID-19 pandemic covariates. Similar to its decline, a resurgence of influenza circulation may additionally take a various path. A reseeding of influenza may drive seasonal epidemics in a number of international locations and improve instances within the prone populations, particularly kids.

*Important discover

medRxiv publishes preliminary scientific reviews that aren’t peer-reviewed and, due to this fact, shouldn’t be considered conclusive, information scientific follow/health-related conduct, or handled as established data.

Journal reference:

  • Francesco Bonacina, Pierre-Yves Bo&eumllle, Vittoria Colizza, Olivier Lopez, Maud Thomas, Chiara Poletto. (2022). Global patterns and drivers of influenza decline through the COVID-19 pandemic. medRxiv. doi: https://doi.org/10.1101/2022.07.15.2227749 https://www.medrxiv.org/content material/10.1101/2022.07.15.22277497v1

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