In a recent study published on the medRxiv * prepress server, researchers investigated the potential factors for a global decrease in influenza virus circulation during the 2019 coronavirus disease pandemic (COVID-19).
Study: Global patterns and influencing factors during the COVID-19 pandemic. Image credit: fizkes / Shutterstock
Fund
Since 2020, with the global spread of severe acute respiratory syndrome (SARS-CoV-2) coronavirus 2, studies showed an apparent shortening of the flu season in tropical countries and some in the northern hemisphere. The trend continued in 2021 and expanded to southern hemisphere countries such as Australia and New Zealand over the next 18 months.
Because both influenza virus and SARS-CoV-2 have a similar route of transmission, the social restriction measures imposed during the pandemic and non-pharmaceutical interventions (NPI) against COVID-19 also hindered the circulation of influenza. flu worldwide. Studies have investigated this phenomenon for specific countries, but not globally. An overall analysis of the observed decrease in influenza circulation during the COVID-19 pandemic could help assess future viral circulation and interpret current transmission trends.
About the study
In the present study, researchers used data from the FluNet influenza database to calculate influenza change for 166 countries and all 13-week quarters between March 2020 and September 2021 relative to a pre-influenza period. -pandemic between 2014 and 2019. There were 20 quarters between winter 2014 to 2015 and autumn 2019 (prepandemic period) and six quarters between spring 2020 and summer 2021 (pandemic period).
The researchers then calculated the percentage of flu-positive cases during six pandemic quarters. In addition, they determined the relationship between the percentage of positive cases for influenza during pandemic quarters and the average percentage of positive cases in the corresponding pre-pandemic quarters.
The team used the random forest-based (RF) -based VSURF algorithm to identify influenza decrease predictors of a significant value from a list of 20 covariates (p.) From public sources and the Transportation Association. International Air Force (IATA). Finally, they retained covariates with minimal variables, with a prediction error less than the minimum prediction error plus a standard deviation (SD). According to predictors of influenza decline, the team classified similar observations in time and space using a regression tree. Finally, they performed a robustness analysis to check for stochastic fluctuations in the data set analyzed using the regression tree and the criteria for including FluNet records.
Study results
FluNet data showed higher peaks in seasonal flu cases in northern countries and lower peaks in southern countries during the pre-pandemic period. The authors observed a sharp drop in the percentage of positive tests for influenza during the COVID-19 pandemic period to a low of 0.04% during July and August 2020.
The authors made 376 observations from 112 countries in the quarter during the pandemic. Compared with two orders of magnitude during the pre-pandemic period, the percentage of positive tests for influenza varied by five orders of magnitude between countries and quarters during the pandemic period. Contrary to expectations, the percentage of positive flu tests was more than 100 times lower for the country’s 135/376-quarter observations. In particular, Japan reported zero positive tests out of the 26,114 tests processed in the spring of 2021. By contrast, the percentage of positive tests was 15% in Haiti during the winter of 2020-21 compared to just 2 , 2% before the pandemic.
During the spring of 2020, the decrease in influenza circulation was less than 90% compared to the pre-pandemic period in 46/65 countries. In the following quarters, this decline became more pronounced in Europe, Mexico, North America and Japan during the winter of 2020-21 and the spring of 2021. Also, the decline became more pronounced for several countries of the southern hemisphere during the summer of 2020 and the summer of 2021.
The regression tree analyzed data from 93 countries and 330 quarters. While North America and Europe had a low decline in influenza despite strict restrictions during the COVID-19 pandemic in the spring of 2020, tropical countries showed a smaller decrease in influenza cases despite poor preparedness for a pandemic. In tropical countries, the incidence of COVID-19 cases and the strength of the response to COVID-19 were also lower compared to European and American nations.
After the spring of 2022, countries in temperate regions grouped together to show a sharp decline in flu circulation. These nations had a high incidence of COVID-19 and strict social restrictions and were well prepared for a pandemic. The analysis also identified four countries without cases of COVID-19, with a low decrease in influenza overall. Interestingly, these countries had not imposed strict social restrictions, but only strict border controls to keep the reduction levels lower.
Conclusions
The study showed a decrease in the circulation of influenza worldwide, and of the 11 predictors of the relative level of influenza, identified the daily cases of COVID-19 and the index of vulnerability to infectious diseases (IDVI) as the two key covariates responsible for this decrease, following Breiman’s rule. More importantly, the study’s analysis pointed to a heterogeneous spatiotemporal decrease in influenza circulation during the COVID-19 pandemic.
In addition, the authors observed a nonlinear association between the observed decrease in influenza circulation and the sociodemographic stage of the covariates of the COVID-19 pandemic. Similar to its decline, a resurgence of flu circulation could also take a different path. A resurgence of the flu could cause seasonal epidemics in several countries and increase cases in susceptible populations, especially children.
* Important news
medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guided by clinical practice or health-related behavior, or treated as established information.