COVID-19 Pandemic Effects on Seasonal Influenza Patterns in West Virginia

Introduction The COVID-19 pandemic began in West Virginia (WV) in March 2020 at the end of the seasonal increase in influenza virus infections. Diagnostic and surveillance testing for influenza continued throughout the pandemic because these viruses cause similar diseases and the effects of SARS-CoV-2 on seasonal influenza patterns were unpredictable. Methods Positivity rates for SARS-CoV-2 and influenza viruses were studied using test results obtained from samples collected across an academic health system from July 2018 through June 2022. Trends in COVID-19 variants were assessed using data obtained from national and local databases. Data were analyzed to detect differences in and correlations between SARS-CoV-2 and influenza positivity rates. Results Test positivity rates were higher for the 602,109 SARS-CoV-2 tests than the 186,563 influenza tests performed during the study period (13.7% vs. 3.5%, p<0.0001). SARS-CoV-2 test positivity rose and fell with the emergence of SARS-CoV-2 variants of differing infectivity, being highest when the omicron variant predominated. Significant negative correlations between the number of SARS-CoV-2 and influenza positive tests were seen throughout stages of the pandemic. The normally expected influenza season did not occur during winter 2020 through spring 2021, and there were essentially no influenza cases during the delta surge. A more typical pattern of seasonal influenza was observed in late 2021 to early 2022 during the omicron surge. Conclusion There was an inverse correlation between SARS-CoV-2 and influenza test positivity rates observed throughout several COVID-19 surges in WV. The reasons for these correlations are unclear.