Measures to prevent spread of SARS-CoV-2 may also mitigate flu

Sept. 18, 2020

Following widespread adoption of community mitigation measures to reduce transmission of SARS-CoV-2, the percentage of U.S. respiratory specimens that tested positive for influenza decreased from more than 20 percent to 2.3 percent and has remained at historically low inter-seasonal levels, the Centers for Disease Control and Prevention (CDC) reported in its Morbidity and Mortality Weekly Report. Data from Southern Hemisphere countries also showed little influenza activity. 

“These findings suggest that certain community mitigation measures might be useful adjuncts to influenza vaccination during influenza seasons, particularly for populations at highest risk for developing severe disease or complications,” the CDC said. 

After recognition of widespread community transmission of SARS-CoV-2, the virus that causes coronavirus disease 2019 (COVID-19), by mid- to late February 2020, indicators of influenza activity began to decline in the Northern Hemisphere. 

Data from clinical laboratories in the United States showed a 61 percent decrease in the number of specimens submitted (from a median of 49,696 per week during September 29, 2019-February 29, 2020, to 19,537 during March 1-May 16, 2020) and a 98 percent decrease in influenza activity as measured by the percentage of submitted specimens testing positive (from a median of 19.34 percent to 0.33 percent). Inter-seasonal circulation of influenza in the United States (May 17-August 8, 2020) is currently at historical lows: a median of 0.20 percent positive tests in 2020, compared with 2.35 percent in 2019, 1.04 percent in 2018 and 2.36 percent in 2017. 

During the period of April-July 2020, only 33 influenza positive test results were detected among 60,031 specimens tested in Australia, 12 among 21,178 specimens tested in Chile, and six among 2,098 specimens tested in South Africa, for a total of 51 influenza positive specimens among 83,307 tested in those countries. 

“Although causality cannot be inferred from these ecological comparisons, the consistent trends over time and place are compelling and biologically plausible,” the CDC said. 

CDC has the report

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