Efforts to prevent COVID-19 led to global decline in flu
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Interventions to prevent SARS-CoV-2 transmission have led to a global decline in influenza during the COVID-19 pandemic, researchers reported in MMWR.
In addition to causing a significant drop in the percentage of respiratory specimens that tested positive for influenza in the early days of the pandemic in the United States, measures such as mask wearing, social distancing, school closures and telework have kept positive tests at “historically low interseasonal levels,” the researchers said. The Southern Hemisphere has experienced a similar effect.
Credit: CDC
If the measures continue through the fall, the influenza season in the U.S. “might be blunted or delayed,” according to the report.
“The global decline in influenza virus circulation appears to be real and concurrent with the COVID-19 pandemic and its associated community mitigation measures,” Sonja J. Olsen, PhD, an epidemiologist in the CDC’s Influenza Division, and colleagues wrote.
Olsen and colleagues reviewed data from around 300 U.S. laboratories in all 50 states, Puerto Rico, Guam and the District of Columbia. They also analyzed influenza laboratory data from surveillance platforms in Australia, Chile and South Africa to determine viral activity in the Southern Hemisphere.
Results showed that the percentage of respiratory specimens that tested positive for influenza dropped from more than 20% to 2.3%, with interseasonal levels also remaining low — 0.2% versus 1% to 2% in previous seasons. In the Southern Hemisphere, there were only 33 positive influenza results among 60,031 Australian specimens, 12 among 21,178 Chilean specimens and six among 2,098 South African specimens from April to July — a positivity rate of .06% (95% CI, .04%-.08%). In comparison, 13.7% of Southern Hemisphere specimens tested positive for influenza during the same period in 2017, 2018 and 2019.
“Initially, declines in influenza virus activity were attributed to decreased testing, because persons with respiratory symptoms were often preferentially referred for SARS-CoV-2 assessment and testing,” Olsen and colleagues wrote. “However, renewed efforts by public health officials and clinicians to test samples for influenza resulted in adequate numbers tested and detection of little to no influenza virus.
“Further, some countries, such as Australia, had less stringent criteria for testing respiratory specimens than in previous seasons and tested markedly more specimens for influenza but still detected few with positive results during months when Southern Hemisphere influenza epidemics typically peak.”
Experts have expressed concern about the prospect of a serious influenza season occurring at the same time as the COVID-19 pandemic.
“In light of the novelty of the COVID-19 pandemic and the uncertainty of continued community mitigation measures, it is important to plan for seasonal influenza circulation this fall and winter,” the researchers wrote. “Influenza vaccination for all persons aged 6 months or older remains the best method for influenza prevention and is especially important this season when SARS-CoV-2 and influenza virus might cocirculate.”
Perspective
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Amesh A. Adalja, MD
Because influenza and COVID-19 are transmitted in similar fashion, it is not surprising that community mitigation activities for one will have an impact on the other. This is clearly demonstrated in the data of the most recent influenza season in the United States, during which COVID-19 mitigation basically disrupted the influenza season earlier. The dramatically lower current influenzas season in the Southern Hemisphere provides further evidence of this phenomenon and likely has significantly diminished the worldwide circulation of the seasonal influenza viruses. Since these measures keep influenza at bay, and will (hopefully) be coupled to high influenza vaccination rates, it may be the case that the Northern Hemisphere flu season in will be significantly blunted, leaving more capacity to deal with COVID-19.
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