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Flu numbers in the U.S. were historically low during COVID-19 in the spring, with deep declines also occurring in the recently completed Southern Hemisphere flu season, CDC researchers found.

Influenza positivity rates in specimens tested (a standard metric of community flu activity) fell 98% in 2020 during March 1-May 16 relative to Sept 29, 2019-Feb. 29, 2020, plummeting from a median of 19.34% to 0.33%, reported Sonja Olsen, PhD, of the CDC in Atlanta, and colleagues.

Indeed, circulation of influenza in the U.S. hit historic lows in summer 2020, with a median of 0.20% positive tests from May 17-August 8 versus 1%-2% from 2017-2019, the authors wrote in the . A graph indicated that influenza positivity rates dropped off sharply, approaching zero by early April -- a time that, in previous seasons, it hovered around 15%.

Olsen and colleagues noted they used March 1 as a benchmark because it was closest to when the U.S. declared COVID-19 a public health emergency, and when "widespread implementation" of community measures such as school closures, social distancing, and mask wearing started around the country.

Because influenza is less transmissible than SARS-CoV-2, these measures "likely contributed to a more substantial interruption in influenza transmission," according to Olsen and colleagues. "Although causality cannot be inferred from these ecological comparisons, the consistent trends over time and place are compelling and biologically plausible."

The group examined data from 300 U.S. clinical laboratories in 50 states, Puerto Rico, Guam, and the District of Columbia that participated in major surveillance systems.

In 2019, flu season started as normal in the U.S., increasing in early November with more than 20% of specimens testing positive for influenza from Dec. 15, 2019 to March 7, 2020.

But by March 22, while the number of samples tested remained high, percent positivity fell to 2.3%, and remained less than 1% since the week of April 5.

Below the equator, the numbers also point to a dramatically reduced flu season. Australia, Chile, and South Africa, where winter is now ending, reported just 55 positive specimens total across the three countries out of 83,307 tested (0.06%, 95% CI 0.04%-0.08%) during April-July compared to 25,000 specimens testing positive of 178,690 in 2019 (13.7%, 95% CI 13.6%-13.9%).

Olsen and colleagues said COVID-related community mitigation measures, if continued through the fall, could keep influenza down in the U.S. this winter. Nevertheless, the authors emphasized the importance of flu vaccination this year, given the likelihood of SARS-CoV-2 and influenza circulating at the same time.

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